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Lake sediment-based sediment yields and erosion rates in the Coast Mountains, British Columbia Owens, Philip Neil 1990

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L A K E S E D I M E N T - B A S E D S E D I M E N T Y I E L D S A N D E R O S I O N R A T E S I N T H E C O A S T M O U N T A I N S , B R I T I S H C O L U M B I A By Phi l ip N e i l Owens B.Sc. (Hons.), Coventry Polytechnic, 1988 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L L M E N T O F T H E R E Q U I R E M E N T F O R T H E D E G R E E O F M A S T E R O F S C I E N C E in T H E F A C U L T Y O F G R A D U A T E S T U D I E S (Department of Geography) We accept this thesis as conforming to the required standard T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A September 1990 ® Phi l ip N e i l Owens, 1990 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia Vancouver, Canada DE-6 (2/88) ii ABSTRACT Lake sediments have been identified as an alternative to contemporary stream monitoring to establish catchment sediment yields and infer erosion rates. This is due primarily to the longer time period over which the former is based, which makes established yields and rates more representative of means or trends in sedimentation. Studies using lake sediments to establish sediment yields have generally assumed that all the sediment contained within a lake is derived from erosion of the catchment under investigation. This study undermines this assumption by constructing a comprehensive lake sediment budget to asses the relative contributions from various sources. Late Holocene (the last 2350 years) rates of sediment yield and erosion are established for 3 small (<1 km^) catchments that straddle timberline (1620 - 1850 m above sea level) in the Coast Mountains of British Columbia. Due to the temporal and spatial variability of sedimentation in lakes, sediment cores for each lake were taken using a multiple-core approach. Chronology was established by the presence of a dated tephra layer. Once the cores were extracted, corrections were made for sediment derived from aquatic productivity (organic matter and biogenic silica), regional aeolian dust input, the erosion of lake banks and for outflow losses. These sources of sediment could account for between 55 and 99% of the sediment contained within the 3 lakes. Lake trap efficiency ranges from low to >70%. Once corrected, estimates of sediment yield range from 4 and 244 kg knT^yr"*. The rate of regional aeolian deposition indicates that, in certain areas, these catchments are undergoing net deposition and not net erosion. The implications for lake sediment-based sediment yields and erosion rates are iii examined. When placed in a regional context sediment yields are more than 1 order of magnitude lower than larger scale basins due to changes in sediment storage. The spatial and temporal representativeness of the data are also evaluated. iv T A B L E O F C O N T E N T S A B S T R A C T H LIST OF T A B L E S v i i LIST OF FIGURES j x A C K N O W L E D G E M E N T S x i i C H A P T E R 1 INTRODUCTION 1 1.1 Conceptual Framework 1 1.2 Lake Sediment Based Sediment Yields and Erosion Rates 4 1.3 Sediment Influx to and Efflux from the Lake 7 1.3.1 Fluvial and colluvial inputs 9 1.3.2 Organic matter 11 1.3.3 Biogenic silica 12 1.3.4 Lake bank erosion 13 1.3.5 Aeolian dust inputs 13 1.3.6 Output 15 1.4 Research Objectives 17 C H A P T E R 2 STUDY A R E A 19 2.1 Location 19 2.2 The Landscapes of Southern British Columbia 19 2.3 Gallie Pond 22 2.4 Middle Lake 23 2.5 Ash Lake 25 2.6 Climate 25 2.7 Geology and Surficial Materials 28 V 2.8 Soils 30 2.9 Vegetation 30 C H A P T E R 3 F I E L D A N D L A B O R A T O R Y M E T H O D S 34 3.1 F ie ld Methods 34 3.1.1 Sampling of the lake sediment 34 3.1.2 Est imat ion of lake bank erosion 40 3.1.3 Est imat ion of aeolian dust inputs 45 3.2 Laboratory Methods 50 3.2.1 Lake sediment 50 3.2.2 Aeolian mater ia l 51 C H A P T E R 4 R E S U L T S 53 4.1 Volume of Lake Sediment 53 4.2 Bu lk Density of Lake Sediment 57 4.3 Organic Mat te r Content of Lake Sediment 60 4.4 Biogenic Si l ica Content of Lake Sediment 64 4.5 Lake Bank Erosion 67 4.6 Input of Aeol ian Mate r ia l 70 4.7 Summary 74 C H A P T E R 5 D I S C U S S I O N 75 5.1 Lake Sediment Budget 75 5.1.1 The mass of accumulated sediment 75 5.1.2 Organic matter content 78 5.1.3 Biogenic silica content 79 5.1.4 Lake bank erosion 80 vi 5.1.5 Aeolian input 81 5.1.6 Outflow losses 83 5.1.7 Summary 86 5.2 Sediment Yields and Erosion Rates 86 5.3 Regional Rates of Sediment Yie ld 90 5.4 Implications for Lake Sediment Based Sediment Yields and Erosion Rates 92 5.5 Temporal and Spatial Representativeness 93 C H A P T E R 6 C O N C L U S I O N S 99 R E F E R E N C E S A P P E N D I X 1 A P P E N D I X 2 A P P E N D I X 3 A P P E N D I X 4 A P P E N D I X 5 C H R O N O L O G Y C L I M A T E D A T A L A K E S E D I M E N T D A T A B A N K E R O S I O N D A T A A E O L I A N M A T E R I A L 100 122 116 122 124 135 vii LIST OF T A B L E S 2.1 Morphometric characteristics of Gallie Pond 24 2.2 Morphometric characteristics of Middle Lake 26 2.3 Morphometric characteristics of Ash Lake 27 4.1 Summary results of sediment depth to Bridge River tephra for the 3 lakes 54 4.2 Summary results of sediment bulk density for the 3 lakes 58 4.3 Summary results of organic matter content for the 3 lakes 63 4.4 Summary results of biogenic silica content for the 3 lakes 65 4.5 Summary results of horizontal erosion pins and stakes for 1 year for the 3 lakes 68 4.6 Summary results of vertical erosion pins and stakes for 1 year for the 3 lakes 68 4.7 Change in lake shoreline of the 3 lakes for different time periods 68 4.8 Mean total aeolian material input for each vegetation strata and for each lake 72 5.1 Volume and mass of total (i.e. mineral and organic) sediment and sedimentation rates in the 3 lakes for the last 2350 years 77 5.2 Lake sediment inputs derived from different sources 5.3 Regional rates of clastic sediment yield 87 5.4 Summary of results and the timeperiods over which they apply 95 A 1.1 Radiocarbon dates of Bridge River tephra 115 A2.1 Monthly mean temperature for climate stations in the viii Pemberton area 118 A2.2 Monthly total precipitation for climate stations in the Pemberton area 118 A2.3 Monthly mean temperature and total precipitation for the Goat Meadows catchment 121 A3.1 Depth to Bridge River tephra, bulk density, organic matter and biogenic silica content of sediments in the 3 lakes in the 3 lakes 123 A4.1 Difference in erosion pin lengths for: a) Gallie Pond 125 b) Middle Lake 126 c) Ash Lake 127 A4.2 Changes in lake shoreline measured using stakes over 1 year for the 3 lakes 128 A4.3 Changes in lake shoreline measured using erosion stakes (calculated using trigonometry) for: a) Gallie Pond 129 b) Middle Lake 130 c) Ash Lake 131 A4.4 Results of vertical erosion stakes for: a) Gallie Pond 132 b) Middle Lake 132 c) Ash Lake 133 A4.5 Lake bank bulk density 134 A5.1 Material contained in the snowpack 136 A5.2 Material contained in the bulk collectors 137 ix LIST OF FIGURES 1.1 Input and output of clastic sediment in a lake system 10 1.2 Paths along which aeolian material may be delivered to a lake 16 2.1 Location of the study area in British Columbia 20 2.2 Location of the 3 catchments 21 2.3 Bathymetry of Gallie Pond 24 2.4 Bathymetry of Middle Lake 26 2.5 Bathymetry of Ash Lake 27 2.6 The stratigraphy of the soils in the Goat Meadows catchment 29 2.7 The vegetation cover of the 3 catchments 31 2.8 Regional biogeoclimatic map 33 3.1 Location of the core sites in: a) Gallie Pond 36 b) Middle Lake 36 c) Ash Lake 37 3.2 The corer used to take samples of surface sediments 38 3.3 The location of lake bank erosion sites and the aeolian bulk collector in: a) Gallie Pond 41 b) Middle Lake 41 c) Ash Lake 42 3.4 Erosion pins and stakes used to calculate horizontal bank erosion 44 3.5 Erosion pins and stakes used to calculate vertical bank erosion 46 X 3.6 Vegetation strata in the 3 catchments and the location of sites used to calculate aeolian dust input 48 4.1 Spatial variation of sediment depth to Bridge River tephra in: a) Gallie Pond 54 b) Middle Lake 55 c) Ash Lake 55 4.2 Spatial variation of sediment bulk density in: a) Gallie Pond 58 b) Middle Lake 59 c) Ash Lake 59 4.3 Spatial variation of organic matter content in: a) Gallie Pond 62 b) Middle Lake 63 c) Ash Lake 63 4.4 Spatial variation of biogenic silica content in: a) Gallie Pond 65 b) Middle Lake 66 c) Ash Lake 66 5.1 Specific sediment yield as a function of drainage area for fluvial suspended-sediment-transport records in British Columbian rivers and lake sediment-based yields in alpine watersheds 91 A 1.1 Distribution of Mount Mazama, Mount Saint Helens Y, and Bridge River tephra in British Columbia 114 A2.1 Location of climate stations in the Pemberton area 117 XI A2.2 Monthly mean temperature for the Pemberton area and the 1989 field season 119 A2.3 Monthly total precipitation for the Pemberton area and the 1989 field season 120 x i i ACKNOWLEDGEMENTS I would like to thank my supervisory committee, who have helped to make this a pleasureable learning experience: Dr Olav Slaymaker, for his advice, encouragement and cheerful approach to all problems and deadlines; Dr Michael Church, for his technical advice and constructive critism; and to Dr June Ryder, from whom I have learnt a lot about the Quaternary history of Southern British Columbia. I would also like to express my appreciation to Dr Catherine Souch, who answered an endless stream of questions (most of the time smiling), and Dr Michael Bovis, for his comments on alpine soil loss and constant wit. Thanks are also due to Wendy Hales, Clive Ward, Mike Rasmussen, Jamie Voogt and Daniel Perrig, for assistance in the field, and to Marty and A l Staehli, Mel and Barbara Maddess, Department of Soil Science (UBC), Peter Jordan, Sue Young, Tony Cheong, Robyn Dowling, Michael Brown, Yvonne Martin, Dr Marwan Hassan and Dr Chris Burn, for reasons to numerous to mention. A special thankyou goes to Steve Rice - my partner in 'pain' - and to my parents. 1 C H A P T E R 1 - I N T R O D U C T I O N The aim of this study is to use lake sediments to establish Late Holocene (defined here as the last 2350 years) rates of catchment erosion and sediment yield within 3 small catchments in the alpine-subalpine ecotone of the Coast Mountains of British Columbia and to assess the proportion of lake sediment which is not derived from erosion of the catchments under investigation. Most of the lake sediment is derived from within the lakes or blown in from outside the catchments and this has implications for estimating sediment yields and erosion rates. 1.1 - Conceptual Framework Sediment is erodible inorganic and organic material at or near the earth's surface (Swanson et al., 1982). Sediment yield is defined as the total sediment outflow from a catchment or drainage basin, measureable at a cross section of reference and in a specified period of time (Vanoni, 1975). Studies of sediment yield are common in the hydrologic and geomorphic literature because they can be used as a measure of the operation of contemporary and palaeohydrological and palaeogeomorphological processes (Wolman and Miller, 1960; Slaymaker, 1977; Walling and Webb, 1983). This is because the amount of material transported by a stream is an index of catchment erosion, and thus mean rates of surface lowering can be estimated (Walling, 1971). Generally catchment erosion is defined as the process of detachment and transport of catchment derived rock and soil materials under the influence of a transporting medium (this is modified slightly in Section 1.3). This distinguishes it from weathering which does not 2 directly involve removal processes. Denudation is the aggregate effect of weathering and erosion. Sediment yield studies are also common because they provide a useful index with which to assess the impact of society on the environment (e.g. Wolman, 1967; Walling, 1974; Binns, 1979). Presently, it is widely held that sediment yield per unit area decreases with increasing basin size (Schumm, 1977; Milliman and Meade, 1983). This concept has, however, been questioned (i.e. Caine, 1974; Trimble, 1983). Church and Slaymaker (1989) found that sediment yield in Britsh Columbia increases with increasing basin size up to 3 x 10^ km^. This is due to secondary remobilisation of Quaternary sediments stored along river banks. Most studies of sediment yield have been based like that of Church and Slaymaker (1989), on contemporary stream monitoring. There are, however, problems associated with this approach (Dearing et al., 1982, 1990). For example, a steady state or balance between material transported by a stream and that produced on the slopes has been assumed to exist. This concept of steady state is an oversimplification because much of the sediment flux is interrupted by locations and periods of sediment storage between the upland slopes and the downstream point where sediment yield is measured (Meade, 1982; Trimble, 1983; Walling, 1983, 1990). There are a variety of storage zones in a catchment, which include the soil mantle on a hillslope, gravel bars in a stream, debris fans and lakes. Vanoni (1975) estimated that for catchments >1 km^ in area, the amount of particulate material that arrives downstream will often be less than 25% of the upstream eroded material. Similarly, Trimble (1981, 1983) found the sediment yield in the 360 km"^ Coon Creek basin, Wisconsin, was only 6% (75 t km'^ yr"-'-) of all erosion estimated to have occurred in that basin between 1853 and 1938. 3 The effect of sediment storage on material transfers can be highlighted by a sediment budget, expressed in the standard form: I -AS = 0 1 where I = input of sediment in a specified time, A t A S = change in storage in A t (positive for an increase in storage) 0 = output of sediment in A t The sediment budgeting approach to catchment geomorphology was originally advocated by Rapp (1960) and subsequently developed by Dietrich and Dunne (1978), Lehre (1981) and Swanson et al. (1982). It has proven to be a powerful theoretical framework as it provides a strong but flexible basis for the design of process monitoring. Most contemporary studies of sediment yield have been based on monitoring over short timescales (usually 1-10 years) and have attempted to extrapolate current process rates. The literature largely ignores the length of record needed for representativeness. In order to extrapolate sediment yields over longer periods of time with some confidence, an alternative source of information is required. Church (1980) has asserted that the most comprehensive records of long term geomorphic history are found imprinted on sediments in depositional environments. However, preservation of such sediment varies and terrestrial deposits are the ones least likely to be maintained unaffected, particularly at high elevations (Hilton-Johnson, 1982). Lakes, with appropriate geometry and hydraulic conditions, may trap a high proportion of inflowing particulates from their surrounding catchments. Once trapped, the 4 sediments tend to remain in the lake for a long time, giving long records and providing opportunities to determine the magnitude and temporal variability of sediment movement in a catchment (Oldfield, 1977; Oldfield and Clark, 1990; Foster et al., 1990). 1.2 - Lake Sediment Based Sediment Yields and Erosion Rates The lake-catchment ecosystem approach may be envisaged as a sediment cascade (Chorley and Kennedy, 1971) in which lakes are viewed as the primary sediment sinks in a catchment. Sediment accumulation in a lake is a consequence of the net balance between input and loss through the outflow (i.e. trap efficiency), which may be related to sediment type, water retention time and lake morphology (Oldfield, 1977). Inputs to a lake are derived from a number of sources. Autochthonous inputs are derived from within the lake, while allochthonous inputs are derived from outside the lake. Equation 1 can therefore be modified: where AS = net mass or volume accumulation of lake sediment during a specified period, At I = the mass or volume of sediment input to the lake in A t AS = (I t + I]) - 0 2 t autochthonous inputs 1 allochthonous inputs 0 the mass or volume of sediment output from the lake in At 5 The sediments deposited are a function of the complex interactions between mechanisms of deposition, resuspension, chemical and biogenic transformations, exchange with water and organisms, and longer term diagenesis (Oldfield, 1977). In the last 20 years or so there have been numerous studies that have estimated sediment yields and catchment erosion rates based on reservoir and lake sediment (e.g. Davis, 1976; Lambert, 1982; Foster et al., 1985). Many studies concerning lake sediment deposition rates have been based on a single core from the deepest part of the lake, which entails making assumptions about focusing of sediment to this point (e.g. Likens and Davis, 1975; Edwards and Rowntree, 1980) as advocated by Lehman (1975). The distribution of sediment in lakes is complex and a function of many interacting processes (Hilton, 1985; Hilton et al., 1986). The heterogeneity of sediment accumulation at the bed of a lake therefore invalidates the general extrapolation of sedimentation rates determined from a single core to the whole lake (Dearing, 1982). Consequently sediment volumes and masses have to be estimated from a three-dimensional body of sediment defined by the lake perimeter, and a network of synchronous levels extrapolated between a suite of cores. This can be achieved by the use of echo sounding or a high density sampling network (e.g. Bloemendal et al., 1979; Foster et al., 1985). Hakanson and Jansson(1983) identify three types of sampling systems: i) deterministic (based on prior understanding of processes); ii) stochastic (random over space); iii) regular grid. 6 Dearing (1982) states that for "simple basins" it may be sufficient to sample on a systematic grid basis. Floating rope stretched between shores has been successfully used to mark out transects up to 500 m long, sampling points being located by anchored buoys or poles pushed into sediment. Studies utilising a large number of cores frequently require a rapid method of correlation of synchronous levels. Dearing (1982) identifies two criteria for satisfactory resolution of synchronous sediment levels in cores: i) areal continuity; i.e. the discriminating sediment property should be deposited in similar proportions to other sediment constituents over the whole lake bed surface so that suites of downcore sediment records are in parallel; ii) areal synchroneity; i.e. the sediment property, once deposited, should either be persistent or immobile, or change in respect of its form or position at a relatively constant rate over the lake bed surface, thus making fluctuations in parallel records synchronous and correlatable. A variety of methods have been put forward for core correlation, including physical and chemical stratigraphic changes (e.g. Battarbee and Digerfeldt, 1976), magnetic susceptibility (e.g. Dearing, 1979), and synchronous horizons marked by micro- and macro-fossils (e.g. O'Sullivan et al., 1973) or tephra (e.g. Souch and Slaymaker, 1986). For a review of core correlation methods see Dearing (1982). Of paramount importance when using lake sediments to calculate sediment yield and erosion rates is the establishment of an absolute chronology of deposition. There are a variety of techniques available depending on the 7 nature and composition of the sediment and the timescale of deposition. Some of the more commonly used techniques include radiocarbon dating (e.g. Battarbee and Digerfeldt, 1976 ; Souch, 1989a), tephra layers (e.g. Souch and Slaymaker, 1986), indicator pollen influx (e.g. Clark, 1986), annual laminae/varves (e.g. O'Sullivan et al., 1982; Saarnisto, 1986), lead-210 (e.g. Krishnaswamy et al., 1971), caesium-137 (e.g. Ashley and Moritz, 1979) and palaeomagnetism (e.g. Thompson and Turner, 1979). For a review of dating methods see Oldfield (1977, 1981). Once an absolute chronology of deposition has been established (generally for a master core cross-correlated with all the other cores) and the mass of sediment in each time increment calculated (based on volume of sediment and mean dry sediment density values for each increment), mean annual dry sedimentation rates can be computed by dividing total mass for each increment by the appropriate time period. 1.3 - Sediment Influx to and Efflux from the Lake The lake-catchment concept as expounded by Likens (1972), Oldfield (1977) and O'Sullivan (1979) utilises and expands on existing concepts. Horton (1945) first advocated the use of the catchment as the fundamental unit within which to study geomorphological and hydrological processes. The catchment provides a convenient and usually clearly defined, topographic unit which may be viewed as an open physical system in terms of inputs and outputs (Chorley, 1969). Lakes may be viewed as an extension of the catchment, because the quantity and quality of the sedimentary deposits contained within a lake will be a function of catchment dynamics as well as internal lake processes (Lundqvist, 1938, 1942). Bormann and Likens (1967, 8 1969) extended the ecosystem concept which was introduced by Tansley (1935) to the ecosystem-catchment concept. This was subsequently extended to the lake system by Likens (1972) and Oldfield (1977) who formulated the lake-catchment-ecosystem concept. This attempts to provide a conceptual framework with which to investigate and measure the energetic and material pathways in a discrete lake-catchment system. To use total annual sediment mass for calculating sediment yield is to assume that all the sediment is derived from the catchment, with no losses of sediment through the outflow. On these assumptions, sediment yields are obtained simply by dividing total annual dry mass by catchment area. However, the assumption of a specific catchment derived source of sediment is unlikely to hold true for all the sediment contained within the lake basin. Foster et al. (1985) identify four sources of material that contribute to the total sediment mass of a lake that are not derived from erosion of the specific catchment under investigation and/or are not spatially distributed over the catchment surface: i) aquatic production of organic matter; ii) aquatic production of biogenic silica; iii) lake bank erosion; iv) atmospheric (aeolian) dust inputs. In addition there are autochthonous and allochthonous inputs of solutes to lake sediment. In a comprehensive mass balance study these inputs and outputs should be evaluated, but in this study they are ignored. From the above, it is possible to formulate a simple mass balance of sediment tranfers in aquatic systems: 9 AS = ( If 4- IC + IQ + Ig + I b + ^ ) - O where AS = net accumulation of lake sediment during a specified time, At I = input of sediment to the lake in A t f = fluvial c = colluvial o = organic matter s = biogenic silica b = lake bank erosion a = aeolian dust 0 = output of sediment from the lake in A t Figure 1.1 shows a schematic diagram to illustrate this sediment budget approach. Material derived from aquatic productivity (i.e. organic matter and biogenic silica) is autochthonous to both the lake and the catchment. Material derived from lake bank erosion is allochthonous to the lake but autochthonous to the catchment, while aeolian dust inputs are generally allochthonous to both the lake and the catchment. The following sections consider each of the terms on the right hand side of equation 3. 1.3.1 - Fluvial and colluvial inputs These two processes are generally coupled together in lake sediment studies, and are believed to supply most material to the majority of lakes in Figure 1.1 - Input and output of clastic sediment in a lake system 11 non-glacier and non-karst environments. The output of the colluvial subsystem was generally believed to be the input to the fluvial subsystem (Leopold et al, 1964), except in areas immediately surrounding the lake. However, in the last 10 years, studies have demonstrated that this may not hold for certain catchments and that the two subsystems may essentially be decoupled (Caine, 1974; Bovis and Thorne, 1981; Caine and Swanson, 1989) for certain particle sizes. This decoupling may be temporally and spatially variable. The implication is that sediment yield will, in general, under- or overestimate upland catchment erosion, depending on whether sediment is going into or out of storage. Similar implications arise due to sediment storage between the upland slopes and the downstream point where sediment yield is being measured. Lake sediment based studies, while suitable for the estimation of sediment yields, may give misleading values for upland catchment erosion, primarily because of storage effects (Owens, 1990). 1.3.2 - Organic matter Aquatic productivity can be an important contributor of material to the limnic system. This is especially true in nutrient-rich (i.e. eutrophic) lakes which promote excessive algal blooms. Most studies that have used lake sediments to derive sediment yields and catchment erosion rates have recognised that this component should not be included in material balance estimates. Values cited in the literature of the proportion of organic matter in lake sediment varies from <1% (Foster et al., 1985) to >40% (Flower, 1980), although in certain lakes values may be much higher. 12 Under certain flow conditions a large amount of incoming stream sediment may be organic material. Thus lake sediment organic matter can be both allochthonous and autochthonous (Von Post, 1862; Jordan and Likens, 1975). In this study material input to lake sediment derived from aquatic productivity is expected to be low; nevertheless this hypothesis must be tested. Only minerogenic material is considered as sediment yield due to catchment erosion. 1.3.3 - Biogenic silica The major fraction of silicon in lake sediment is derived from the external contribution of silicate minerals (Hakanson and Jansson, 1983). However, it has been suggested (e.g. Foster et al., 1985) that the biogenic precipitation of silica may contribute to the minerogenic fraction of virtually all lakes. Diatoms (microscopic single-celled algae) assimilate silicon from lake water for incorporation into their frustules (Hutchinson, 1957). Upon death, the frustules settle out to the sediments, the rate of preservation of which varies (Flower, 1980). Dearing et al., (1987) found that biogenic silica represented up to 20% of the minerogenic sediment yield for Lake Havgardssjon in Sweden. Flower (1980) estimated diatomaceous inputs to Lough Neagh, Northern Ireland, sediments of up to 30%, while McManus and Duck (1985) found that this component represented 10 to 40% of the inorganic sediment accumulated in certain Scottish reservoirs. As with organic matter, there is also an input to the limnic system of an allochthonous component - terrestrial and stream diatoms. In this study, 13 allochthonous and autochthonous silica are not considered as sediment yield or erosion from the catchment. 1.3.4 - Lake bank erosion Many lakes exhibit signs of bank erosion caused by fluctuating water levels, wave action, frOst action, heaving by ice, and physical disturbance by humans and animals. Very few quantitative data exist for rates of lake bank retreat. Grew (cf. Foster, 1987) estimated that erosion of bank material contributed 13 t yr" 1 (10-15%) to the sediment of Merevale Lake, England (0.065 km^). Due to the vegetated and artificially lined nature of the bank of this lake, sediment is supplied from only a small portion of the lake shoreline. This serves to illustrate that imperceptible lake bank erosion processes can produce significant allochthonous inputs, especially in lakes with large width to length ratios and the catchments of which are either relatively small or experience only low rates of erosion. In this case material input to the lake system derived from lake bank erosion is considered part of the sediment yield but not erosion from the catchment. 1.3.5 - Aeolian dust inputs Atmospheric fallout of dust particles to the lake surface may make a significant contribution to the annual increment of deposited organic and inorganic material. Water bodies have 100% capture efficiency for particles settling on their surface (Pye, 1987). Pye (1987) states that atmospheric dust particles are derived from a number of sources that include fires, gas to particulate conversions, industrial emissions, sea salts, cosmic dust, volcanic 14 dust and deflation of sediments and soils at the earth's surface. Hidy and Brock (1971) estimate that the total annual global dust production from the last source is between 61 and 366 x 10^ t. Contemporary reported rates of continental dust deposition vary from <10 to c.200 t knT^yr"-'-. Present-day aeolian infall to the alpine zone of many mountain ranges has been identified as an important geomorphic and pedogenic process (Dumanski et al., 1980; Kotarba, 1987; Darmody and Thorn, 1987). Similarities in grain size distribution of aeolian materials and sediments in alpine and subalpine lakes have convinced several authors (i.e. Caine, 1974; Andrews et al., 1984; Harbor, 1985) that aeolian infall is the dominant sediment source of these lakes. Regionally, dry and wet fallout in areas above 1000 m has been shown to be an order of magnitude lower than below this altitude due to inversion effects (Slaymaker and Zeman, 1975). Jones (1982) calculated that maximum contemporary aeolian deposition for the Goat Meadows catchment ranged between 12 and 48 x 10"^  kg m'^ yr"-'-. Using a mass balance approach to sediment transfers on hillslopes, Jones implied that the catchment was undergoing net accumulation rather than net erosion. Souch (1984) has however, questioned the validity of these figures on the grounds that measurements were made at sites of maximum accumulation only. Aeolian material may be transported to the lake system along a variety of paths: i) primary aeolian material transported directly to the lake, derived from inside the catchment; ii) primary aeolian material transported directly to the lake, derived from outside the catchment; 15 iii) secondary aeolian material transported to the lake system, initially derived from outside the catchment and temporarily stored on the hillslopes and then retransported by aeolian processes; iv) secondary aeolian material transported to the lake system by colluvial and fluvial processes. This material was originally derived from outside the catchment and temporarily stored on the hillslopes. This may be represented by a mass balance equation and illustrated diagrammatically (Figure 1.2): I = I x + I 2 + I 3 + I 4 : 4 where I = total input to the lake Ij.4 = aeolian inputs described above 1 = I a + I 4 Essentially only component 12 is initially derived from inside the catchment and considered catchment erosion. However, its separation from the other components is difficult. 1.3.6 - Output The amount of sediment deposited in a lake will be determined by the incoming sediment and trap efficiency. If a lake has no outflow then the lake can be assumed to be 100% trap efficient. However, if a lake has an 16 Figure 1.2 - Paths along which aeolian material may be delivered to a lake o Lake sediment Net change in lake sediment in time At I-Aeolian material blown into the lake 1- derived from outside the catchment 2- derived from inside the catchment 3- initially derived from outside the catchment and temporarily stored on the hillslopes 4- as 3 but remobil ised by fluvial and c o l l u v i a l processes 17 outflow(s) then it is probable that material is lost from the system. It is thus necessary to estimate outflow losses based on the calculation of trap efficiency. In principle monitoring would be possible, but in practise it is often not feasible. Therefore, normally trap efficiency is calculated from empirically derived graphs, such as that developed by Brune (1953), which relate trap efficiency to the capacity : inflow ratio of the reservoir and an approximate index of sediment particle size. For reservoirs in the US draining catchments between 0.1 and c. 500,000 km^, Brune (1953) showed that trap efficiency varied between <5% and >95% for material in the sand and silt size range. Heinemann (1984) discusses some of the problems involved in obtaining accurate estimates of trap efficiency. Trap efficiency is likely to change through time as the water storage capacity of the lake changes. This may be due to water level fluctuations and/or natural sedimentation processes. An increase in sediment depth, for example, is likely to decrease trap efficiency. Souch (1984) showed that a variety of lake level changes have occurred in the study area. Trap efficiency and outflow losses can be assumed to have changed through time. 1.4 - Research Objectives In an area such as the Coast Mountains, where stream gauging stations and long sediment records are rare (Slaymaker, 1987), lake sediments may prove an appropriate way to investigate catchment sediment yields, storage effects and a first approximation catchment erosion. This study attempts to establish contemporary rates of catchment erosion and sediment yield within three small catchments in the alpine-subalpine ecotone of the Coast Mountains, by using the lake-catchment concept. 18 The lakes selected (Gallie Pond, Middle Lake and Ash Lake) were chosen as part of ongoing research into the clastic sediment yield of the Lillooet River system at different spatial and temporal scales (Slaymaker and Gilbert, 1972; Gilbert, 1973; Slaymaker, 1977; Hart, 1979; Jones, 1982; Souch, 1984). > Using a detailed sediment budgeting framework, the relative proportions of different lake sediment sources will be calculated and examined in the context of lake sediment based sediment yields and erosion rates. In addition, the hypothesis of scale controls of sediment yield for British Columbia will be tested (Slaymaker, 1987; Church and Slaymaker, 1989; Church et al.,'1989) (see section 1). 19 C H A P T E R 2 - S T U D Y A R E A 2.1 - Location Gallie Pond, Middle Lake and Ash Lake are three small lakes which straddle the alpine-subalpine ecotone, approximately 120 km north of Vancouver, south western British Columbia, in the Pacific Ranges of the Coast Mountains (Figure 2.1). The alpine zone is here defined as the altitudinal zone above the upper limit of continuous forest (Love, 1970). The catchments of the three lakes are not glacierised. Gallie Pond drains into Middle Lake, which in turn, along with Ash Lake, drain into Ryan River and this into Lillooet River (Figure 2.2). 2.2 - The Landscapes of the Southern Coast Mountains Ryder (1981) states that the landscapes of the southern Coast Mountains of British Columbia embodies elements of three distinct temporal and spatial scales. At the regional level, the mountains, major structural lineaments, fragments of erosion surfaces and associated summit levels are the most extensive features. These are the product of tectonic processes and subaerial denudation operating over the Tertiary period. They provide the framework upon which more localised forms have subsequently developed. During the Pleistocene Epoch, valley and mountain ridges were modified by glacial processes. The morphology of the resultant landforms exhibits some regional variability, with distinct features relating to the aggregate effect of glaciations throughout the Quaternary. Immediately following deglaciation there was a period of intense geomorphic activity, termed the paraglacial period (Church and Ryder, 1972), during which erosional and depositional processes were controlled Figure 2.1 - Location of the studv area in British Columbia (adapted from Barrett, 1988) M Mosquito creek H Hummingbird creek 22 primarily by the susceptibility of glacial drift to redistribution under non-glacial conditions. Generally over the Holocene Epoch only minor modification of valleys has occurred, the landforms which have resulted are entirely local in origin (e.g. talus, slope and channel erosion, weathering phenomena etc.). Studies to date (e.g. Clague, 1981, 1989; Ryder and Clague, 1989) have shown that the Cordilleran ice sheet reached its maximum extent in southern British Columbia approximately 14,000 to 14,500 years B.P. Deglaciation was in progress by 13,500 years B.P. and the ice sheet wasted rapidly (Clague, 1981). Locally, palaeoclimatic reconstruction based on lake sediments in Gallie Pond (Souch, 1984, 1989b) has shown that deglaciation occurred before 10,500 years B.P. The early post-glacial was probably wet and cold. A warmer and drier episode corresponding with the generally documented hypsithermal interval has been shown to be present from 10,500 to ca. 6340 years B.P. The local ecosystem was significantly more arboreal and winter snowpacks greatly reduced. Neoglaciation appears to have begun shortly after 6300 years B.P. with cooler and wetter conditions, although in the southern Coast Mountains, temperatures, and therefore timberlines, were higher than at present until ca. 5200 years B.P. (Clague, 1981). Two later phases of neoglaciation have also been identified in the region (Ryder, 1981; Ryder and Thompson, 1986; Souch, 1990). 2.3 - Gallie Pond Gallie Pond is a small oligotrophic lake, the outlet of which defines the lower limit of the Goat Meadows catchment (0.023 km^). The elevation of the lake is approximately 1850 m above sea level and occurs in a rock-bounded hollow on a glacially scoured ridge between Lillooet River and 2 3 Miller Creek. The lake is fed by two streams, Hummingbird and Mosquito (Figure 2.2). Losses occur through an ephemeral outlet stream on the north-west margin of the lake, and groundwater drainage through the lake sediment (Gallie, 1983). The lake is constrained at its outlet by a rock sill which has prevented scouring and erosion of the sediments. Radiocarbon dating of the basal sediments suggests that the lake has been in existence throughout the Holocene Epoch (Souch, 1984). Table 2.1 and Figure 2.3 describe the morphometric characteristics of the lake during the summer of 1989. The lake was subdivided into two environments,by Souch (1984): a deeper central portion underlain by fine lacustrine silts; and, around the edge a "till shelf, a sill covered by a coarse boulder deposit. The origin of this feature is uncertain, but it may possibly have been formed through the action of surface ice, and periodic changes in water level. Sedimentation occurs only in the deeper (390 m^) central portion of the lake and calculations are based only on this. 2.4 - M i d d l e L a k e The elevation of Middle Lake is approximately 1710 m above sea level. It is located in a hollow in Pleistocene deposits. Middle Lake is a small oligotrophic lake, the outlet of which defines the lower limit of a 0.202 km^ catchment. The lake is fed by one main stream, which starts at the outlet of Gallie Pond. There are also several small ephemeral streams that become active during snowmelt and/or precipitation events. Losses occur through an outlet stream on the south-west margin of the lake and through gaps between large boulders that constitute part of the lake bank on the south-west margin of the lake. Groundwater drainage may occur through the lake sediment, but, there was no evidence for this during the field season. Table 2.1 - Morphometric characteristics of Gallie Pond Site location 5 0 ° 2 4 ' N 122°57'W Altitude (approx.) 1850 m a.s.l. Surface area 901 m^ Maximum length 54 m Maximum width 21 m Maximum water depth 1.23 m Mean water depth 0.56 m Water volume 505 m^ N.B. Data refer to the outlet floor level of the lake Figure 2.3 - The bathymetry of Gallie Pond 2 5 Table 2.2 and Figure 2.4 describe the morphometric characteristics of the lake during the summer of 1989. The lake contains two deeper palaeochannels, which cross, and suggest that this lake has been dry for some of the Holocene Epoch. The occurrence of Bridge River tephra in lake sediment indicates that it has been in existence for the last 2350 years (see Appendix 1). It is possible that the lake may have been dry some time during this period, however, there is no discontinuity in the sediment above the tephra. The lake also contains some small islands that may be ephemeral, but at present are vegetated, and numerous large boulders that are probably derived from the talus to the south of the lake. 2.5 - Ash Lake The elevation of Ash Lake is 1624 m above sea level. It is a small oligotrophic lake , the outlet of which defines the lower limit of a 0.022 km^ catchment, located in Pleistocene deposits. Two streams and the water from the lake outlet are confluent at the northern end of the lake (Figure 2.5). The lake is fed by ephemeral streams to the south and west of the lake margin and possibly by water which seeps through a marshy area to the south-east of the lake. Table 2.3 and Figure 2.5 describe the morphometric characteristics of the lake during the summer of 1989. This lake also contains some large boulders, as well as pieces of large organic debris lying on top of and within the lake sediment. There is also a network of palaeochannels which indicates that at some stage the lake has been dry, and Bridge River tephra, the implication of which is similar to that for Middle Lake (section 2.4). 2.6 - Climate Table 2.2 - Morphometric characteristics of Middle Lake Site location 50°24 'N 122°57'W Altitude (approx.) 1710 m a.s.l. Surface area 600 m~ Maximum length 35 m Maximum width 28 m Maximum water depth 0.48 m Mean water depth 0.16 m Water volume 96 N.B. Data refer to the outlet floor level of the lake Figure 2.4 - Bathymetry of Middle Lake Palaeochannel Iders Contour interval 0.1 m Table 2.3 - Morphometric characteristics of Ash Lake Site location Altitude (approx.) Surface area Maximum length Maximum width Maximum water depth Mean water depth Water volume 50°24 'N 122°58'W 1624 m a.s.l. 618 m 2 43 m 22 m 0.55 m. 0.23 m 142 m 3 N.B. Data refer to the outlet floor level of the lake Figure 2.5 - Bathymetry of Ash Lake 28 The present mesoscale climate is cold, perhumid. Temperatures for the area ranged from -40°C to + 2 3 ° C between 1979 and 1980 (although this included an unusual outbreak of very cold continental arctic air), with the annual mean for 1979 to 1980 about 0 to 1°C (Gallie, 1983). Active alpine glaciers are common here because the Pacific Ranges are oceanic. Present glaciers extend downslopes to elevations of 1500 m above sea level, but intervening ridges, such as the one on which the Goat Meadows catchment is located, have remained ice free for the last 10,000 years. Annual precipitation exceeds 1800 mm of which 70 to 80% falls as snow (Gallie and Slaymaker, 1985). Continuous snow cover persists for 7 to 9 months of the year (Gallie and Slaymaker, 1984). The average May 1 snowpack in the Goat Meadows catchment is approximately 1500 mm water equivalent (Gallie, 1983). Appendix 2 contains mean monthly temperature and precipitation data for climate stations in the Pemberton area and for the study site. 2.7 - Geology and Surficial Materials The local bedrock consists of an association of metasediments, which were mapped by Woods worth (1977) as Late Cretaceous Gambier Group. This forms a roof pendant on quartz diorite of the Coast Mountain Complex (Roddick, 1976). The bedrock in the three catchments is discontinuously covered by a stoney dioritic Pleistocene till (Souch, 1984) which is overlain by a number of fine textured Holocene deposits, including two ash layers (Figure 2.6) of which the lower is Mount Mazama and the upper is Bridge River (see Appendix 1). The two tephra layers are discontinuous and are concentrated in local sediment traps. Variable amounts of fine textured, organic rich loess deposits are interlayered. These deposits are generally 0.1 2 9 Figure 2.6 - Characteristic stratigraphy of the soils of the Goat Meadows catchment (adapted from Barrett, 1981) loess upper ash loess lower ash loess ablation t i _ l _ l _ _ _ . [lodgementi ' t i l l 1 loess upper ash loess ablation t i l l _ _ _ , lodgement! . t i l l ' o o 3 i 30 to 0.3 m thick, however, Barrett and Slaymaker (1989) found that the depth to till may exceed 1 m in the immediate vicinity of Ash Lake. Here the parent material of the soil appears to be primarily a clay-silt lacustrine deposit, which is rich in peat. In active sites, colluvium overlies the basal till, especially along the glacially oversteepened cliffs which run along the southern margin of the catchments (Figure 2.7). In the Middle Lake catchment talus on the north facing slopes extends to the valley bottom and forms the southern margin of Middle Lake. 2.8 - Soils The soils within the Goat Meadows catchment have been described by Gallie (1983) and Souch (1984) according to the Canadian system of classification (Canada Soil Survey Committee, 1978). It is inferred in this study that they are generally similar in the Middle Lake and Ash Lake catchments. The soils are areally dominated by Orthic Dystric and Orthic Sombric Brunisols and Cumulic Regosols, although in tree islands, and stands of coniferous trees in Ash Lake catchment, soils are generally Orthic Humo-Ferric Podzols. On saturated footslopes and areas around the lakes Rego Humic Gleysols dominate. Barrett (1981, 1988) identified many of the soils within the study area as hydrophobic. Hydrophobicity may determine areas where overland flow will be preferentially generated. 2.9 - Vegetation Vegetation in this area is transitional between coastal Mountain Hemlock biogeoclimatic zones to the south-west and interior Engleman Spruce-Subalpine Fir zones to the north-east (Krajina, 1969; Biel et al., 1976) 31 Figure 2.7 - The vegetation cover of the 3 catchments (based on false colour aerial photographs) 32 (Figure 2.8). The catchments are located near the upper altitudinal limit of parkland subzone at the alpine-subalpine ecotone (Brooke, 1965). At Goat Meadows catchment, discontinuous tree islands grade into krummholz life-forms 100 m further upslope. The limit of arboreal species occurs at about 2100 m above sea level. The local vegetative mosaic is heterogeneous due to steep microclimatic gradients and a variety of geomorphic processes active in this environment. It is dominated by heath, sedge, forb, moss, Lutkea, lichen, Casiope and grass in addition to tree islands and, in the lower two catchments, stands of coniferous trees (Figure 2.7 illustrates the distribution of vegetation in the 3 watersheds based on a simple 3-fold classification). Gallie (1983) identified five vegetation associations in the Goat Meadows catchment and found that a strong covariance exists between surficial materials, soils and vegetation associations. 33 Figure 2.8 - Regional biogeoclimatic map (adapted from Ministry of Forests, 1985) Alpine Tundra and G l a c i e r s Mounta in Hemlock E n g e l m a n n S p r u c e — Suba lp ine Fir C o a s t a l W e s t e r n Hemlock Interior Douglas Fir L a k e s Pemberton S t u d y area 0 I N A 2 0 k m J I 34 C H A P T E R 3 - F I E L D A N D L A B O R A T O R Y M E T H O D S This chapter presents descriptions of the methods used to collect samples and the subsequent laboratory analyses in order to fulfill the research objectives discussed in Section 1.4. The methodology followed was similar for all lakes; any specific differences are noted. 3.1 - Field Methods 3.1.1 - Sampling of the lake sediment Due to the spatial and temporal variability of sedimentation in lakes (Section 1.2), sediment volumes and masses were estimated by a high density sampling network. Cores of lake sediment were taken at sampling sites selected using a grid system. For Gallie Pond only the deeper central portion of the lake (Section 2.3) was sampled using a 3 m x 3 m grid system. Souch and Slaymaker (1986) state that a grid size of 4 m x 4 m or greater would probably underestimate sediment accumulation in Gallie Pond by missing areas of greatest accumulation. For Middle Lake and Ash Lake, sampling was based on a 5 m x 5 m grid size. This grid size was chosen based on logistical constraints of sampling time and time required for laboratory analysis (the surface areas of Middle Lake and Ash Lake are 600 m^ and 618 m^ repectively, while the surface area of the central portion of Gallie Pond is 390 m^). The proportional sedimentation area represented by one core in Gallie Pond, Middle Lake and Ash Lake is 17, 38 and 36 m^, respectively. While the density of cores in Middle and Ash lakes is less than half that of Gallie Pond, this is not expected to influence the spatial representativeness of the cores. 35 The grids in all 3 lakes were established from baselines laid along the northern and eastern edges of the lakes, from which transects were run across the lakes. In order to minimise disturbance of the lake sediment, the core tubes were installed by moving systematically from east to west along each transect and from the north to the south of the lakes. Figure 3.1 shows the location of the cores in each lake. In total 56 cores were collected between 2 5 t h August and 2 n d September 1989: 23 from Gallie Pond, 16 from Middle Lake and 17 from Ash Lake. The coring apparatus consisted of 30.0 cm lengths of plastic ABS piping of internal diameter 10.0 cm and wall thickness 6 mm. This size of pipe was chosen due to the large volume of sediment obtained per unit depth and the thick tube wall which provided strength. The core tubes were beveled at one end to allow for easier penetration into the sediment, and were inserted into the sediment by use of a corer designed by the author specifically for these small shallow lakes, and constructed by Jan Skapski (technician, Department of Geography, UBC). Figure 3.2 illustrates the corer. Once the core tube was attached to the corer (by fibre tape) it was driven into the sediment with a mallet. The corer contains air holes to allow air and water to escape during tube installation so as to minimise sediment compaction due to trapped air and lake water. The core tubes were driven, where possible, approximately 30 cm into the sediment. This was to incorporate Bridge River tephra which was known to occur at sediment depths of between 5 and 15 cm in Gallie Pond (Souch, 1984) and between 10 and 25 cm in Middle and Ash lakes (Souch, pers. comm.) for the purpose of core synchroneity and chronology (Appendix 1). In some locations, due to the nature of the sediment, the 10.0 cm diameter core tubes were unable to hold lake sediment during core removal. In this situation 5.0 cm internal 36 Figure 3.1 - Location of the core sites in a) Gallie Pond b) Middle Lake and c) Ash Lake a) Gallie Pond Ti l l shelf * Coring location b) Middle Lake Pataeochannel • Coring location Boulders Figure 3.1 (contd.) - c) Ash Lake Figure 3.2 - The corer used in this study to take samples of surface sediment Handle 1 5 0 c m -t-1-i i 7.5 c m o Hood 2.3 c m 0 . 3 c m Iwall t h i c k n e s s ! 8 cm Handle 7.2 c m 1 0 c m o o Diameter • A i r / w a t e r e s c a p e holes _-J Weld ing diameter (3 mm wall thickness) ABS piping was used, the pipe being hammered into the sediment without the use of the corer using a procedure outlined by Souch (1984). At one site this also proved ineffective and 2.5 cm internal diameter (1.5 mm wall thickness) ABS piping was used. In Gallie Pond there were no obstacles in the lake to prevent coring, but in Middle Lake and Ash Lake boulders and large organic debris lying on top of and within the sediment prevented coring at some sampling sites and alternative locations were found. After the core tubes were installed the air holes were sealed to create a vacuum. The cores were then removed, the bottom end being capped with a lid while the core tube was still in the water - this prevented loss of sediment due to suction at the air/water interface. The core bottom and lid were then sealed with fibre tape and the core tube was separated from the corer. Surplus water on top of the sediment in the core tube was carefully drained off by syphoning to minimise mixing and disruption at the sediment/water interface. It is possible that some sediment in suspension was lost during this process, but it is assumed to be negligible. The depth of sediment in each tube was established and the core tubes were then labelled and, due to the high water content of the sediment, left to air dry for a few days. When dry, the open tube end was sealed with plastic wrap and aluminium foil and the core was then transported. During the procedure described above, the core tubes were kept in an upright position to minimise sediment loss and disturbance. After removal, wooden poles were used to mark the locations of the core sites which were mapped. At other suitable locations (i.e. lake bank, till shelf boundary, palaeochannels, rocks etc.) additional mapping points were 4 0 established. At each site water depths were also taken and this enabled the bathymetry of the lakes to be established (Figures 2.3, 2.4 and 2.5). 3.1.2 - Estimation of lake bank erosion There are a variety of onsite and remote sensing methods that have been used to measure the rate of bank erosion (e.g. Hooke, 1979; Thorne, 1981; Leeks, 1984). Due to the small size of the study lakes and the high spatial variability of bank erosion (Thorne, 1981), erosion pins and stakes were used to estimate bank erosion of the shorelines of the study lakes. Erosion pins were first used by Wolman (1959) and have been used with success by numerous subsequent researchers (e.g. Thorne, 1978; Hooke, 1979). The erosion pins used in this study were 21.0 cm metal nails with a diameter of 8 mm and a flat head of diameter 15 mm. The erosion stakes were 60.0 cm long, 20 mm square and made of cedar. This type of wood is strong and unlikely to warp due to heat and moisture changes. Sites were located approximately every 5 m at Middle Lake and Ash Lake and every 10 m at Gallie Pond (the shoreline lengths of Middle Lake, Ash Lake and Gallie Pond are 120 m, 126 m and 153 m, respectively). Figure 3.3 illustrates the actual location of bank erosion sites at each lake, taking into account the effect of bouldery banks, channel inlets etc. At most sites, depending on the height and nature of the bank, between 1 and 3 erosion pins were pushed horizontally into the vertical face of the bank by hand (Figure 3.4). This was to yield data on the vertical distribution of erosion in addition to rates of horizontal retreat. The distance from the outside of the pin head to the point where the pin enters the bank sediment was measured using a metal tape measure. It is estimated that the distance could be measured to within +0.5 mm. 41 Figure 3.3 - The location of lake bank erosion sites and the aeolian bulk collector in a) Gallie Pond b) Middle Lake and c) Ash Lake a) Gallie Pond Tilt shelf • Horizontal bank erosion site • Vertical bank erosion site Bulk aeolian collector b) Middle Lake Palaeochannel Boulders • Hor izontal bank erosion site • Ver t ica l bank eros ion site Bulk aeol ian c o l l e c t o r Figure 3.3 (contd.) - c) Ash Lake 4 3 As an independent method at each site, bank erosion stakes were also installed. This provided an alternative technique to provide a check on the accuracy of the pin method. The erosion stakes were located on top of the lake bank at distances of 1 m and 2 m from where the uppermost erosion pin entered the bank (Figure 3.4). The second stake was primarily used as a replicate. At all sites the centres of the stakes were located exactly on the 1.00 m and 2.00 m horizontal distance mark and to the right of the tape (the operator being on the bank facing towards the lake). The stakes were driven vertically into the ground using a mallet. So as to obtain greater precision of measurement, small 20 mm nails of diameter 1 mm and head diameter 2 mm were driven into the top of the cedar stakes. These provided the reference from which distances were subsequently measured. In addition to the horizontal distance from bank edge to the bottom of each stake (i.e. 1 m and 2 m), the height of the stakes (i.e. from the ground surface to the pin head) on the side furthest from the lake, and the distance and angle from the pin head to the lake bank edge were measured (see Figure 3.4). The angle was measured with a Suunto Instruments inclinometer. This provided an additional method of calculating the distance from the lake bank to the centres of the stakes, as it was judged that inaccuracies would result from measuring the distance along the ground surface at different times in the field season due to the hummocky nature of the ground surface and the effect of vegetation change. In total 75 erosion pins and 75 erosion stakes were installed using the format illustrated in Figure 3.4, the location of which is referred to as "horizontal erosion sites" in Figure 3.3. At some sites an alternative method had to be used as no clearly identifiable bank could be found. At these sites 4 4 Figure 3.4 - Erosion pins and stakes used to calculate horizontal bank erosion P L A N W a t e r D = -7 L 2 + H 2 - 2 L H c o s Y 4 5 erosion pins and stakes were driven vertically into sediment (Figure 3.5) at intervals of 0.5 m (stake 3 marking the water line at the time of installation as an arbitrary reference). Heights were measured from the pin head or nail head to the ground surface on the side furthest from the lake. 56 pins and stakes were installed using this technique at sites referred to as "vertical erosion sites" in Figure 3.3. The erosion pins and stakes at each site and for each lake were initially measured when they were installed between July 30*n and August 4^n 1989 when the lakes became ice free. They were remeasured at the end of the field season between 2 1 s t and 22nc* September 1989, and again approximately one year later between 2n c* and 3rc* August 1990. At "horizontal erosion sites" bank height was also measured with a measuring tape. At selected sites samples of a known volume were taken for the estimation of bulk density (Section 3.2). 3.1.3 - Estimation of aeolian dust input In order to calculate the direct input of aeolian material to the lakes and to determine the regional input of material to the catchments (Section 1.3.5), a sampling framework based on vegetation/ground cover strata was established. This framework was chosen so as to give areally representative data on spatial variations of the amount of aeolian dust input. This sampling design was implemented before snowmelt to avoid bias in site location. Vegetation/ground cover strata were used as these have been identified as causing spatial variations in aeolian accumulation rates (Zeman, 1973; Darmody and Thorn, 1987). 4 6 Figure 3.5 - Erosion pins and stakes used to calculate vertical bank erosion Pin 3 Stake 1 X Dist a n c e s a re in m. W a t e r 4 7 For each catchment 3 sites were located in each of the strata mapped in Figure 2.7 (2 being replicates) with 1 site being located in the centre of each lake (Figure 3.3 and 3.6). In total 30 sites were established. The sites were initially picked randomly, however problems in the field necessitated the sampling design to be altered (including the addition of 3 sampling sites); this is discussed later in this section. Aeolian input was assessed for 2 time periods - the snow-covered and the snow-free period - using 2 different methods adapted from Jones (1982). For the snow-covered period a Mount Rose snow sampler was used to assess the amount of aeolian material contained within the snowpack. This assumes that no snow in the pack is derived from previous years. During the summer of 1989, all the snow in the catchments had melted by late September. Between 5^n and 8^n July 1989, 4 cores were taken 1 m from each site in a north, east, south, west direction, so as to encompass local spatial variations in the snowpack. All the cored material was then melted into a 4 1 sample bottle and filtered in the field through pre-weighed 0.45 Mm Sartorius cellulose nitrate membrane filter papers using polycarbonate Sartorius SM16510 filtration apparatus and a hand held Nalgene vacuum pump. After each site was filtered all the equipment was washed with distilled water which was then filtered. Each site required the use of several filter papers. All samples were sealed in envelopes and returned to the laboratory for further analysis. The location of each site was fixed by means of a wooden pole and referenced by measuring tape and compass to obvious landmarks. At some sites the corer collected vegetation and/or soil at the end of the core barrel. While this was carefully removed with a knife and discarded, it Figure 3.6 - Vegetation strata in the 3 catchments and the location of sites used to calculate aeolian dust input 49 may cause this technique to overestimate the aeolian material contained in the snowpack. At some locations there was no snow and alternative site locations were chosen which had the same vegetation stratum. In the Goat Meadows catchment all the tree islands had lost their snow cover and therefore snow cores were not taken in this stratum in this catchment, and additional sites were located in the other 2 strata. Snow cores were not taken at physically inaccessible sites; therefore the sites chosen were not strictly random, but are considered spatially representative of the study area. For the snow-free period, bulk collectors were installed at each site. Additional sites were also located in the 3 tree islands in the Goat Meadows catchment that were not snow cored, and in the center of each lake (Figure 3.3). The bulk collectors were modified from designs described by Zeman (1973) and Jones (1982), and collected both dry and wet fallout. The collectors consisted of 184 mm diameter polyethylene heavy duty funnels attached to 4 1 heavy duty polyethylene bottles by fibre and electrical tape. At the base of the funnel stem 1 mm mesh netting was attached so as to prevent insects from entering the collectors and to trap material greater than 1 mm diameter. The height of the funnel top above the ground surface was 36.0 cm, this was so that raindrop splash and other forms of hillslope sediment transport did not contaminate the containers. The collectors were fixed to the ground with stakes. The collectors were installed as soon as the sites were ice-free between 25^n and 29^n July 1989 and emptied at the end of the field season between 2 3 r d and 26^n September. They remained in the field for ca. 60 days. The samples were filtered as described earlier. Each site required several filter 50 papers. The material trapped on the wire mesh was removed by tweezers to plastic specimen bottles. All equipment was thoroughly washed with distilled water to remove any dust, which was then filtered. 3.2 - Laboratory Methods 3.2.1 - Lake sediment The lake sediment was extruded from the core tubes by a tight fitting pusher constructed for each pipe type. The cores were then split in half lengthways and allowed to dry over a period of a few weeks, during which they were described in terms of the lithostratigraphic units present, texture, colour (using the Munsell colour chart) and any other features of interest. The Bridge River tephra layer, which occurred in most cores, was used for cross correlation of the cores and for the establishment of an absolute chronology. Appendix 1 describes the techniques used to identify and date this tephra layer. The outer layer of the cores, which may have been smeared along the core tube during insertion and extraction, were discarded and the central portions remaining were used for further analyses. For each core a known volume of sediment was taken between the core surface and the Bridge River tephra layer and oven dried at 105°C for 24 hours. It was then cooled in a desiccator with silica gel for 2 hours and weighed on a Mettler H20 scientific balance to determine dry bulk density. This material was then split into 2 equal samples, each being weighed as above. Sample A was then placed in a muffle furnace at 550°C for 3 hours to determine loss-on-ignition, a surrogate for organic matter content (Lavkulich, 1981) and for the assessment of the clastic sediment bulk density (Hillel, 1981). 550°C 51 was chosen to burn off organic matter as Johnson (1977) asserts that the majority of organics volatilise at 2 temperatures - 300°C and 510°C. Sample B was used to determine biogenic silica. As ashing of sediment causes diatom valves to become resistant to alkali digestion due to the conversion of opaline silica to crystabilite (Goldberg, 1958), organic matter was oxidised by the addition of 25 ml of 30% hydrogen peroxide (H2O2) at 95°C (Flower, 1980). Although Engstrom and Wright (1983) state that some refractory organics may not be totally oxidised, due to the oligotrophic nature of the lakes and the decomposed nature of the sediments in this study, this is. unlikely to be a problem. After the removal of carbonates and amorphous oxides by the addition of 5 ml 3.ON hydrochloric acid (HCI), the samples were then treated with 50 ml 0.2N Analer sodium hydroxide (NaOH) solution at 95°C and biogenic silica was determined following procedures adapted from Flower (1980) and Engstrom and Wright (1983). 3.2.2 - Aeolian material The aeolian material filtered in the field was oven dried at 105°C for 24 hours, cooled in a desiccator for 2 hours and weighed on a Mettler H20 balance to determine dry weight (the weight of the filter paper being known). All the filter papers for each site were placed in crucibles in a muffle furnace at 550°C for 2 hours to determine organic matter content by loss-on-ignition (the flash-point of the filter paper being 170°C). Although some problems have been identified with this procedure and an alternative technique proposed (Jones, 1984), the large number of samples to be analysed, the weight of the samples and the nature of the analysis, validate the use of the muffle furnace. All material > 1 mm diameter trapped on the 52 wire mesh inside the bulk collectors was similarly oven dried and ashed to determine organic matter and clastic sediment content. 53 C H A P T E R 4 - R E S U L T S This chapter presents the results obtained from the field and laboratory procedures described in Chapter 3 and analysis. Where possible an assessment is made of sources of error. Chapter 5 is a discussion on the implications of the results and analysis. 4.1 - Volume of Lake Sediment The volume of sediment accumulated in the lakes over the last 2350 years was determined by a high density sampling network of sediment cores. The sediment depth to Bridge River tephra is presented in Appendix 3 and summarised in Table 4.1. In Gallie Pond, Bridge River tephra is undisturbed and continuous throughout the lake, with distinct boundaries with the over- and underlying sediment, indicating little mixing or redistribution of sediments once in the lake. In many of the cores this deposit consists of 2 distinct units: the lower, much coarser unweathered shards of ash; the upper, a much finer deposit, a product of ash fallout intermixed with watershed sediments. Of 23 cores extracted 18 contain this tephra deposit at sediment depths of between 4 cm and 16 cm (Figure 4.1a). Above and below this deposit are lacustrine silts which are massive and contain little organic matter. Those above the tephra layer become more unconsolidated near the surface, while those below show an increase in organic matter with depth and some laminations. Mazama tephra is present in 3 cores (Dl, F l and M2) at greater depth (10, 17 and 27 cm, respectively). In Middle Lake, of the 16 cores removed only 5 contain Bridge River tephra (Figure 4.1b), the depth of which varies between 8.5 and 23.0 cm. 54 Table 4.1 - Summary results of sediment depth (cm) to Bridge River tephra for the 3 lakes L A K E n X S.D. S.E. R A N G E Gallie 18 9.7 3.6 0.8 4.0 - 16.0 Middle 5 15.5 5.5 2.5 8.5 - 23.0 Ash 6 11.6 3.5 1.4 5.5 - 15.5 where S.D. = standard deviation and S.E. = standard error of the mean Figure 4.1 - Spatial variation of sediment depth to Bridge River tephra in a) Gallie Pond, b) Middle Lake and c) Ash Lake a) Gallie Pond / - Til l shelf C o n t o u r in terva l 5 cm Figure 4.1 (contd.) b) Middle Lake 12.0. y * V B o u W e r s • C o r e locat ion and depth to tephra in 56 The absence of the tephra in many of the cores may be due to the short length (30.0 cm) of the core tubes not incorporating this deposit or may reflect an absence of tephra at particular sites altogether. Cores that do not contain tephra in Middle Lake and Ash Lake are generally from the deeper, central areas and therefore the latter explanation is preferred. Freezing of these shallow lakes would disturb the sediment and may explain the loss of Bridge River tephra in the sediment record. As this deposit provides a clearly identifiable synchronous layer which is extrapolated between cores to enable sediment volumes and masses to be estimated, cores that do not contain this layer have no spatial or temporal control and therefore cannot be placed within a 3-dimensional body of sediment as defined by the lake, the surface of the sediment and Bridge River tephra. For this reason, only cores that contain Bridge River tephra are used to construct the volume of sediment accumulated in Middle Lake over the last 2350 years, and for further analysis (e.g. bulk density, organic matter and biogenic silica content). The lake sediment of Middle lake is variable both spatially (across the lake) and temporally (with depth). Those cores with tephra are mainly comprised of massive lacustrine silts above this layer, although there are variations in colour (reflecting variations in organic matter content) and particle size (reflecting variations in water level and watershed hydrologic conditions). The cores taken from Ash lake to a large extent reflect the situation in Middle Lake. Only 6 of 17 cores removed from the lake contain Bridge River tephra (Figure 4.1c) at sediment depths of between 5.5 and 15.5 cm, the implications of which are similar to Middle Lake. The spatial and temporal 57 variability of sediment type is high. Those cores with tephra are mainly composed of massive lacustrine silts above this layer, although there are variations in organic matter content and particle size. From Figure 4.1 the sediment volume in Gallie Pond can be calculated by planimetry. Assuming that sediment depth at the till shelf boundary is 0, the total volume of sediment for this time period is 22.2 m^. For Middle Lake and Ash Lake due to the small number of locations giving depth to tephra, sediment volume is based on the surface area of the lake (600 and 618 m^, respectively) and 0.5 of the mean depth to tephra (7.8 ± 2 . 3 and 5.8 ± 1 . 8 , respectively). This is to take account of the fact that depth to tephra based on the mean values presented in Table 4.1 would probably overestimate total sediment volume, as sediment depth is likely to decrease proportionally from the core locations towards the lake shoreline, where it will approach 0. Under these assumptions the volume of sediment in Middle Lake and Ash Lake is 46.8 ± 1 3 . 8 and 35.8 ± 1 1 . 1 m^, respectively. As the sediment contained in these lakes approximates an irregular cone, the volumes of sediment in Middle and Ash lakes calculated above are probably overestimated by a factor of 1.5. Due to the absence of tephra in many cores and the assumptions above, emphasis should be placed on the order of magnitude of the volumetric estimates, not the values per se. 4.2 - Bulk Density of Lake Sediment Lake sediment was oven dried at 105 °C to determine dry bulk density (Section 3.2.1). This enables the volume of sediment accumulated in each lake to be converted into sediment mass. The results are presented in Appendix 3 and Table 4.2. The mean bulk density of Gallie Pond, Middle Lake and Ash Lake sediment above Bridge River tephra is 729, 674 and 58 Table 4.2 - Summary results of sediment bulk density (kg m"3) for the 3 lakes L A K E n X S.D. S.E. R A N G E Gallie 18 729 79 18.6 622 - 866 Middle 5 674 152 67.9 483 - 888 Ash 6 529 60 24.0 468 - 629 Figure 4.2 - Spatial variation of sediment bulk density (kg m"3) in a) Gallie Pond, b) Middle Lake and c) Ash Lake a) Gallie Pond Figure 4.2 (contd.) b) Middle Lake 4 5 7 Boulders • C o r e loca t ion and bulk densi ty l k g m " 3 l 6 0 529 kg m'6 (and standard deviations 79, 152 and 60 kg m"d), respectively. The standard error of the mean of the Middle Lake data is higher than that for the other 2 lakes. This reflects the small sample size and the high variability of the data from this lake. Although Middle Lake and Ash Lake sediment has a lower bulk density than Gallie Pond, this may reflect differences in sample size: 5, 6 and 18, respectively. The value of Gallie Pond sediment is significantly different than that of sediment in Ash Lake. This may be due to the woody nature of the organic matter in the latter. The samples analysed in this study are integrated subsamples for each core taken between the sediment surface and Bridge River tephra. The variability of the integrated sample was assessed by a more detailed analysis of core M l . Three samples were taken between the appropriate limits at set intervals, the bulk densities being 1017, 762 and 659 kg m" ,^ where 1017 kg m"^ and 659 kg m"^ are the lower and uppermost samples, respectively. The mean and standard deviation of these samples is 813 and 184 kg m" .^ This illustrates that there is a high variability in bulk density within a core. Bulk density increases with depth, probably due to sediment compaction. The spatial variability in bulk density for the 3 lakes is shown in Figure 4.2. For Gallie Pond, there does not appear to be any significant pattern, although slighty higher values occur in the deeper areas of the lake. With Middle Lake and Ash Lake there also does not appear to be any significant spatial pattern, although this in part reflects the small sample sizes. 4.3 - Organic Matter Content of Lake Sediment 61 The organic matter content of lake sediment was determined by weight loss after ignition at 550 °C for 3 hours (Section 3.2.1). This enables clastic sediment bulk density to be calculated. Appendix 3 presents data for each sample analysed and this is summarised in Table 4.3. Mean organic matter content (% by weight) varies between lake, from 10.3% at Gallie Pond to 21.9% at Middle Lake: Ash Lake lies between this range at 16.6%. The high mean value for Middle Lake is influenced by the exceptionally high value for core MC4: 40.4%. The mean mass of organic matter per unit volume reflects the pattern descibed above. Although the trend in organic matter content expressed as kg m"^ and % by weight is similar, differences between the 2 data sets reflect differences in dry bulk density values (Appendix 3). The spatial variation in organic matter content (% by weight) is shown in Figure 4.3. For Gallie Pond values are greatest in the deepest and central part of the lake, a pattern in accordance with the variation of sediment depth to Bridge River tephra (Figure 4.1) and bulk density (Figure 4.2). In Middle Lake and Ash Lake spatial variations appear to follow no pattern: this is probably due to the small sample sizes. In Middle Lake, however, organic matter content does seem to increase westwards, away from the main inflowing channel to the east. The transfer of samples between containers may result in some error in the data. To test this, 12 randomly picked samples from the 3 lakes were weighed throughout sample analysis to check for weight change due to loss or contamination. The sample weight change was < ± 2% of the original weight for all samples. The within-core variability between Bridge River tephra and the sediment surface was assessed by analysis of core M l from Table 4.3 - Summary results of organic matter content % by weight (and kg m"^ ) for the 3 lakes L A K E n x SJX S.E. R A N G E  Gallie 18 10.3(72.8) 1.5(15.2) 0.4(3.6) 6.7-13.1(38-96) Middle 5 21.9(148.6) 11.8(70.4) 4.9(9.8) 10.9-40.4(97-268) Ash 6 16.6(86.7) 2.5(8.1) 1.0(6.6) 12.5-19.4(79-100) Figure 4.3 - Spatial variation of organic matter content (% by weight) of surface sediments in a) Gallie Pond, b) Middle Lake and c) Ash Lake Figure 4.3 (contd.) b) Middle Lake m a t t e r content I % I c) Ash Lake •j • 64 Gallie Pond (Section 4.2). Values ranged from 5.2 to 13.3% with the mean 9.5%. 4.4 - Biogenic Silica Content of Lake Sediment The proportion of biogenic silica in the lake sediment was determined by weight loss after digestion with sodium hydroxide (Section 3.2.1). The results are presented in Appendix 3 and summarised in Table 4.4. The percent by weight of biogenic silica in the lake sediment is similar for all 3 lakes: being lowest in Gallie Pond (7.3%) and highest in Ash Lake (12.0%). Data variability in all 3 lakes is high, indicated by high standard deviation values. This, in addition to the small sample size for Middle and Ash lakes, results in high standard error values for these 2 lakes. This is especially so for Ash Lake and as a result the reliability and confidence of the data for this lake is limited. The biogenic silica content expressed as kg m~ 3 shows a similar trend, any differences being due to different mean dry bulk density values for the lakes. Within-core variability is illustrated by a more detailed analysis of core M l from Gallie Pond. Values range from 4.7 to 9.1% (48 to 69 kg nT 3 ) , with a mean of 7.2% (56 kg nT 3 ) . The spatial variation in biogenic silica content (% by weight) is illustrated in Figure 4.4. In Gallie Pond values are greatest in the deepest and central portions of the lake. This is in agreement with the bulk density and organic matter content data. In addition, there are pockets of sediment with greater and lower silica content. In Middle Lake and Ash Lake, due to the small sample size, no trend can be identified. Due to the method used, it is difficult to quantify sources of error. Measurement was accurate to within +0.01 g for each of 3 sets of 65 Table 4.4 - Summary results of biogenic silica content % by weight (and kg m"d) for the 3 lakes L A K E n x SJX SJL R A N G E  Gallie 18 7.3 (53.3) 3.4 (25.2) 0.8 (5.9) 0.9-15.0 (6-99) Middle 5 10.8 (70.6) 2.3 (15.2) 1.0 (6.8) 8.3-14.4 (56-89) Ash 5 12.0 (64.6) 5.0 (29.4) 2.2 (13.1) 3.7-18.1 (20-95) Figure 4.4 - Spatial variation of biogenic silica content (% by weight ) of sediment in a) Gallie Pond, b) Middle Lake and c) Ash Lake a) Gallie Pond Till shelf C o n t o u r i n t e r v a l 5% Figure 4.4 (contd.) b) Middle Lake c) Ash Lake 6 7 measurement per sample (i.e. ± 0 . 0 3 g per sample). The technique used relies on the fact that sodium hydroxide dissolves all the diatom frustules (amorphous silica) and nothing else. Krause et al. (1983) conclude from laboratory tests that the alkali dissolution method used causes only minor degradation of clays and other mineral silicates (<1.5% of the total silica in the mineral samples). 4.5 - Lake Bank Erosion As described in Section 3.1.2 the contribution of bank material to lake sediment was assessed by the use of erosion pins and stakes, the results of which are presented in Appendix 4 and summarised in Tables 4.5, 4.6 and 4.7. From Table 4.5 it can be seen that at horizontal erosion sites the pin and stake data are different. The pin and stake (method 2) data indicate that the banks are undergoing mean net deposition of 0.7 mm yr"l and 6.9 mm yr"l, respectively, while the stakes (method 1) indicate mean net erosion of 3.9 mm yr ' l . The amount of erosion or deposition also varies at each lake for each technique. These differences are probably the result of the errors inherent in each of the techniques used. The mean results for the pins are not significantly different than zero, while some results for the stakes are. Also, the pin results are comparable with the pooled measurement error ( ± 1 . 5 mm), while for the stakes they are greater. Intuitively, the pins are expected to be the most accurate of the three techniques. This is reinforced by the low standard error of the pin data compared to the other techniques, which implies greater confidence in this method. Stake method 2 involves problems due to bank topography, while stake method 1 involves the calculation of distances using indirect measurements and trigonometry (section 3.1.2). Many of the stakes appear 68 Table 4.5 - Summary results of horizontal erosion pins and stakes for 1 year for the 3 lakes PINS L A K E n x S T A K E S (1) S T A K E S (2) SD SE n x SD • SE n x SD SE Gallie 19 0.3 3.9 0.9 10 -14.0 48.0 15.2 11 11.1 46.7 14.0 Middle 25 1.4 4.4 0.9 22 0.3 12.8 2.7 22 6.0 7.8 1.7 Ash 25 0.2 6.0 1.2 21 -3.5 23.9 5.2 20 5.6 11.8 2.6 All 3 69 0.7 4.9 0.6 53 -3.9 26.7 3.7 53 6.9 22.3 3.1 net change of length is in mm - indicates bank erosion Stakes (1) - lengths calculated by trigonomtery Stakes (2) - lengths measured along bank tops Table 4.6 - Summary results of vertical erosion pins and stakes for 1 for the 3 lakes year L A K E n X SD SE Gallie 12 0 9.9 2.8 Middle 6 -10.7 26.3 10.7 Ash 28 23.0 27.1 5.0 All 3 46 12.6 26.9 4.0 Table 4.7 - Change in shoreline (mm) of the 3 lakes during different time periods (the time period for each lake is different - see Appendix 4) based on erosion pins S N O W - F R E E S N O W - C O V E R E D 1 Y E A R L A K E n x SD SE n x SD SE n x SD SE Gallie 19 2.1 2.9 0.7 19 -1.8 2.9 0.6 19 0.3 3.9 0.8 Middle 25 1.4 4.0 0.8 25 0.0 3.9 0.8 25 1.4 4.4 4.4 Ash 24 2.5 5.4 1.1 24 -2.4 8.9 1.8 25 0.2 6.0 1.2 All 3 68 2.0 4.3 0.5 68 -1.4 6.0 0.7 69 0.7 4.9 0.6 - indicates net erosion 6 9 to have been disturbed during the measurement period from September 1989 to August 1990 (see Appendix 4). This was probably due to snow creep downslope and resulted in many of the stakes being tilted towards the lake. This would explain the net erosion indicated by stake method 1, as this technique measures the distance from the lake bank to the top of the stake (see Figure 3.4). Stake disturbance was most apparent around Gallie Pond which clearly biases the overall result for this technique. The high variability of the stake data is indicated in the high standard deviations compared to the pin data: 26.7 mm and 22.3 mm for stake method 1 and 2, respectively, compared to 4.9 mm for the pins. Due to the above, only the pin data will be considered further. The vertical bank erosion data is shown in Table 4.6. Site inspection showed that the banks did not change much over the measurement period. The data shows no trend with high standard deviations indicating variability and high standard errors lending little confidence to the data. The overall yearly mean for all three lakes indicates net deposition of 12.6 mm. Although high this is consistent with data presented in Table 4.5. For these reasons the vertical erosion data are not discussed further. Table 4.7 shows that all 3 lakes appear to be experiencing net deposition, the magnitude of which varies between lakes and between different time periods. The trend in all 3 lakes is similar: net deposition in the snow-free period and net erosion in the snow-covered period. Over a 1 year period, when account is taken of the bulk density (Table A4.5), height of the bank and the distance between sites, the dry mass of bank expanded material for Gallie Pond, Middle Lake and Ash Lake is 1.04 kg, 4.5 kg and 0.8 kg, 70 respectively. These figures are dependent on a few bulk density measurements and indicate order of magnitude shoreline expansion. A possible explanation of the data described above is that changes in bank vegetation between measurements is masking real bank changes. This possibility will be taken up in detail in Chapter 5. Another possible source of error involved in this technique is due to the thermal properties of the pins and tape measure used. The pins were measured not only at different times in the year but also at different times in the day. The probability of the air temperatures being different during measurements is high. As the pins and tape measure are metal they are likely to expand and contract as air temperatures change. A pin and the tape measure used in this study were found to expand by 0.5 - 1.0 mm when subjected to a temperature change from 15 °C to 31 °C over a period of several hours. This range of temperature was possible over the study period. Although the data imply that net bank deposition is occurring, changes in vegetation and temperature of the field apparatus, in addition to operator measurement errors, could explain this result. It seems probable that the error inherent in the techniques used to measure bank erosion of the 3 study lakes is greater than any net change occurring. However, it could be argued that material is being supplied to lake sediment by bank erosion, but that the banks are undergoing net deposition. Due to the thick vegetation cover of much of the bank face around the lakes, indicating stability, and the results presented above, it is assumed in this study that the contribution of bank material to lake sediment is negligible during the 1 year of measurement. 4.6 - Input of Aeolian Material 7 1 Appendix 5 presents data on the input of aeolian material to the 3 lakes and catchments. The data are summarised and expressed in g m" 2 in Table 4.8. The mean annual total amount of material for each catchment is similar: 29.80, 27.99 and 38.57 g m" 2 for Gallie Pond, Middle Lake and Ash Lake catchments, respectively. The mean annual total amount of aeolian material for each stratum varies both within and between catchments. The 9 9 range of values is 14.62 g m"^ to 55.86 g m"-J. These yearly values (i.e. snow-core + bulk collector data) may underestimate true values as the bulk collectors (installed for ca. 60 days) were removed before the first snowfall. There is no snow core data for the 3 lakes. The proportion of organic and mineral material also varies spatially. In general, talus, bare soil and bedrock sites have a higher proportion of mineral compared to organic material, while the reverse is true of tundra and tree sites. The material contained within the snowpack is 1 or 2 orders of magnitude greater than that collected in the bulk traps for most vegetation strata for the 3 catchments. Again, in general, the proportion of mineral to organic matter content for both the snow-core and bulk collector data reflect the total for each stratum. The material caught by the bulk collectors located in the centre of each lake, while different for each lake, show a similar trend. While the absolute values vary, the proportion of organic material is greater than mineral material. Total material values for Gallie Pond, Middle Lake and Ash Lake are 2.29, 0.54 and 1.19 g m"2, respectively. The ratios between lakes is different than the corresponding ratios between catchments. However, the order of magnitude within each data set is similar, and emphasis should be placed on this. 72 Table 4.8 - Mean total aeolian material input (g m"z) for each vegetation stratum and for each lake 1 Y E A R * SNOW BULK** C A T C H -M E N T V E G Min Ore: Tot Min Org Min Org Goat Tu 5.83 8.79 14.62 5.24 7.76 0.59 1.03 Meadows Ta 39.15 5.83 44.98 26.73 4.30 12.42 1.53 Tr — — 0.53 1.61 L — — 0.97 1.32 All 29.80 Middle Tu 7.12 12.49 19.61 6.75 11.65 0.37 0.84 Lake Ta 43.02 3.81 46.83 41.98 3.30 1.03 0.51 Tr 3.23 14.31 17.54 2.90 13.83 0.33 0.48 L 0.18 0.36 All 27.99 Ash Tu 7.80 47.76 55.56 7.61 47.38 0.19 0.38 Lake Ta 8.48 6.86 15.34 8.01 6.19 0.47 0.67 Tr 2.90 41.91 44.81 2.61 41.05 0.29 0.86 L 0.30 0.89 All 38.57 V E G - vegetation Min - mineral Org - organic Tot - total material (i.e. mineral + organic) Tu - tundra Ta - talus, bare soil and bedrock Tr - tree islands and stands of conifers L - lake All - mean total of Tu, Ta and Tr * 1 Y E A R is the sum of snow-core and bulk collector material: it is assumed that this constitutes 1 year bulk is only material <1 mm. Material >1 mm was mainly composed of insects that had entered the collectors and mineral material derived from within the watersheds 73 Appendix 5 reveals the small amount of material collected at each site using the different techniques. At all sites < 1 g, and in most cases < 0.1 g, of material was collected. As a result, there are likely to be measurement errors. The Mettler H20 balance used was re-zeroed every 20 measurements. The average balance drift during each set of measurements was ± 0 . 0 0 0 1 g. To assess operator variance in filter paper and sample weighing, 18 samples were prepared and weighed as described in Section 3.2.2 in October 1989. The same samples were then reweighed using the same procedures in February 1990. The mean difference in the sample weights was ± 0 . 0 0 0 2 g. Mineral and organic matter content for each sample was assessed by loss-on-ignition (Section 3.2.2). Blank filter papers were ashed and the weight of the residue was 0.0001 g. This may be paper residue or may reflect balance drift or operator variance. As most sample weights are 1 to 4 orders of magnitude greater than the sources of error discussed above, they are assumed negligible. There were other sources of error that were not quantified. For example, transfer of samples between containers (e.g. bulk collector, filtering apparatus, envelopes, crucibles etc.) during sample collection and laboratory analysis is likely to result in loss of material. While every effort was made to minimize this, the amount of sample lost was not evaluated. Sample contamination is also a possibility. Jones (1982) demonstrates that samples processed in the Muffle furnace are subject to weight loss due to deflagration and oxidation reactions of constituent minerals at high temperatures. Furthermore, not all organic matter was oxidised and some seed and needle parts remained after processing. One other source of error not evaluated resulted from the ignition of the filter papers. Sometimes this was explosive and, based on observation, material was ejected from the crucibles. 74 While there appear to be many possible sources of error, the data presented in Appendix 5 and Table 4.8 are believed to be of the correct order of magnitude and spatially representative of aeolian input in the study area. 4.7 - Summary To calculate the volume of sediment accumulated in the study lakes over the last 2350 years sediment cores were taken. Bridge River tephra, used for core correlation and the establishment of an absolute chronology of sediment deposition, is found at varying depths in all 3 lakes. In Middle and Ash lakes, however, most cores did not contain this deposit. Mean bulk density of lake sediment ranges from 529 to 729 kg m"3. These results are used in Chapter 5 to translate sediment volume to dry sediment mass. To calculate the clastic sediment bulk density, lake sediment organic matter was determined. Mean values ranged from 10.3 to 21.9% (72.6 to 148.6 kg m"3). The mean biogenic silica content of lake sediment ranged from 7.3 to 12.0% (53.3 to 64.6 kg m"3). The input of sediment to the lake system from lake bank erosion is negligible, while that from aeolian sources is more substantial. Chapter 5 develops data presented in this chapter to calculate catchment sediment yields and erosion rates based on the assumptions given in Chapter 1. 75 C H A P T E R 5 - DISCUSSION This chapter develops the data presented in Chapter 4 in order to fulfill the objectives of the thesis, namely, the use of lake sediments to establish sediment yields and erosion rates for 3 small alpine/sub alpine catchments in the Coast Mountains and the assessment of the proportion of lake sediment that is not derived from erosion of the catchments under investigation and/or is not spatially distributed over the catchment surface. Once corrections have been made to the mass of sediment contained within the lakes for the sources above, sediment yields and erosion rates will be placed within a regional framework to test the hypothesis of scale controls on sediment yield. The relative proportions of the different sources of lake sediment will be discussed in view of the use of lake sediments for the establishment of sediment yields and erosion rates. Finally the temporal and spatial representativeness of the data will be evaluated. 5.1 - Lake Sediment Budget It is appropriate at this point to restate Equation 3, as it is this sediment budget equation upon which this thesis is built: A S = ( I f + I c + I 0 + I s + I b + I a ) -0 This section will utilise data presented in Chapter 4 to calculate the relevant terms of this equation for the 3 lakes for the last 2350 years. 5.1.1 - The mass of accumulated sediment The mass of sediment accumulated over the last 2350 years can be calculated from the volume of sediment accumulated in the lake over this time period (Section 4.1) and the mean bulk density of the sediment (Section 7 6 4.2), the values of which are restated in Table 5.1. The mass of sediment accumulated in Gallie Pond, Middle Lake and Ash Lake is 16184, 31341 and 18938 kg, respectively. The range of values for the mass of sediment in each lake shown in Table 5.1 reflects the variability of the volume (and hence the depth to tephra data) and bulk density data, while those for sedimentation rates also reflect the standard deviation of the date for Bridge River tephra (Appendix 1). The mass values are similar in terms of the order of magnitude but vary between lakes. The value for Middle Lake is almost double that of the other 2 lakes. This is mainly due to the higher values of sediment depth to tephra for this lake. The values for Gallie Pond can be compared directly to those of Souch (1984) who used a similar methodology. She estimated that the volume and mass of sediment deposited over the last 2350 years to be 19.2 m^ and 18521 ± 5790 kg, respectively. While slightly different, the 2 sets of data are comparable and fall within 1 standard deviation of each other. The sedimentation rate calculated by Souch of 0.022 ± 0.007 kg m"2yr"-'-compares to that calculated in this study of 0.018 ± 0.004 kg m" 2yr"l. Differences reflect variations in depth to tephra, bulk density and surface area data for the 2 studies. There are no data with which to compare the results of Middle and Ash lakes. In addition, due to the method used to calculate the volume of sediment in these 2 lakes (Section 4.1), sediment masses and sedimentation rates should be treated with caution, and emphasis should be placed on the order of magnitude of the results, rather than the specific values per se. However, Souch (pers. comm.) identified Bridge River tephra in these 2 lakes at depths similar to those reported in this study, which, in general, were greater than those found in Gallie Pond. This would support the greater volumes and masses stated in Table 5.1. 77 Table 5.1 - Volume and mass of total (i.e. mineral and organic) sediment and sedimentation rates in the 3 lakes for the last 2350 years B U L K SEDIMENTATION V O L U M E DENSITY MASS R A T E L A K E (m3) (kg m"3) (ke) (ke m2vr"-h Gallie 22.2 729 ( ± 1 5 7 ) 16184 (12698-19668) 0.018 (0.014-0.022) Middle 46.5 ( ± 3 3 . 0 ) 674 ( ± 3 0 4 ) 31341 (4995-77751) 0.022 (0.003-0.056) Ash 35.8 ( ± 2 1 . 6 ) 529 ( ± 1 2 0 ) 18938 (5808-37253) 0.013 (0.004-0.026) ± = ± 2 standard deviations ( ) indicates maximum-minimum values (see text) 78 The sedimentation rates for the 3 lakes are similar and the trend is in accordance with the catchment area draining into each lake. When compared to lowland lakes (e.g. Foster et al., 1985; Dearing et al., 1990) and alpine lakes (e.g. Andrews et al., 1975, 1985) in temperate environments, the sedimentation rates for the 3 lakes are low. This probably reflects the low energy environment of these catchments, which is discussed in Section 5.2. 5.1.2 - Organic matter content Organic matter content was determined by direct analysis of lake sediment. The mean organic matter content of the 3 lakes ranges from 10.3% (Gallie Pond) to 21.9% (Middle Lake). The data for Gallie Pond can be compared to Souch (1984). In this study mean organic matter content was estimated to be 10.3% (S.E. = 0.4), while Souch (1984) estimated it to be 7.2% (S.E. = 0.05). Differences reflect differences in sample size (18 and 104, respectively) and this is shown by the standard error of the mean. Values for the other 2 lakes are higher and this may be due to the lower altitude of the other lakes which would favour conditions more conducive to aquatic productivity. As a result of the oligotrophic nature of the lakes, most autochthonous organic material produced is probably oxidised in the water column before burial in the sediments (Mackareth, 1966). The ratio of organic carbon to nitrogen (C:N) can be used to interpret sources of organic matter (Hutchinson, 1957). In general, organic matter derived from peaty material either in the lake sediments or from marginal bogs adjacent to the lake or influent streams, has a C:N ratio of between 40 and 50:1, whereas autochthonous material produced by the decomposition of plankton within 79 the lake itself has a C:N ratio of approximately 12:1 (Wetzel, 1983). Souch (1984) found values for surface sediments above Bridge River tephra of < 17:1. This indicates a predominantly autochthonous source for the organic matter, although there may be inputs from allochthonous sources. Arnett (1978) estimated that the percentage of organic matter accounted for 3 to 10% of total stream sediment load for 16 small moorland basins in north-east England. The values for the 3 lakes fall within the range cited in the literature for lakes in lowland and alpine environments. In surface sediments in Lough Neagh, Northern Ireland, Flower (1980) calculated that organic matter content ranged from 15 to 40%, and was generally between 20 and 25%. Andrews et al. (1985) estimated the organic matter content of surface sediments in Blue Lake, Colorado Front Range, ranged between 5 and 10%. 5.1.3 - Biogenic silica This component was also determined by direct analysis of lake sediment. Values of mean content range from 7.3 to 12.0% (by weight). The variability between and within lakes is low. Biogenic silica is proportionally more abundant in organic sediments, being lowest in Gallie Pond. Biogenic silica concentration in lake sediments of between 10 and 20% has been fairly commonly reported in the literature (e.g. Bradbury and Winter, 1976; Engstrom, 1983). Flower (1980) found diatom silica in sediment cores from Lough Neagh, Northern Ireland, to range between ca. 7 and 15%. In addition to autochthonous components of organic matter and biogenic silica derived from diatoms and other aquatic organisms, lake sediment may 80 also be composed of biochemically precipitated carbonate minerals, amorphous and crystocrystalline Fe and Mn oxyhydroxides, sulphides, phosphates and sorbed or coprecipitated elements (Engstrom and Wright, 1983). Hence the aquatic productivity component calculated above and in Section 5.1.2 is likely to be an underestimate of true values. 5.1.4 - Lake bank erosion The contribution of sediment to the lakes from the erosion of the lake banks was shown to be negligible at the resolution of measurement. The bank erosion data indicate that the banks are undergoing net deposition. A possible explanation for this is that changes in bank vegetation between measurements is masking real bank changes. When the pins were installed and first measured in late July 1989, the lakes had recently become ice-free and the air temperatures were cool. At the end of the snow-free period in late September when the pins were remeasured for the first time, the bank vegetation had grown. The vegetation was thick (mainly moss, lichen and grass) and covered most of the bank face of Gallie Pond and Ash Lake. Middle Lake on the other hand contains banks with less vegetation cover. Growth in vegetation over the summer would result in an apparent expansion of the bank, which the erosion pin technique would record as net deposition. When the pins were remeasured in August 1990, although this was almost at the same time of year as when the pins were first measured, it was apparent that the lakes had become ice-free earlier than in 1989. This inference was based on the extent of perennial snow-patches in the catchments, the sparse snow cover on nearby glaciers and the very warm temperatures for this time of year (Slaymaker, pers. comm.). The vegetation on the lake banks may have been thicker than on the same date in 1989 but 81 less than in late September 1989. This change in vegetation would account for the apparent net deposition in the snow-free period, net erosion during the snow-covered period and net deposition over the 1 year period. 5.1.5 - Aeolian input The input of aeolian material to the lakes and their catchments was assessed: the former to calculate the direct input of material to the lake system; the latter to calculate the deposition rate within the catchments, as this has implications for estimated sediment yields and erosion rates (Section 5.2). The input of aeolian material to the lakes was measured only during the snow-free period. Total material values for Gallie Pond, Middle Lake and Ash Lake are 2.29, 0.54 and 1.19 g m"2, respectively. As the organic matter content of the lake sediments has already been evaluated (Section 5.1.2) only the minerogenic component of aeolian input needs to be considered. This is 0.97, 0.18 and 0.30 g m" 2 for the 3 lakes. To assess the input of aeolian material to the lakes during the longer snow-covered period, the mean of the mean values of the snow-core data for the other strata could be used (e.g. bare soil/talus/bedrock, alpine tundra and trees). However, aeolian material can be derived from a number of sources both within and outside the catchments (Section 1.3.5). This is especially so in areas of low vegetation cover which have been shown empirically (Woodruff and Siddoway, 1965; Thorn and Darmody, 1985) and theoretically (Chepil, 1951) to favour aeolian erosion. Therefore bare soil/talus/bedrock sites are likely to contain locally (within the catchment) and regionally (outside the catchment) derived material. The other sites are located in areas of good vegetation cover with little or no mineral sediment sources and the material collected is probably locally derived organic material and regionally derived 8 2 organic and mineral material. This is reinforced by. the proportions of organic and mineral material shown in Table 4.8. If only vegetated sites are considered the mean mineral input for the 3 catchments for the snow-covered period ranges from 4.83 to 5.25 g m"2: the similarity between catchments lends support to the argument above. On these assumptions the annual deposition of aeolian material in the lakes is based on the sum of the minerogenic component of the bulk collector data for each lake and the mean of the mineral component of the mean snow-core data for vegetated strata. Thus total mineral aeolian inputs to Gallie Pond, Middle Lake and Ash Lake are 5.60, 3.01 and 3.34 kg yr~*. Expressed as a percentage of the total dry mass of sediment in each lake, values are 81.3, 22.6 and 41.4%, respectively. This trend is in accordance with the catchment:lake ratio, which is 26:1, 337:1 and 36:1 for Goat Meadows, Middle Lake and Ash Lake catchments, respectively. While regional aeolian deposition is similar between catchments, the proportional inputs by colluvial and fluvial processes will be a function of the catchment:lake ratio, assuming that the magnitude of these processes is similar between catchments. The higher value for Gallie Pond may also reflect its exposed position on the ridge top, while the other lakes are located at lower elevations and in sheltered valley bottoms. Izmailow (1984) found similar patterns in the Polish Tatra Mountains. The values presented above compare well with those calculated by Jones (1982) for the Goat Meadows catchment. This may be due to the fact that the methodology used in this study is adapted from that used by Jones. She found that the mineral material in bulk collectors ranged from 0.46 - 2.9 g m" 2 for the summer of 1981, while that in the snow-pack ranged from 0.4 -83 4.0 g m . This gives a combined maximum-minimum range of 0.86 - 6.9 g m - 2 y r - l This compares favourably with the values presented in Appendix 5, Table 4.8 and calculated above. Differences may be due to sample size (27 and 33 in this study and 9 and 4 in Jones (1982) for snow-core and bulk collector data, respectively), the time at which snow-cores were taken (5-8 July 1989 and 1-2 July 1981) and the time during which bulk collectors were left in the field (ca. 60 days and ca. 45 days, respectively). In addition, there is probably between-year variability in rates of aeolian deposition due to variations in climate (Appendix 2). Despite differences, the 2 sets of data are similar and in both cases aeolian material contained in the snow-pack is greater than summer deposition, with non-vegetated sites having the coarsest and greatest amount of material. 5.1.6 - Outflow losses As all 3 lakes contain outlet streams, there will be a loss of sediment from the lake systems. To calculate sediment yields and erosion rates it is necessary to estimate the trap efficiency of the lakes. There are many empirically derived trap efficiency curves which are based on different lake and catchment parameters. The most commonly used trap efficiency curve is that developed by Brune (1953). Other curves (e.g Chen, 1975) and mathematical models (e.g Curtis and McCuen, 1977) are available and are probably more reliable (c.f. Heinemann, 1984). However, the information required for these methods is not available, while that for Brune's curve is, and it is this method that is therefore used. Trap efficiency is based on the ratio of the capacity of the lake and the annual inflow to it. In this study the lake capacity is taken as the volume of water calculated in Chapter 2, while the annual inflow is based on the runoff component of the water 84 balance for Goat Meadows catchment as calculated by Gallie (1983) and the catchment areas as calculated in Chapter 2. For all lakes the trap efficiency will vary for different particle sizes. Brune's curve takes this into account by having curves for 3 different particle size ranges. In Gallie Pond the lake storage capacity : annual inflow (C:I) ratio is high (0.022). For sands and coarse silts the trap efficiency is estimated to be >70%, while for fine silts and dispersed clays it is <50%: for a combination of the 2 size ranges it is >60%. For Middle Lake the C:I ratio is very low (0.0005) due to the large catchment area and the small storage capacity of the lake, and falls below the critical value on the Brune curve indicating a trap efficiency of 0% for all particle sizes. This is unlikely to be true for the coarser size range and the volume of sediment extracted is proof of this. For Ash Lake a medium to low C:I ratio (0.006) indicates trap efficiencies for coarse, mixed and fine particle sizes of 45, 32 and 17%, respectively. Lake trap efficiency is variable over time. An increase in lake sediment will decrease trap efficiency, if all other variables remain constant, by decreasing the storage capacity of the lake. Sediment accumulated since Bridge River tephra will have resulted in a decrease in the trap efficiency of the lakes over the last 2350 years. Based on the volume of sediment calculated in Section 4.1 it is possible to calculate the trap efficiency of the lakes 2350 years ago. For Gallie Pond the C:I ratio increases slightly (0.0023) and the trap efficiencies increase accordingly by a few % for each of the particle size ranges. For Middle Lake the increase in the C:I ratio (0.0007) has no effect on the trap efficiency, while for Ash Lake the C:I ratio and trap efficiencies increase to 0.008 and by 8%. Based on 85 descriptions of soils and other sediment sources in the catchments (Jones, 1982; Gallie, 1983; Souch, 1984; Barrett, 1988) the fine sand and coarse silt is probably the dominant size range of inflowing stream sediments. For this size range the trap efficiency for the 3 lakes over the last 2350 years has probably ranged between 70 and 75%, 0 and 0% and 48 and 53% for Gallie Pond, Middle Lake and Ash Lake, respectively. ' These values are dependent on the runoff component of the annual water balance for the Goat Meadows catchment (1000 mm), which may not strictly apply to the other 2 catchments. For example, if the annual runoff is 500 mm then the trap efficiency for Ash Lake is >60%, while if the annual runoff increases to 1500 mm, the trap efficiency becomes <40% for the present storage capacity of the lake. For Middle Lake to have a trap efficiency >0%, annual runoff must be <500 mm. The use of the Brune curve has been cited as possibly overestimating values for finer sediment (e.g. Gottschalk, 1965; Chen, 1975) and underestimating values for coarser sediments (e.g. Chen, 1975), while in the silt size range it is probably reliable (Heinemann, 1984). To provide a check on the Brune curve, Heinemann's empirically derived 1981 curve was used. While the result for Middle Lake is no different, for the other 2 lakes trap efficiencv over the last 2350 years is lower: <60% in Gallie Pond and 23 -33% for Ash Lake. Although in this study the values obtained from Brune's curve are used, the results obtained from Heinemann's curve indicate the inaccuracy that is likely using such curves. Furthermore, most of the runoff to the lakes is delivered in a short period of time during snowmelt. The minimum residence time for Gallie Pond can be calculated from outflow discharge data given in Gallie (1983). Simple 86 calculations of settling times based on Stokes' law indicate that for Gallie Pond, assuming a non-stratified water column, material >2u m would settle out under these conditions. This assumes no resuspension of sediment in these shallow lakes. This suggests that the trap efficiencies calculated above may be underestimates of true values. 5.1.7 - Summary The data presented and described above is summarised in Table 5.2. Clearly the 4 sources evaluated constitute a significant portion of the sediment contained in the 3 lakes. The proportion of each source varies within and between lakes, although the trend is similar, with aeolian dust being dominant. Organic matter and biogenic silica content are similar, with the former always greater. Lake bank input is negligible at the resolution of measurement but may supply material over a longer time period. Based on Equation 3, the data in Table 5.2 suggest that fluvial and colluvial input to the lakes varies between 1.1 and 30.0% over the post Bridge River period, the implications of which are discussed in the following sections. 5.2 - Sediment Yields and Erosion Rates To use total lake sediment mass for the calculation of sediment yields and erosion rates is to assume a specific catchment based source of sediment, with no losses of sediment through the outflow. On these assumptions sediment yields are simply obtained by dividing total dry sediment mass by catchment area and by the time period over which the sediment has accumulated. The sediment yield for the Goat Meadows, Middle Lake and Ash Lake catchments would be 299, 61 and 366 kg km'^yr"-1-, respectively under these assumptions. However, the assumption of a specific catchment Table 5.2 - Lake sediment inputs (% of mineral and organic lake sediment) derived from different sources ORGANIC BIOGENIC L A K E A E O L I A N L A K E M A T T E R SILICA B A N K DUST* T O T A L Gallie 10.3 7.3 0 81.3 98.9 (7.3-13.3) (0.5-14.1) (65.5-105.8) (73.3-133.2) Middle 21.9 10.8 0 22.6 55.3 (0.0-45.5) (6.2-15.4) (8.9-144.6) (15.1-205.5) Ash 16.6 12.0 0 41.4 70.0 (11.6-21.6) (2.0-22.0) (20.6-138.0) (34.2-181.6) * mineral component only () the range in parenthese represents ± 2 standard deviation for organic matter and biogenic silica data and maximum-minimum values for aeolian data (this data also takes into account the standard deviation of the date for Bridge River tephra) 88 derived source of sediment is unlikely to hold true for all the sediment contained within the lake basins. This thesis has examined the contribution of 4 sources of sediment to the total sediment mass of a lake that are not derived from erosion of the catchments under investigation and/or are not spatially distributed over the catchment surface. Table 5.2 illustrates that these 4 sources of material may supply between 55 - 99% of the total sediment accumulated over the last 2350 years in the 3 lakes (this does not include the possibe maximum-minimum range). In this thesis material supplied to the lakes by aquatic productivity (i.e. organic matter and biogenic silica) and regional aeolian input is not considered as either sediment yield or erosion, while inputs from lake bank erosion are considered as sediment yield but not erosion. Due to the fact that the input of material by the erosion of lake banks is assumed to be negligible, sediment yield and catchment erosion rates are essentially the same under these assumptions (this will be discussed in Section 5.3). Fluvial and colluvial sources account for 1.1, 44/7 and 30.0% of the sediment in Gallie Pond, Middle Lake and Ash Lake, respectively. When corrections are made for trap efficiencies and sediment sources, revised sediment yields and erosion rates are 4.4 to 4.7 kg km"2yr"^ for the Goat Meadows catchment, >29.5 kg km" 2yr"l for Middle Lake catchment and 207 to 244 kg km" 2yr"l for the Ash Lake catchment (the range of values takes into account changes in trap efficiency). These values are based on the modal values given in Table 5.2 and suggest an increase in sediment yield with decrease in elevation. The Goat Meadows catchment is above treeline and has the lowest sediment yield, while the Ash Lake catchment is below treeline and has a sediment yield 2 orders of magnitude greater. This has also been suggested for catchments in the Colorado Front Range (Caine, 89 1974; Bovis, 1978) where studies have shown that above treeline material is essentially contained within catchments, while below, sediment yields are higher as a result of larger scale fluvial and colluvial processes. This is discussed further in the next section. However, in addition to the direct regional aeolian inputs to the lakes there is also aeolian input to the catchments as a whole. The soils of the catchments have been identified as being primarily windblown in nature (Jones, 1982; Gallie, 1983; Souch, 1984, pers comm.) (Figure 2.6). This would indicate that aeolian material is accumulating in the catchments. Material eroded by fluvial and colluvial processes, therefore, may be essentially regional aeolian material which has been stored on the hillslopes and later remobilised (term I4 in Equation 3). Based on the mean of the mean minerogenic component of vegetated sites (Section 5.1.5) annual regional aeolian input is 5810, 5180 and 5350 kg km"2yr"^ for Goat Meadows, Middle Lake and Ash Lake catchments, respectively. The similarity of the 3 estimates lends support to this as a regional aeolian input. It is possible to estimate the magnitude of aeolian deposition over the post-glacial period from the accumulation of aeolian material as a loess capping overlying the till in many of the soil profiles of the Goat Meadows catchment. A reasonable estimate of accumulation is 20 cm. When account is taken of ash layers, organic matter content, gravel and boulders (Jones, 1982; Gallie, 1983; Souch, 1984), only about 30% or 6 cm depth can be ascribed to wind deposition. An average bulk density of 1100 kg m"^ (Jones, 1982) may be used to convert this volume to an average accumulation rate over the Holocene Epoch of 6300 kg km" 2 y r _ 1 (Souch, 1984). While this 90 value is sensitive to the assumptions made, it is comparable with the order of magnitude of the aeolian inputs calculated above. This result is based only on the cumulic soils in the catchment. A significant proportion of each of the 3 catchments is talus and bedrock. As regional aeolian deposition is generally uniform over the catchment surface, areas with no soil cover are probably the dominant sediment source areas for fluvial and colluvial processes. Talus and bedrock covers 49, 55 and 23% of Goat Meadows, Middle Lake and Ash Lake catchments, respectively. Based on process measurement and the soil profiles, the regional aeolian input is ca. 5000 kg km" 2yr"l (this is probably an underestimate). Only 13, 2 and 32% of the aeolain material deposited on talus and bedrock areas over the last 2350 years is required to account for all of the material accumulated in the lakes over this time period. Figures of regional aeolian deposition are 1 to 3 orders of magnitude greater than lake sediment-based sediment yields. This implies that aeolian material derived from outside the catchments of study could account for all of the mineral material removed from the catchments and contained within the lakes. It also implies that in certain areas the catchments are undergoing net deposition and not net erosion, and that fluvial and colluvial processes are primarily remobilising aeolian material stored on the hillslopes. Clearly catchment erosion is not synonymous with sediment yield. The implications of this are discussed in Section 5.4. 5.3 - Regional Rates of Sediment Yield The sediment yields for the 3 catchments in this study are orders of magnitude lower than regional rates for larger scale basins (Table 5.3). This 91 Table 5.3 - Regional rates of clastic sediment yield (adapted from Slaymaker, 1987; Hickin, 1989; Jordan and Slaymaker, in press) A R E A SEDIMENT YIELD RIVER BASIN (km2) (t km^yr' 1 )  This study 0.02 - 0.20 0.004 - 0.2 Central Creek 2.4 5.3 Miller Creek 21.6 34 Seymour 148 60 Lillooet* 3150 538 Squamish 3600 500 Fraser (at Hope) 217000 81 * based on advance of Lillooet delta from 1913 - 1948 (see Jordan and Slaymaker, in press) Figure 5.1 - Specific sediment yield as a function of drainage area for fluvial suspended-sediment-transport records in British Columbian rivers and lake sediment-based yields in alpine watersheds (adapted from Church and Slaymaker, 1989) 10 — -. -• • • 0.01 0.1 1 10 100 1,000 10,000 100,000 1,000,000 D r a i n a g e area I k m ' I WSS>: $*i£m main trend tor u n d i s t u r b e d basins — — trend tor c o n t e m p o r a r y g l a c i e r i z e d bas ins • Ga l l i e Pond • M idd le L a k e 92 supports a hypothesis for British Columbia of scale controls on sediment yield (Slaymaker, 1987; Church et al., 1989; Church and Slaymaker, 1989) and geomorphic activity in general (Section 2.2, Ryder, 1981). Specific sediment yield increases at all spatial scales up to 3 x 10^ k m 2 (Figure 5.1). This results from the dominance of secondary remobilisation of Quaternary sediments stored along river valleys over primary denudation of the land surface. Not only does this challenge the conventional view that specific sediment yield decreases downstream due to sediment going into storage, but also questions the view that sediment yield is the consequence of erosion from the land surface and reflects the denudation rate for the prevailing climate and geology (Church and Slaymaker, 1989). This last point is reinforced by the 3 catchments in this study which demonstrate an input of aeolian material well in excess of lake based sediment yields, and a remobilisation of this material by fluvial and colluvial processes. In addition, small catchments such as those examined in this thesis may essentially have a stream subsystem decoupled from the hillslope subsystem, with material eroded from the hillslopes going into storage. In larger catchments the 2 subsystems become linked by episodic large scale mass movements such as debris flows and avalanches, the magnitude of which can be significant (Jordan, 1987). In the Lillooet River basin, Jordan and Slaymaker (in press) used a sediment budget approach to highlight the importance of debris flows and landslides in the Quaternary volcanic complex as a source of clastic sediment. 5.4 - Implications for lake sediment-based sediment yields and erosion rates 93 In the last 20 years or so there have been numerous studies that have estimated sediment yields and inferred erosion rates based on reservoir and lake sediments (Section 1.2). Few however, have differentiated between the different sources which make up the lake sediment record, and even fewer have attempted to quantify these sources by direct analysis of lake sediment and process measurement. This thesis has illustrated that some sediment sources should not be considered in sediment yield and ersoion estimates by considering 4 such sources. Contributions from these sources ranges from 55 - 99% of the minerogenic and organic sediment in 3 lakes. Clearly, studies that consider all the sediment within a lake (or reservoir) as derived from erosion within the catchment of study and do not make corrections for such sources, give misleading results which are generally overestimates of true values. Furthermore, while sediment masses corrected for such sources and losses give more realistic estimates of sediment yield, to use such data to infer upland erosion rates may also be misleading. The sediment yield of a stream contributing to lake inputs is a function of hillslope erosional processes and of the balance between erosion and deposition in stream channels. The relationship between upland erosion and sediment yield is complex since not all material detached from hillslopes and channels will reach lake basins. In order for lake sediment based sediment yields to be converted to upland erosion rates it is necessary to know the sediment delivery ratio to the lake. To infer erosion rates without taking storage effects into consideration may lead to under or overestimates of true values depending on whether sediment is going into or out of storage (Owens, 1990). 5.5 - Temporal and spatial representativeness 94 This study has attempted to integrate transfer rates based on different timescales (Table 5.4). As the contribution of aquatic productivity to the lake sediment record was assessed by direct analysis of lake sediment, the time period over which this applies is 2350 years. The outflow losses were similarly based over 2350 years by taking into account variations in trap efficiency due to sediment accumulation. It does however, assume that other variables have remained constant. This is unlikely to be true as Souch (1984) has shown, based on a detailed analysis of the sediments in Gallie Pond, that climatic conditions have varied over this time period with the onset of wetter conditions and cool neoglacial periods (Section 2.2). As a consequence lake water levels, catchment runoff and hence trap efficiency are likely to have varied. The input of material from lake banks and aeolian sources was determined by process measurement over a 1 year period, extrapolated over 2350 years. This is dangerous as any errors will be exaggerated by extrapolation. It is assumed that bank erosion supplied no material to lake sediments over the 1 year of measurement, but may over the longer time period supply significant amounts of material to the lakes. Regional aeolian input was estimated to supply a significant amount of material to the lakes and catchments and the results of this thesis depend on these data. Climate data for the Pemberton and study areas is presented in Appendix 2. It is appropriate to compare the airport data for 1989 with the long term record. Mean monthly temperature for the 1989 field season seems comparable with the record for 1912 - 1990. Monthly total precipitation for the 1989 field season does, however, differ from the long term record, with total precipitation in August 1989 being double the long term mean for this Table 5.4 - Summary results of lake sediment budget, sediment yields and aeolian deposition rates and the timescales over which they apply C A T C H - SEDIMENTATION L A K E SEDIMENT SOURCES M E N T R A T E (% of total sediment) (kg vr' 1 ) O S B A - Total Goat 6.88 10.3 7.3 0 81.3 98.9 Meadows (2350) (2350) (2350) (1) (1) Middle 13.33 21.9 10.8 0 22.6 55.3 Lake (2350) (2350) (2350) (1) (1) Ash 8.06 16.6 12.0 0 41.4 70.0 Lake (2350) (2350) (2350) (1) (1) TRAP SEDIMENT A E O L I A N A E O L I A N SOIL EFFICIENCY YIELD F A L L O U T A C C U M U L A T I O N (%) (kg km-Syr' 1) (kg km'^yr"1) (kg k m ' V 1 ) 70 - 75 4.4 - 4.7 5810 6308 (2350) (2350) (1) (10500) 0 - 0* >29.5* 5180 (2350) (2350) (1) 48 - 53 207 - 244 5350 (2350) (2350) (1) * probably an underestimate oftrue value 96 month. This may reflect the normal variability in precipitation in this area for the 1912 - 1990 time period. On the other hand, it may indicate that August 1989 was unusually wet. This may have implications for aeolian dust inputs, as well as for fluvial and colluvial sediment transfers. The lakes from which these sediment yields are derived are small with very low sedimentation rates and are therefore sensitive to inputs of material from different sources. To extrapolate the relative proportions of the different sediment sources to larger lakes may be misleading if linear relations are assumed between lake area and the magnitude of the sediment source. The relation between lake area and the magnitude of the sediment source is probably non-linear and it is therefore necessary to establish isometric relations between the 2 parameters before extrapolating results. Thus, while the results presented in this thesis are believed to be representative of the catchments examined, care must be taken in applying these results to other catchments. 97 CHAPTER 6 - CONCLUSION Lake sediments have been identified as an alternative to contemporary stream monitoring to establish catchment sediment yields and infer erosion rates. This is primarily due to the longer time period over which lake studies are based. Studies using lake sediments to establish sediment yields have generally assumed that all the sediment contained within a lake is derived from erosion of the catchments under investigation and is spatially distributed over the catchment surface. This study questioned this assumption by constructing a comprehensive lake sediment budget to assess the relative contributions from various sources and by establishing contemporary rates of sediment yield and erosion for 3 small catchments. A significant portion of lake sediment was composed of organic matter and biogenic silica, most of which is derived from within the lake itself. Values are similar between lakes, with values for the lower 2 lakes being higher than Gallie Pond. In all lakes organic matter content is higher than biogenic silica content, the combined values of which range from 18 to 33% of total lake sediment accumulated over the last 2350 years. Lake banks were shown to be undergoing apparent net deposition rather than net erosion over a 1 year period due to changes in vegetation between measurement. It was assumed that at the resolution of measurement, the contribution of material to lake sediment is negligible, but may be more significant over a longer time period. Of the sediment sources examined, regional aeolian deposition was dominant. Process measurement over 1 year suggests that this source could account for between 23 and 81% of the lake sediment. This was 98 corroborated by aeolian deposition rates estimated from catchment soils. When combined, these sources of sediment may supply between 55 and 99% of the sediment contained in the 3 lakes. These values may be underestimates, as long term bank erosion is not included. In addition, the mass of sediment in each lake is probably an overestimate of true values. Lake trap efficiency ranges from low to >70%. These values may also be underestimates. Emphasis should be placed on the order of magnitude of the results and not actual values. This has important implications for studies that use lake sediments to calculate sediment yields and infer erosion rates. In these catchments, most of the sediment contained in lakes is not derived from erosion of the catchments. Studies that do not account for such sources are likely to overestimate true values. Once corrected, estimates of sediment yield range from 4 to 244 kg k m - 2 y r _ l and show an increase in yield with a decrease in elevation. Values above timberline are 1 to 2 orders of magnitude lower than below. When placed in a regional context, sediment yields are orders of magnitude lower than for larger basins. This supports a hypothesis for British Columbia of increasing sediment yield with increasing basin area up to 3 x 10^ k m 2 due to remobilisation of Quaternary sediments. Based on process measurement and the aeolian capping of the catchment soils, aeolian material deposited over part of the catchments could account for all the material removed from the catchments and stored in the lakes. In addition, in certain areas, the catchments appear to be undergoing net deposition and not net erosion. This calls into doubt the use of lake sediment-based sediment yields for inferring erosion rates. 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John Wiley and Sons Ltd., Chichester, 129-152. Walling, D.E. and Webb, B.W., 1983: Patterns of sediment yield. In Gregory, K . J . , (Ed.), Background to Palaeohydrology. John Wiley and Sons Ltd., Chichester, 100-116. Westgate, J.A. , 1977: Identification and significance of late Holocene tephra from Otter Creek, southern British Columbia, and localities in west-central Alberta. Canadian Journal of Earth Science, 14, 2593-2600. Wetzel, R.G., 1983: Limnology. Saunders College Publishing, Philadelphia, 767p. Wolman, M.G. , 1959: Factors influencing the erosion of cohesive river banks. American Journal of Science, 257, 204-216. Wolman, M.G. , 1967: A cycle of sedimentation and erosion in urban river channels. Geografiska Annaeler, 49A, 385-395. Wolman, M.G. and Miller, J.P.,1960: Magnitude and forces in geomorphic processes. Journal of Geology, 68, 54-74. Woodruff, N.P. and Siddoway, F .H . , 1965: A wind erosion equation. Proceedings of the Soil Science Society of America, 29, 602-608. Woodsworth, J . , 1977: Geology of the Pemberton (92-J) map area. Geological Society of Canada Open File Report 482, Ottawa, 1 map sheet. Zeman, L . J . , 1973: Chemistry of Tropospheric Fallout and Streamflow in a Small Mountainous Watershed Near Vancouver, Britsh Columbia. Unpublished PhD Thesis, University of British Columbia, Vancouver, 154p. 112 APPENDIX 1 - CHRONOLOGY An absolute chronology of sediment deposition in Ash Lake, Middle Lake and Gallie Pond was established based on the occurrence of a distinct tephra layer. Tephra is a general term for airborne pyroclastic material ejected during the coarse of a volcanic eruption. Tephra layers can be considered instantaneous on a geologic scale and thus form regional isochronous stratigraphic markers. Three volcanic eruptions that occurred during the Holocene distributed tephra over southern British Columbia - Mount Mazama, Mount Saint Helens Y and Bridge River. Figure A l . l illustrates the distributions of these tephra. It can be seen that only Mount Mazama and Bridge River tephra have been identified in deposits in the vicinity of the study area. There are a variety of parameters that can be easily used in the field that enable different tephras to be differentiated, and these include stratigraphic position, thickness, colour, degree of weathering, grain size and sedimentary structure, in addition to distinct phenocryst assemblages and glass shard habits (Sarna-Wojcicki et. al., 1983). Reasoner and Healy (1986) were able to identify and differentiate Mazama and Bridge River tephra in the field relatively easily at Yoho National Park, British Columbia. Mazama tephra is the older deposit and therefore occurs at a greater depth than Bridge River tephra. The former has a slight reddish colour (5YR 7/2), while the latter is light grey (2.5YR 7/10). Bridge River is also coarser than Mazama tephra due to its closeness to source; Mount Meager complex, British Columbia, and Crater Lake, Oregon, respectively. Bridge River glass shards are typically "chunky" and display lineated, lensoid gas vesicules, whereas Mazama glass shards are commonly thin bubble-wall fragments. 113 In lake cores from Gallie Pond, Souch (1984) identified both Mount Mazama and Bridge River tephra on the basis of the "in field" description described above. In some of the cores taken in this study, tephra layers were positively identified as Bridge River or Mazama tephra by Souch (pers. comm.) and by myself using the techniques described above. The dates of the two tephra layers have been established by the use of radiocarbon dating. Table A 1.1 shows the range of dates cited for Bridge River tephra, which for the purpose of this study, is assumed as 2350 years B.P. 114 Figure Al . l - Distribution of Mount Mazama, Mount St. Helens Y and Bridge River tephra (adapted from Westgate, 1977; Mathewes and Westgate, 1980; Sarna-Wojcicki et. al., 1983) • Study area Table A 1.1 - Radiocarbon dates of Bridge River tephra (adapted from Souch, 1984) Reference Study site Lab. No. Date Nasmith et. al. 1967 Fulton, 1971 Westgate, 1977 Read, 1977 Jesmond, B.C. GSC-529 2440 ± 140 Mica Creek, B.C. GSC-1532 2450 ± 139 Plinth Mt., B.C. S-581 2550 ± 8 0 Meager Mt., B.C. GSC-2587 2350 ± 5 0 116 A P P E N D I X 2 - C L I M A T E D A T A Climate data for the Pemberton area dates from 1912 at Pemberton Meadows. This station was replaced by Pemberton BCFS in 1969, and this by Pemberton Airport in 1984 (Figure A2.1). No data are available for 1968 and for most of 1966 and 1967. Data were obtained from Atmospheric Environment Service, Vancouver, Canada. Mean monthly temperature and total precipitation for each station are presented in Table A2.1 and A2.2, respectively, along with the standard deviation and length of record. Mean temperature for each month is the mean of the mean maximum and mean minimum temperature for that month. Total precipitation is the sum of total rainfall and water-equivalent total snow fall. Mean monthly temperature and total precipitation are illustrated in Figures A2.2 and A2.3, respectively, along with data for the 1989 field season from Pemberton Airport and the study area. This enables the comparison of data from each station and the assessment of the representativeness of the 1989 field season. Table A2.3 presents monthly mean temperature and total precipitation for the study area (Goat Meadows watershed) for 1979 and 1980 (Gallie, 1983), 1981 (Jones, 1982) and 1989. For 1989, total precipitation was assessed by a measuring cylinder of diameter 6.0 cm and height 30.0 cm. This was periodically measured and emptied. Mean temperature was assessed by a maximum/minimum thermometer. This was shielded from the sun and allowed free air circulation. There are no data for July 1989 due to equipment malfunctions. Figure A2.1 - Location of climate stations in the Pemberton + Study site • Pemberton Meadows • Pemberton B C F S • Pemberton Airport 118 Table A2.1 - Mean monthly temperature (°C) for climate stations in the Pemberton area MEADOWS BCFS AIRPORT 1912 - 1967 1969 - 1984 1984 - 1990 Mean SD n Mean SD n Mean SD n J A N -5.4 4.8 52 -4.8 3.0 14 -2.6 1.4 4 F E B -2.1 3.1 51 -0.6 2.3 14 -2.2 2.9 4 M A R 2.9 1.7 50 3.1 1.6 14 4.2 1.4 3 APR 8.4 1.6 49 7.9 1.6 14 8.7 1.1 5 M A Y 13.1 1.8 50 12.3 1.2 15 11.8 0.7 5 J U N 15.9 1.3 50 15.8 1.7 16 16.1 1.1 5 J U L 18.2 1.3 51 18.4 1.2 16 18.1 1.1 6 A U G 17.1 1.1 52 18.1 1.5 14 18.3 1.1 6 SEP 13.1 1.4 52 13.0 1.7 15 13.7 1.3 6 OCT 7.7 1.5 51 7.0 1.5 15 8.4 1.2 5 N O V -1.5 2.3 53 0.8 2.1 14 2.7 1.3 4 D E C -3.1 2.9 53 -4.2 3.8 16 -1.7 1.7 4 A N N 7.3 1.0 7.2 0.7 8.2 1.5 3 Table A2.2 - Mean monthly total precipitation (mm) for climate stations in the Pemberton area MEADOWS BCFS AIRPORT 1912 - 1967 1969 - 1984 1984 - 1990 Mean SD n Mean SD n Mean SD n J A N 135.3 81.1 53 175.9 106.3 13 142.5 48.0 4 F E B 90.2 49.0 51 154.4 50.1 14 61.1 32.0 5 M A R 68.3 36.4 51 94.1 41.7 15 81.4 37.1 4 APR 40.1 23.5 49 60.4 39.0 15 73.3 21.8 3 M A Y 33.5 23.5 47 43.5 18.4 15 58.4 19.0 5 J U N 32.8 21.2 49 44.8 25.6 16 48.7 34.6 5 J U L 24.7 17.3 49 34.6 26.3 16 39.2 27.1 5 A U G 29.7 22.7 52 39.3 25.1 15 38.5 34.4 5 SEP 58.0 40.6 51 77.6 39.7 15 36.7 22.4 6 OCT 122.9 63.0 53 127.4 77.7 15 95.9 89.4 4 N O V 141.5 74.8 52 162.3 79.1 13 133.0 19.9 4 D E C 160.4 72.3 53 183.3 83.7 14 112.9 15.1 2 A N N 937.4 172.1 1197.6 183.0 887.3 5.51 2 119 Figure A2.2 - Mean monthly temperature for the Pemberton area and for the 1989 field season 120 Figure A2.3 - Mean monthly total precipitation for the Pemberton area and the 1989 field season 200-1 MONTH 121 Table A2.3 - Monthly mean temperature and total precipitation for the Goat Meadows watershed M O N T H M E A N T E M P E R A T U R E T O T A L PRECIPITATION (°C) (mm) 1979 1980 1981 1989 1979 1980 1981 1989 July 11.0 9.5 6.0 — 46.8 8.1 — 32.2 August 12.0 8.0 12.0 10.8 35.1 23.4 >23.0* 75.0 September 7.5 — 2.0 12.4 180.5 133.7 >21.0* 17.0 1979 and 1980 data are based on Gallie (1983) 1981 data are based on Jones (1982) * - underestimates due to equipment malfunction 122 APPENDIX 3 - L A K E SEDIMENT D A T A This appendix presents data on the composition of lake sediment. The sediment analysed is the sediment above the Bridge River tephra deposit. Bulk density was determined by oven drying composite subsamples between the appropriate limits at 105 °C for 24 hours. This sample was then divided in 2: 1 sample placed in a muffle furnace at 550 °C for 3 hours to determine loss-on-ignition, a surrogate for organic matter content; the other treated with hydrogen peroxide, hydrochloric acid and sodium hydroxide (Section 3.2.1) to determine biogenic silica content. Not all cores removed from the lakes contained Bridge River tephra, and only those that did were analysed. 123 Table A3.1 - Sediment depth to Bridge River tephra, bulk density, organic matter and biogenic silica content of sediment in the 3 lakes L A K E Gallie Middle Ash SED. B U L K ORGANIC BIOGENIC D E P T H DENSITY MAT;] PER SILK 6A CORE cm k g m - 3 kgm"^ ' % kgm" 6 % B l 8.5 787 81 10.3 75 9.6 B2 5.5 717 48 6.7 23 3.2 D2 7.5 693 38 10.3 44 6.3 E l 8.5 643 70 10.9 41 6.4 F l 5.5 846 96 11.3 81 9.6 G l 9.5 659 78 11.8 99 15.0 H I 4.0 663 59 8.9 53 8.0 11 5.0 756 72 9.5 42 5.5 J I 8.0 726 68 9.4 41 5.6 J2 8.5 678 77 11.3 71 10.5 KI 11.5 708 87 12.3 6 0.9 K2 10.5 622 81 13.1 44 7.0 K3 10.5 629 61 9.7 36 5.8 L l 14.5 839 90 10.7 77 9.2 L2 12.5 866 92 10.6 33 3.8 L3 16.0 678 62 9.2 40 5.9 M l 16.0 813 77 9.5 56 7.2 M2 12.0 791 74 9.4 97 12.3 B3 12.5 584 151 25.9 84 14.4 CI 8.5 888 97 10.9 89 10.0 C3 23.0 713 128 18.0 57 8.3 C4 15.5 483 268 40.4 56 11.6 D2 18.0 702 99 14.2 67 9.6 A l 10.5 469 79 16.9 71 15.1 B2 5.5 629 79 12.5 84 13.4 C2 14.0 532 88 16.6 53 9.9 D2 11.0 468 91 19.4 G2 15.5 549 83 15.2 20 3.7 H I 13.0 524 100 19.0 95 18.1 124 A P P E N D I X 4 - B A N K E R O S I O N D A T A This appendix presents data on lake bank erosion obtained using different techniques. Most of the data presented are differences in pin or stake lengths for different time periods (snow-free, snow-covered and 1 full year) and these are inferred as changes in bank profile (- indicates erosion, + indicates deposition). The units of measurement are mm. Table A4.1 contains data on erosion pins, while Table A4.2 and A4.3 present erosion stake data using different methods. Table A4.4 contains data from vertical erosion sites. In addition at chosen sites around each lake samples of bank material of a known volume were taken. These were subsequently oven dried for 24 hours at 105 °C. The results are presented in Table A4.5. 125 Table A4.1 - Difference in erosion pin lengths (mm) for a) Gallie Pond b) iddle Lake and c) Ash Lake a) Gallie Pond SITE PIN No. 30/07/89 - 21/09/89 - 30/07/89 -21/09/89 02/08/90 02/08/90 1 1 3 -1 2 2 1 1 2 3 2 0 0 0 3 1 8 -7 1 2 6 2 8 4 1 0 -9 -9 2 6 -2 4 5 1 -1 0 -1 2 1 -3 -2 6 1 -4 -3 -7 7 1 3 -5 -2 8 2 3 -2 1 1 1 0 1 9 1 3 0 3 14 1 1 0 1 15 2 6 -2 4 1 1 0 1 2 1 0 1 3 1 -5 -4 indicates net erosion T a b l e A4.1 (contd.) - b) Middle Lake 31/07/89 - 21/09/89 - 31/07/89 -SITE PIN No. 21/09/89 02/08/90 02/08/90 1 1 8 4 12 2 3 4 7 2 1 1 -1 0 3 1 0 2 2 2 1 1 2 4 1 -2 0 -2 5 1 -1 0 -1 2 1 -4 -3 6 1 -2 0 -2 2 0 -2 -2 7 1 5 0 5 2 -2 -8 -10 8 1 0 1 1 2 7 0 7 9 1 10 -12 -2 2 9 -3 6 10 1 1 0 1 2 1 3 4 12 1 1 1 2 2 6 0 6 13 1 -3 1 -2 2 1 1 2 14 1 -6 8 2 15 1 -2 3 1 2 -1 0 -1 - indicates net erosion results are in mm Table A4 .1 (contd.) - c) Ash Lake 01/08/89 - 22/09/89 - 01/08/89 -SITE PIN No. 22/09/89 01/08/90 01/08/90 1 1 -1 2 1 2 -2 1 -1 2 1 2 0 2 2 1 -3 -2 3 1 3 2 5 4 1 2 -1 1 2 - - 3 11 1 -3 7 4 2 -1 3 2 12 1 5 -4 1 2 6 -6 0 13 1 4 3 7 2 5 -1 4 14 1 3 -2 1 2 6 -29 -23 15 1 4 -2 2 2 -5 4 -1 16 1 0 -1 -1 2 11 -1 10 17 1 0 -1 -1 2 0 0 0 18 1 1 0 1 2 -2 3^ 1 19 1 -1 0 -1 2 22 -31 -9 - indicates net erosion results are in mm 1 2 8 Table A4.2 - Change in lake shoreline (mm) measured using erosion stakes over 1 year for the 3 lakes A S H -2 3 4 23 5 -8 * SITE S T A K E No. G A L L I E MIDDLE 1 1 85 21 2 76 24 2 1 18 * 2 * 0 3 1 2 10 2 * 10 4 1 * -3 2 * 10 5 1 * 2 2 * 7 6 1 8 -4 2 * 2 7 1 8 * 2 0 * 8 1 * 0 2 * 6 9 1 * * 2 * * 10 1 -2 2 0 11 1 2 12 1 2 2 -2 13 1 15 2 4 14 1 21 12 2 -2 * 15 1 2 4 2 -96 15 16 1 2 17 1 2 18 1 2 19 1 2 14 30 -8 -3 * 0 -4 34 * * 2 5 * -5 8 12 2 0 - indicates net erosion * indicates disturbed stake blank indicates site with no stakes 129 Table A4.3 - Change in lake shoreline (mm) measured using erosion stakes (calculated using trigonometry) for a) Gallie Pond, b) Middle Lake and c) Ash Lake a) Gallie Pond 30/07/89 - 21/09/89 - 30/07/89 -SITE S T A K E No. 21/09/89 02/08/90 02/08/90 1 1 * 2 9 23 32 2 1 9 16 25 2 24 * * 3 1 -33 -47 -80 2 -148 * 4 1 -34 * * 2 -8 * * 5 1 -41 * 2 35 * * 6 1 -56 4 -52 2 -53 7 1 -21 -43 -64 2 -6 -65 -71 8 1 28 * * 2 -10 * * 9 1 -1 * * 2 8 * * 14 1 -16 17 1 2 2 10 12 15 1 -1 7 6 2 3 48 -51 - indicates net erosion * indicates disturbed site Table A4.3 (contd.) - b) Middle Lake 31/07/89 - 21/09/89 - 31/07/89 -SITE S T A K E No. 21/09/89 02/08/90 02/08/90 1 1 200 -180 20 2 4 19 23 2 1 -10 * * 2 -10 2 -8 3 1 23 -20 3 2 2 -2 0 4 1 109 -92 17 2 5 3 8 5 1 5 -6 -1 2 -9 4 -5 6 1 29 -28 1 2 -14 13 -1 7 1 -21 * * 2 * * * 8 1 18 -37 -19 2 11 -13 -2 9 1 12 * * 2 7 * 10 1 28 -45 -17 2 58 -85 -27 12 1 -13 -3 -16 2 3 -12 -9 13 1 -13 21 8 2 -15 26 11 14 1 -7 11 4 2 * * • * 15 1 -26 30 4 2 -6 18 12 results are in mm - indicates net erosion * indicates disturbed stake Table A4.3 (contd.) - c) Ash Lake 01/08/89 - 22/09/89 - 01/08/89 -SITE S T A K E No. 22/09/89 01/08/90 01/08/90 1 1 -7 11 4 2 9 -12 -3 2 1 -2 0 -2 2 0 -6 -6 3 1 14 -24 -10 2 4 19 23 4 1 31 -47 -16 2 25 * * 11 1 22 -24 -2 2 11 -10 1 12 1 16 -48 -32 2 12 -54 -42 13 1 33 * * 2 13 -54 -41 14 1 116 -44 72 2 19 -20 -1 15 1 17 * * 2 7 * * 16 1 32 -38 -6 2 -85 88 3 17 1 125 * * 2 80 -110 -30 18 1 15 -17 -2 2 19 -20 -1 19 1 37 -30 7 2 13 -3 10 results are in mm - indicates net erosion * indicates disturbed stake 132 Table A4.4 - Results of "vertical erosion stakes" (mm) for a) Gallie Pond, b) Middle Lake and c) Ash Lake a) Gallie Pond PIN OR 30/07/89 - 21/09/89 - 30/07/89 -SITE S T A K E No. 21/09/89 02/08/90 02/08/90 10 SI 1 S2 26 * * S3 7 * * P l 3 * P2 2 -3 -1 P3 4 -3 1 11 SI 2 * * S2 -1 * * S3 16 -31 -15 P l 4 * * 12 SI 12 -21 -9 S2 2 -2 0 S3 2 -8 -6 P l 0 -1 -1 P2 3 22 25 P3 0 2 2 13 SI 6 * * S2 1 * * S3 5 * P l -23 32 9 P2 -5 3 -2 P3 -7 4 - -3 b) Middle Lake PIN OR 31/07/89 - 21/09/89 - 31/07/89 -SITE S T A K E No. 21/09/89 02/08/90 02/08/90 11 SI -13 -51 -64 52 -4 -2 -6 53 5 -5 0 P l -2 3 1 P2 1 1 2 P3 0 3 3 P - pin S - stake - indicates net erosion * indicates disturbed stake or pin Table A4.4 (contd.) - c) Ash Lake SITE PIN OR 01/08/89 - 22/09/89 - 01/08/89 -S T A K E No. 22/09/89 01/08/90 01/08/90 5 SI 0 69 69 S2 4 74 78 S3 4 28 32 P l 0 5 5 P2 -2 3 1 6 SI 9 23 32 S2 10 37 47 S3 4 23 27 P l 0 4 4 P2 2 0 2 7 SI -7 10 3 S2 -10 35 25 P l -4 6 2 P2 4 -2 2 8 SI -33 18 -15 S2 -9 69 60 S3 36 25 61 P l 2 1 3 P2 1 -11 -10 9 SI 9 51 60 S2 2 63 65 S3 -2 17 15 P l 12 -2 10 P2 1 -1 0 10 SI -3 47 44 S2 -6 25 19 S3 * * * P l 0 0 0 P2 0 2 2 P - pin S- stake - indicates net erosion * indicates disturbed pin or stake results are in mm Table A4.5 - Lake bank bulk density (kg m' "3) for the 3 lakes L A K E SITE* B U L K DENSITY Gallie 4 525.8 9 898.4 11 1634.5 Middle 9 419.9 11 677.4 Ash 1 200.3 6 83.7 17 129.8 * site refers to bank erosion site 135 A P P E N D I X 5 - A E O L I A N M A T E R I A L This appendix presents data on the input of aeolian material to the 3 lakes and catchments. The data presented in Table A5.1 and A5.2 are the total weights and the proportion of mineral and organic material of the filtered samples collected in snow-cores and bulk aeolian collectors for each site. The bulk aeolian collector data is further divided to material > 1 mm and < 1 mm in diameter as separated by the wire mesh inside each trap. Material > 1 mm was mainly composed of trapped insects and at some sites material > 1 mm in diameter, probably derived from within the watersheds. The surface area of the funnel used in each bulk trap and the barrel of the snow corer are 0.0266 m 2 and 0.0044 m 2 , respectively. As 4 snow cores were taken at each site, the surface area over which snow was sampled at each site is 0.0176 m 2 . The bulk collectors were left in the field for ca. 60 days. Although they were removed before the first snow fall, it is assumed that the total aeolian material collected by the snow cores and bulk collectors represent 1 year of deposition. 136 Table A5.1 - Material contained within the snow pack at each site CATCH-M E N T SITE T O T A L (B) MINERAL (sr) (%) ORGANIC (sr) (%) Goat 1 0.0220 0.0050 22.7 0.0170 77.3 Meadows 2 0.1471 0.0811 55.1 0.0660 44.9 3 0.0364 0.0169 46.4 0.0195 53.6 4 0.0227 0.0025 11.0 0.0202 89.0 5 0.0085 0.0039 45.9 0.0046 54.1 6 0.0068 0.0029 42.6 0.0039 57.4 7 0.4153 0.3817 91.9 0.0336 8.1 8 0.1154 0.0819 71.0 0.0335 29.0 9 0.0577 0.0098 17.0 0.0479 83.0 Middle 1 0.0894 — Lake 2 0.1195 0.0211 17.7 0.0984 82.3 3 0.0277 0.0044 15.9 0.0233 84.1 4 0.0352 0.0049 13.9 0.0303 86.1 5 0.0252 0.0174 69.0 0.0078 31.0 6 0.1114 0.0568 51.0 0.0546 49.0 7 0.0177 0.0117 66.1 0.0060 33.9 8 0.0963 0.0274 28.5 0.0689 71.5 9 0.5548 0.5250 94.6 0.0298 5.4 Ash 1 0.3527 0.0131 3.7 0.3396 96.3 Lake 2 0.3302 0.0743 22.5 0.2559 77.5 3 0.0793 0.0493 62.2 0.0300 37.8 4 0.1291 0.0106 8.2 0.1185 91.8 5 0.1707 — 6 0.0430 0.0131 30.5 0.0299 69.5 7 0.0678 0.0474 70.0 0.0204 30.0 8 0.0403 0.0090 22.3 0.0313 77.7 9 0.2551 0.0124 4.9 0.2427 95.1 Table A5.2 - Aeolian material collected in bulk collectors M A T E R I A L < 1 M M DIAMETER C A T C H - T O T A L M I N E R A L ORGANIC M E N T SITE (e) te) (%) (s) (%): Goat 1 0.0563 0.0165 29.3 0.0398 70.7 Meadows 2 0.0424 0.0141 33.3 0.0283 66.7 3 0.0380 0.0125 32.9 0.0255 67.1 4 • 0.0371 0.0120 32.3 0.0251 67.7 5 0.0832 0.0546 65.6 0.0286 34.4 6 0.0249 0.0084 33.8 0.0165 66.2 7 0.4187 0.3649 87.1 0.0538 12.9 8 0.9576 0.8935 93.3 0.0641 6.7 9 0.0408 0.0229 56.1 0.0179 45.9 10 0.0602 0.0243 40.4 0.0359 59.4 11 0.0407 0.0085 20.9 0.0322 79.1 12 0.0701 0.0096 13.7 0.0605 86.3 13 0.0609 0.0258 42.4 0.0351 57.6 Middle 1 0.0211 0.0085 40.3 0.0126 59.7 Lake 2 0.0210 0.0071 34.0 0.0139 66.0 3 0.0220 0.0104 47.3 0.0116 52.7 4 0.0186 0.0050 26.9 0.0136 73.1 5 0.0200 0.0100 50.0 0.0100 50.0 6 0.0484 0.0158 32.6 0.0326 67.4 7 0.0208 0.0138 66.3 0.0070 33.7 8 0.0290 0.0084 29.0 0.0206 71.0 9 0.0822 0.0584 71.0 0.0238 29.0 10 0.0143 0.0048 33.6 0.0095 66.4 Ash 1 0.0227 0.0047 20.7 0.0180 79.3 Lake 2 0.0065 0.0065 100.0 0.0000 100.0 3 0.0257 0.0096 37.4 0.0161 62.6 4 0.0408 0.0091 22.3 0.0317 77.7 5 0.0264 0.0080 30.3 0.0184 69.7 6 0.0148 0.0039 26.4 0.0109 73.6 7 0.0312 0.0144 46.2 0.0168 53.8 8 0.0338 0.0132 39.1 0.0207 60.9 9 0.0247 0.0060 24.3 0.0188 75.7 10 0.0318 0.0080 25.2 0.0238 74.8 T A B L E A5.2 (contd.) M A T E R I A L > 1 mm DIAMETER C A T C H - T O T A L M I N E R A L ORGANIC M E N T SITE (s) (e) (%) (e) ( %) Goat 1 0.0211 0.0020 9.5 0.0191 90.5 Meadows 2 0.0275 0.0021 7.6 0.0244 92.4 3 0.0452 0.0.024 5.3 0.0428 94.7 4 0.0172 0.0013 7.6 0.0159 92.4 5 0.0028 —— — 6 0.0347 0.0025 7.2 0.0322 92.8 7 0.0776 0.0399 51.4 0.0377 48.6 8 0.0386 0.0348 90.2 0.0038 9.8 9 0.0059 0.0012 20.3 0.0047 79.7 10 0.1590 0.0148 9.3 0.1442 90.7 11 0.1309 0.0038 2.9 0.1271 97.1 12 0.8584 0.0187 2.2 0.8397 97.8 13 0.0890 0.0127 14.3 0.0763 85.7 Middle 1 0.0474 0.0023 4.8 0.0451 95.2 Lake 2 0.0368 0.0022 6.0 0.0346 94.0 3 0.0104 0.0022 21.2 0.0082 78.8 4 0.0184 0.0020 10.9 0.0164 89.1 5 0.0011 0.0011 100.0 0.0000 0.0 6 0.0292 0.0014 4.8 0.0278 95.2 7 0.0004 8 0.0246 0.0022 8.9 0.0224 91.1 9 0.0079 0.0021 26.6 0.0058 73.4 10 0.0058 0.0012 20.7 0.0046 79.3 Ash 1 0.0650 0.0033 5.1 0.0617 94.9 Lake 2 0.0168 0.0010 5.9 0.0158 94.1 3 0.0038 0.0038 100.0 0.0000 0.0 4 0.1239 0.0048 3.9 0.1191 96.1 5 0.0144 0.0007 4.9 0.0137 95.1 6 0.0112 0.0013 11.6 0.0099 88.4 7 0.0069 0.0005 7.2 0.0064 92.8 8 0.0096 0.0019 19.8 0.0077 80.2 9 0.0429 0.0018 4.2 0.0411 95.8 10 0.0108 0.0028 25.9 0.0080 74.1 

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