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Clastic sediment sources and suspended sediment yield in a Coast Mountain watershed, British Columbia Hart, Jackson Sanford 1979

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CLASTIC SEDIMENT SOURCES AND SUSPENDED SEDIMENT YIELD IN A COAST MOUNTAIN WATERSHED, BRITISH COLUMBIA  JACKSON SANFORD HART B.Sc,  Trent University, 197b  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ARTS in THE FACULTY.OF GRADUATE- STUDIES (The Department of Geography)  We accept this thesis as conforming to the required  standard  THE UNIVERSITY OF BRITISH COLUMBIA September, 1979 ©  Jackson Sanford Hart, 1979  In presenting  t h i s thesis in p a r t i a l f u l f i l l m e n t of the requirements for  an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference  and  study.  I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may by his representatives.  be granted by the Head of my Department or  It i s understood that copying or publication  of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission.  September,  1979.  Department of Geography The University of B r i t i s h Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5  Abstract This d i s s e r t a t i o n investigates the c l a s t i c sediment sources and suspended sediment y i e l d s of clear-cut and undisturbed areas of a g l a c i a l l y 2 -modified, 33 km Columbia.  watershed i n the southern Coast Mountains of B r i t i s h  S u r f i c i a l materials were mapped and the character of sediment  movement interpreted by morphologic evidence.  Basin sediment y i e l d s  through snowmelt, summer low flow and f a l l storm periods were related to . the magnitude of p r e c i p i t a t i o n inputs, storage changes of snow and the a v a i l a b i l i t y of c l a s t i c sediment to the. channel system.  In the undisturbed  area sediment activated along steep t r i b u t a r y streams of the t i l l - m a n t l e d v a l l e y walls below t r e e l i n e was considered most important to basin scale yield.  In the alpine zone mass wastage a c t i v i t y i s widespread yet effects  mainly a r e d i s t r i b u t i o n of materials on slopes; sediment supply to the f l u v i a l ' system i s l i m i t e d by the extensive presence of coarse materials and the lower drainage density.  Snowmelt period sediment export exceeded that  during summer low flow and October storm periods; however, approximately 60 percent of the t o t a l sediment y i e l d observed took place during a November rain-on-snow event of an estimated 10 year recurrence i n t e r v a l . During the October storm period, when sediment y i e l d s from the forested and clear-cut slopes could be i s o l a t e d , sediment removal from the clear-cut slope was greater by approximately eight times and t h i s accelerated erosion was attributed almost e n t i r e l y to road e f f e c t s .  iii Contents  ^ Page  Abstract  i i  Contents  i i i  L i s t of Tables  v  L i s t of Figures  vi  L i s t of Photographs  vii  Acknowledgements  viii  1. Introduction 1.1. L i l l o o e t River watershed studies 1.2. Wasp Creek watershed study.... 1.3. Related studies 2. Description of the study area 2.1. Location and area 2 . 2 . Physiographic s e t t i n g 2.2.1. Bedrock geology 2.2.2. Glaciation and g l a c i e r i z a t i o n . 2.2.3. Watershed form... 2.3. Climate 2. U. Vegetation... 2 . 5 . Land use  1 1 3 ^+ 7 T 7 7 7 9 9 10 11  3. Methods 3.1. Introduction.... 3.2. Precipitation 3.3. Stage and discharge  12 12 12 12  3.3.1.  '-  12  Measurement  Discharge record length and synthesis 3.3.3. Hydrograph separation 3.3.U. Flood frequency analysis 3.U. Sediment source mapping 3.U.I. Undisturbed area. 3.U.2. Clear-cut area 3.5. Sediment y i e l d  13 13 13 13 13  3.5.1.  Network...  1^  3.5.2.  Sample c o l l e c t i o n Sample analysis. Sediment y i e l d computation Interpolation method Sediment r a t i n g curve method Mean sediment concentration-discharge, method Upper Wasp Creek y i e l d estimate for the November storm.. Y i e l d from the clear-cut area  3.3.2.  3-5-3. 3.5-^-  a) b) c) d) e)  1^  1^  :  k. Hydrometeorologic observations k.l. Introduction h.2. Precipitation h.2.1. Gauge relations k.2.2. Seasonal variation? h.3Stage and discharge 1+.3.1. Stage-discharge relations k.3.2. Seasonal discharge v a r i a t i o n k.3.3. Flood frequency analysis U.3.J+. Hydrologic response k.k. Conclusion  1^ 15 15  15 16 19 19 19 •  21 21 21 21 21 2U 2*+  28 30 30 33  iv 5. Sediment sources and a v a i l a b i l i t y 5.1. Introduction.. 5.2. Sediment sources 5.3. Sediment transfers - undisturbed area  35 35 35 39  5.3.1.  G l a c i a l processes  39  5.3.2.  Slope processes Large scale mass movements Rockf a l l s Debris avalanches Debris flows Snow avalanches Slow mass movements  39 39 39 *+0 b2 b2  a) b) c) d) e) f) 5.3.3.  39  ^3  F l u v i a l processes  5.*+. Sediment transfers - clear-cut area 5.^+.l. Slope processes a) Debris avalanches b) Mass movements along roads 5-4.2. F l u v i a l processes 5.5. Sediment sinks - undisturbed area 5.6. Sediment sinks - clear-cut area 5. T- Sediment a v a i l a b i l i t y - undisturbed area 5.8. Sediment a v a i l a b i l i t y - clear-cut area  bb bb U5 U5 U6 hi bl ^7 bQ  6. Sediment y i e l d 6.1. Introduction 6.2. Sediment y i e l d r e s u l t s 6.2.1. Snowmelt period - May to July 6.2.2. Summer period - August and September 6.2.3. F a l l storm period - October and early November 6.3. Sediment discharge regimen 6.3.1. Upper Wasp Creek 6.3.2. Lower Wasp Creek 6.3.3. Wasp Creek t r i b u t a r y streams G.b. Conclusion  51 51 51  57  7- Discussion and conclusions.... 7.1. V e r t i c a l zonation of geomorphic processes 7.2. Seasonal analysis 7.3. E f f e c t s of t e r r a i n disturbance l.b. Conclusions •: 7.5. Further work  58 58 58 59 6l 62  Bibliography  63  Photographs  66  Appendices A. Wasp Creek seasonal hydrographs. B. Sample analysis procedure C. Suspended sediment data  •  51 51 53  53 5b 55 57  71 72 73 7^  L i s t of Tables Page 25  Table 1.  Wasp Creek watershed hydrometeorologic  Table 2.  Sediment sources of Wasp Creek basin  36  Table 3.  Surface conditions of clear-cut area  36  data summary  Table B . l .  Sample analysis procedure  73  Table C . l .  Suspended sediment y i e l d data summary  7k  Table C.2.' Sediment concentration data for Wasp Creek t r i b u t a r y streams.  75  vi L i s t of Figures  Page  Frontispiece: Oblique a e r i a l view to southwest over the Wasp Creek watershed, August 2 , 1 9 ^ 7 - B.C. Government photograph BC 399= 67  ix  Figure 1.  L i l l o o e t River drainage system above L i l l o o e t Lake..  2  Figure 2.  Wasp Creek watershed bedrock geology  8  Figure 3.  P r e c i p i t a t i o n and temperature data for Pemberton Meadows, B.C.,  19U1-1966  1  0  Figure k. Discharge-sediment concentration relationships for the three largest storm runoff events at lower Wasp Creek  17  Figure 5- R a i n f a l l , discharge and sediment concentration graphs for the October 17 storm, lower Wasp Creek  18  Figure 6. catches  Regression r e l a t i o n of Ryan River and Wasp Creek r a i n gauge  ^2  Figure 7-  Tenquille Lake snow course data, 1,700 m  23  Figure 8.  Pemberton Meadows p r e c i p i t a t i o n data, 230 m  23  Figure 9-  Lower Wasp Creek discharge r a t i n g curve  26  Figure 1 0 . Upper Wasp Creek discharge rating curve. Figure 1 1 . a) Generalized mean of mean daily discharge for L i l l o o e t River ( 1 9 2 8 - 1 9 6 8 ) ; b) mean daily discharge f o r L i l l o o e t River i n 1975--.  27 •  2  9  Figure 12.. Frequency of maximum daily discharge i n t e n s i t i e s for r i v e r s of the L i l l o o e t River drainage system  31  Figure 13. Wasp Creek hydrologic response to storm events  32  Figure lk. Hypsometric analysis of Wasp Creek basin Figure 15. Idealized cross-section of Wasp Creek v a l l e y wall and s t r a t i graphic column . .  33  Figure 1 6 . Wasp Creek watershed s u r f i c i a l geology  37  Figure 1 7 - Wasp Creek clear-cut  38  Figure 1 8 . Colour i n f r a r e d a e r i a l photograph of study.area with overlay showing drainage network  ^1  Figure 1 9 - Longitudinal p r o f i l e of Wasp Creek  35  ^  Figure 2 0 . V e r t i c a l zonation of sediment sources i n upper Wasp Creek watershed  ^9  Figure 2 1 . Wasp Creek watershed c l a s t i c sediment transfer model  50  Figure 2 2 . Upper Wasp Creek sediment rating curves  52  Figure 2 3 . Upper Wasp Creek flow and sediment discharge duration curves for June 2 1 to July 3 1 , 1975  5^  Figure 2h. events  56  Figure A . l .  Lower Wasp Creek sediment y i e l d response to storm runoff Wasp Creek seasonal hydrographs  72  vii L i s t of Photographs Photograph 1.  View up Basin C showing extensive colluvium  Photograph 2. Debris avalanche scar along drainage l i n e on steep, t i l l - m a n t l e d v a l l e y wall of Wasp Creek Photograph 3.  Page 67 67  Small debris flow tracks on c o l l u v i a l slope near t r e e l i n e . 68  Photograph h. View up Wasp Creek showing coarse colluvium on right and a l l u v i a l cone on l e f t  68  Photograph 5.  Debris flow deposit along Ryan River  69  Photograph 6.  View up Wasp Creek above the upper Wasp gauging station. . 69  Photograph 7-  Road cut-slope i n clear-cut area exposing g l a c i a l t i l l . .. 70  Photograph 8.  Gullying of road surface during snowmelt period  70  viii Acknowledgements I would l i k e t o acknowledge w i t h g r a t i t u d e t h e c o n s i d e r a b l e a s s i s t a n c e p r o v i d e d by my ject.  s u p e r v i s o r Dr. Olav Slaymaker w i t h a l l a s p e c t s o f t h i s  pro-  The a d v i c e o f Dr. Robert W i l l i n g t o n a t the d e s i g n stage p r o v e d most  valuable.  Dr. M i c h a e l Church's comments a t t h e d e s i g n stage and h i s  cisms o f t h e f i n a l d r a f t were a p p r e c i a t e d by a d o p t i o n .  The  c o u l d not have been conducted w i t h o u t t h e e f f o r t s o f my  field  criti-  f i e l d work assistants,  Dr. Ian L a i d l a w and Mr. E r i c L e o n a r d , under sometimes t r y i n g c o n d i t i o n s . F i n a n c i a l a s s i s t a n c e was  p r o v i d e d by a g r a n t from the Department o f  Employment and Immigration Slaymaker.  The  and a N a t i o n a l R e s e a r c h C o u n c i l g r a n t t o Dr.  i n f r a - r e d imaging o f the study a r e a was  s u b s i d i z e d by the Canada Centre f o r Remote S e n s i n g . Denis B f i e r e  conducted  and  I am g r a t e f u l t o  Mr.  of the F a c u l t y of F o r e s t r y f o r h i s a s s i s t a n c e w i t h the pre-  p a r a t i o n f o r t h i s photo r u n .  The B r i t i s h Columbia F o r e s t S e r v i c e k i n d l y  s u p p l i e d h e l i c o p t e r time and Mr. J a c k M c C l e l l a n o f t h i s agency c o n t r i b u t e d l o g i s t i c a l support w h i c h g r e a t l y a i d e d t h e f i e l d e f f o r t s .  The  co-operation  o f L & K L o g g i n g L t d . and Mr. Lawrence V a l l e a u o f V a l l e a u Logging L t d . e s s e n t i a l t o work i n the Ryan R i v e r b a s i n and i s much a p p r e c i a t e d . I extend my  g r a t i t u d e t o Ms.  Finally,  Christina Mayall for f i e l d assistance, for  p r o o f r e a d i n g t h e f i n a l d r a f t and f o r her f o r e b e a r a n c e p r o t r a c t e d process.  was  d u r i n g a somewhat  Frontispiece: Oblique a e r i a l view to southwest over the Wasp Creek watershed, August 2 , 19U7. B.C. Government photograph BC 3 9 9 : 6 7 .  1  1.  Introduction The steep mountain slopes of the Canadian C o r d i l l e r a are overlain by  a mantle of g l a c i a l t i l l , a legacy of.Pleistocene g l a c i a t i o n .  Relative to  work i n non-glaciated areas t h i s t e r r a i n has received scant attention with respect to problems of erosion and sedimentation  and so research to better  understand the behaviour of the natural system and consideration of related management problems are c l e a r l y required. L i l l o o e t River watershed and the present  Research i s underway i n the study was  developed to provide com-  plementary information for the p a r t i a l l y clear-cut slopes of a t r i b u t a r y basin. 1.1.  L i l l o o e t River watershed studies 2  The L i l l o o e t River system above L i l l o o e t Lake drains a 3 , 8 0 0 km area of the southern Coast Mountains of B r i t i s h Columbia (Figure l ) . The main 2  valley bottom, occupying an area of 110 km slopes r i s i n g to alpine areas above the basin r e l i e f ranges from 19k Bridge Peak.  , i s flanked by steep, forested  1,500  m to  2,000  m at L i l l o o e t Lake to the  m elevation.  m summit of  2,936  Areas of perennial snow and i c e , predominantly i n the western  region of the basin, occupy about 7 percent of the t o t a l area 1973).  The  (Gilbert,  The v a l l e y bottom i s occupied by g l a c i o - f l u v i a l and f l u v i a l deposits  and the v a l l e y walls are mantled by Pleistocene and Holocene g l a c i a l colluvium and minor alluvium.  till,  The Coast Mountain plutonic complex, com-  prised p r i n c i p a l l y of quartz d i o r i t e and granodiorite, .underiiesrthe^lar-rgest f r a c t i o n of the basin.  Other i n t r u s i v e rocks are present of Jurassic  and older (?) to T e r t i a r y age, Cretaceous metasedimentary and metavolcanic rocks and T e r t i a r y and Quaternary volcanics (Woodsworth,  1977).  A variety of hydrologic and geomorphic studies has - been c a r r i e d out within the L i l l o o e t River watershed.  In 1969 studies were i n i t i a t e d i n .  alpine and subalpine areas of the M i l l e r Creek basin which addressed water, solute and sediment y i e l d s from g l a c i e r i z e d and non-glacierized areas and Slaymaker,  1975;  Zeman and Slaymaker,  1975;  Slaymaker,  1977)-  (Woo  Hydro-  logic and geochemical research i s on-going i n the basin (Braun, i n prep.; Slaymaker and G a l l i e , 1 9 7 9 ; T e t i , i n prep.). watercourses include Ponton's (1972 a&b)  Studies along the larger  investigation of the sectional  and downstream hydraulic geometry of the Green and Birkenhead Rivers Teversham's  (1973)  (see also Teversham and Slaymaker,  1976)  and  investigationc  of vegetation response to f l u v i a l a c t i v i t y on the L i l l o o e t River v a l l e y bottom..  Between 1970 and 1972 Gilbert (1973) examined the sedimentary en-  vironment of the L i l l o o e t Lake d e l t a .  This work provided measures of t o t a l  2  3 c l a s t i c sediment y i e l d based on long-term sedimentation rates i n L i l l o o e t Lake. Gilbert calculated mean rates of delta front advance between 1858 and 1969 and i d e n t i f i e d a t r i p l i n g of rate i n the post-19^8 period.  The ac^-  celerated sedimentation coincides with r i v e r t r a i n i n g works on the v a l l e y bottom between 19^+6 and 1 9 5 1 , a 2.5 m lowering of L i l l o o e t Lake i n 1 9 5 2 , and the increased land use of logging of the forested slopes, agriculture on the v a l l e y bottom and associated road construction.  The r i v e r t r a i n i n g  and lake lowering operations, which resulted i n an increased channel gradient along a  50  km reach above the lake from  0.0008  to  0.0010,  are of singular  importance, yet only p a r t i a l l y account for the y i e l d increases (Slaymaker and G i l b e r t ,  Over the period  1972). 2  from less than h km 2  k km  I9U6-I969  the logged area was increased  2 (0.1$)  to  8l  km  (2.1%)  and a g r i c u l t u r a l land use from  2  to  U8.5  km.  (1.3$).  The magnitude of these changes suggests that they  may also be s i g n i f i c a n t at the watershed  scale.  Gilbert's work described a r e l a t i v e l y long-term sedimentary record which integrated the contributions from the d i f f e r i n g sediment source areas. The need for investigation of these contributing areas was thus indicated. The complex physiographic setting coupled with the effects of human d i s t u r bance of the t e r r a i n render this a task requiring comprehensive and long-term investigation.  The present study addresses one aspect of t h i s problem.  1.2. Wasp Creek watershed  study  A study of the Wasp Creek watershed, t r i b u t a r y to the Ryan River (Figure l ) , was developed to provide preliminary, c l a s t i c , sediment source and suspended sediment y i e l d information for the mountain slopes of the L i l l o o e t watershed.  The study area was  selected for i t s physiographic s i m i l a r i t y to  extensive areas of the L i l l o o e t watershed and because the logging of a lower /  slope had been carried out i n a manner consistent with normal practice i n the region.  Both p a r t i a l l y clear-cut and undisturbed areas were monitored  through a range of hydrologic conditions from May to November, 1 9 7 5 -  Since  these areas were not e n t i r e l y i s o l a t e d f o r hydrologic measurements and d i f f e r i n size and physical character the approach taken i s descriptive r a ther than a paired experimental watershed  study.  The s p a t i a l scale of the study integrates the response of a d i v e r s i t y of sediment sources both above arid below t r e e l i n e .  These sources were exa-  mined i n an e f f o r t to evaluate t h e i r r e l a t i v e importance to sediment production at the watershed  scale.  appropriate to t h i s enquiry.  An intermediate-sized watershed was  thought  At a larger scale the contributions from the  k  mountain slopes and the effects of t e r r a i n disturbance are masked by sediment y i e l d s from other areas while with decreasing scale the representativeness of the basin i s reduced. The s i x month study period permits only q u a l i t a t i v e observations of the kinds and r e l a t i v e importance of the sediment sources i n the undisturbed area.  Without longer term information the unit source area and  watershed scales of a c t i v i t y cannot be wholly reconciled. However, the recency of logging a c t i v i t y permits more d e f i n i t i v e evaluation of i t s effects to date. The central objectives of the study may be summarized as follows: 1.  to define kind and magnitude of observable sediment sources;  2. to d i f f e r e n t i a t e sediment sources and suspended sediment y i e l d s of undisturbed and clear-cut areas; and 3. to r e l a t e observed sediment y i e l d to environmental determinants i n a series of independent hydrologic events. 1.3.  Related studies The sediment sources and y i e l d s of the forested and logged slopes of  the B r i t i s h Columbia Coast Mountains have to date received l i t t l e attention. Reference may be made to evidence from adjacent areas of the northern P a c i f i c Mountain System - the Coast and Cascade Ranges of Washington and Oregon, the Olympic Mountains of Washington and the Coast Mountains of southeastern Alaska.  These are the areas of closest physiographic and  climatic s i m i l a r i t y to'"the'.study^region. Research i n Alaska has been i n both glaciated and non-glaciated areas underlain by g r a n i t i c and metamorphic bedrock.  In Oregon and Washington research has been conducted on s o i l s de-  rived from a variety of underlying bedrock types of non-glaciated areas. Both regions have annual p r e c i p i t a t i o n receipts of about 2,500 mm;  recorded  i n t e n s i t i e s are high i n coastal Alaska and the Cascade Range and somewhat lower along the more subdued Oregon Coast Range. Several investigations of the undisturbed, forested slopes have been carried out throughout•these  areas (e.g. Wooldridge, 1 9 6 ^ ; Rothacher et a l ,  I96T; Swanston, 1 9 6 7 ; Williams, 1 9 6 7 ; Brown and Krygier, 1 9 7 1 ; Fredriksen, 1970;  Swanson and Swanston, 1 9 7 7 ) .  The s i g n i f i c a n t sediment transfer pro-  cesses are s o i l creep, debris*"slides and flows, surface erosion, and channel bed and bank erosion.  Factors c o n t r o l l i n g t h e i r rates of operation are  numerous and produce widely disparate r e s u l t s . generalizations emerge.  However, two  important  F i r s t l y , work i n Alaska indicates that mantles of  g l a c i a l t i l l are less erodible than bedrock- or c o l l u v i a l l y - d e r i v e d s o i l s  5  (Swanston, may  Secondly, considerable  1969)-  v a r i a b i l i t y of sediment y i e l d -  occur both between basins during a single storm (Williams, 196k)  and  from year to year for the same basin (Rothacher et a l , 1 9 6 7 ; Brown and Krygier,  1971)-  An assessment of the e f f e c t s of t e r r a i n disturbance must  be approached with caution against an expected background of high natural v a r i a b i l i t y observable  even within the short term of most studies.  short study period order of magnitude differences of y i e l d coupled  For a with  s i t e observations are required to ascribe causal factors. Most writers have recognized substantial acceleration of erosion with logging and road construction.  Clear-cutting alone  (without slash  burning) has not been found to s i g n i f i c a n t l y increase surface erosion (Fredriksen,  Brown and Krygier,  1970;  1971);  root decay lessens shear strength and may debris s l i d e s (Bishop and Stevens,  I96U).  of y i e l d are a t t r i b u t e d to the steepening  however, on unstable  slopes  cause U-5 times as many shallow The more s i g n i f i c a n t increases of slopes, d e f l e c t i o n of flow  and exposure of s o i l which accompanies road construction (Dyrness, 1 9 6 7 ; Fredriksen,  Swanson and Dyrness,  1970;  1975).  Anderson  (1970),  i n review  of results from Oregon and Northern C a l i f o r n i a , cites y i e l d increases from clear-cut areas up to a factor of k and relates 80 percent of these to surface: erosion and mass movements associated with roads. Jeffrey  (1968)  has observed that logging effects have created problems  in B r i t i s h Columbia comparable to those elsewhere.  The roads rather than  the clear-cut slopes are i d e n t i f i e d as the p r i n c i p a l causes of accelerated erosion..  In the southern Coast Mountains O'Loughlin  (1972)  found shallow  debris slides and avalanches on'forested and logged slopes to be most simil a r i n kind to those operating on the g l a c i a l t i l l slopes of Alaska.  southeastern  Although the slopes were r e l a t i v e l y stable the incidence of  f a i l u r e increased by a factor similar to that reported i n other areas.  The r e l a t i v e effects of logging appear to transcend  logged  physiographic  controls. 2  O'Loughlin estimated that 95 t/km  /yr might represent the amount of  sediment activated by debris s l i d e s and avalanches in the Coast Mountains. Slaymaker and McPherson material may  (1977)  point out that much of t h i s r e d i s t r i b u t e d  be retained i n temporary storage and not r e a d i l y removed by  f l u v i a l action.  As w e l l , the generally low a v a i l a b i l i t y of material i n  the stream-banks of intermediate-sized watersheds may than are expected.  lead to lower y i e l d s  These observations underscore the need to r e l a t e i n -  dividual" .''processes to watershed scale y i e l d s - for assessments of the  effects of human disturbance of the t e r r a i n t h i s i s p a r t i c u l a r l y important. As f o r the slopes below t r e e l i n e , the alpine zone of the Coast Mountains has received l i t t l e attention with respect to c l a s t i c movement.  sediment  A :'f ew studies of individual processes have been conducted (e.g.  Mackay and Mathews, 197^ a&b; Patton, 1 9 7 6 ) and some preliminary sediment y i e l d data are available from both g l a c i e r i z e d and non-glacierized areas (Mokievsky-Zubok, pers. comm.; Slaymaker,  1977).  However, to explain  transfer processes i n the study area reference i s made to observations made elsewhere both i n the Canadian C o r d i l l e r a (see review by Slaymaker, 197^)  and i n other mountainous regions (see review by Caine,  197^).  2. Description of the study area 2.1. Location and area 2  The study area i s a 33 km  watershed m  the southern Coast Mountains  of B r i t i s h Columbia about 130 km north of Vancouver.  Figure 1 shows the  northward drainage of Wasp Creek into the Ryan River and thence to the L i l l o o e t River. 2.2.  Physiographic setting A b r i e f description of the basin physiography i s given here.  section 5 the s u r f i c i a l geology and geomorphic processes  In  are discussed i n  greater d e t a i l . 2.2.1.  Bedrock geology  Woodsworth  (1977)  mapped the Wasp basin as p r i n c i p a l l y granodiorite  and quartz d i o r i t e of the Mesozoic plutonic complex (Figure 2 ) .  A fault  zone traverses the basin from northwest to southeast to form one boundary of a narrow zone of lower Cretaceous metasedimentary and metavolcanic rock of the Gambier Group underlying less than 20 percent of the basin.  The  constituent materials of t h i s unit are mapped as andesitic to d a c i t i c t u f f , breccia, agglomerate, andesite, a r g i l l i t e , conglomerate, lesser marble, greenstone, and p h y l l i t e ; however t h e i r r e l a t i v e occurrence i n the basin i s not known.  Figure 2 i s taken from Woodsworth's 1:250,000:-scale''map and  should be interpreted accordingly. 2.2.2.  Glaciation and g l a c i e r i z a t i o n  The southern Coast Mountains were affected by several major g l a c i a t i o n s during the Pleistocene Epoch.  It i s known that ice of the Vashon Stade of  the Fraser Glaciation over-rode most peaks of t h i s region (Ryder, 1972)  and  there i s some evidence to suggest that at least one preceding g l a c i a t i o n thicker and more extensive than this most recent one  (Tipper,  was  1971).  The maximum Fraser i c e sheet elevation over the Wasp Creek area has not been conclusively established. at the top of Mt. Meager surface elevation of km to the south.  (2,6^5  2,250  Mathews (pers. comm.) report's an e r r a t i c  m) hO km to the northwest and a probable ice  m i n the Garibaldi Lake area (Mathews,  1950)  50  Since the main ice stream sloped to the south along the  L i l l o o e t River v a l l e y the 2 , ^ 0 most l i k e l y over-ridden.  m maximum elevation of the Wasp basin  was  The rounded form of the basin summits would sup-  port t h i s conclusion. Since retreat of Pleistocene i c e , alpine g l a c i e r s have occupied,  per-  haps discontinuously, the upper north-facing slopes of the study area. Mathews'  (1950)  work indicates that these Holocene alpine g l a c i e r s achieved  8  Figure 2. WASP C R E E K WATERSHED BEDROCK GEOLOGY  ::::::::::  : : : : : : : : : :  —— — — —  clear-cut divide treeline road  —1000—  index contour  contour interval 100m  granodiorite ;'ri~i~rrfl  quartz diorite metasedimentary and metavolcanic (Gambier Group) assumed fault line  •••Iv.'.'.M  (after Woodsworth, 1977) 1 _i  i  2 Kilometres — l  9  stages of maximum advance during the early eighteenth and middle nineteenth centuries i n the Garibaldi Park area.  In the Wasp basin fresh morainic  deposits above the 1,1+00 m elevation attest to g l a c i a t i o n within the l a s t century of about 20 percent of the t o t a l study area.  A e r i a l photo analysis  provides some - inconclusive evidence at two s i t e s of e a r l i e r , more extensive advances which may correlate with the eighteenth century maxima of the " L i t t l e Ice Age" i n f e r r e d by Mathews f o r some glaciers i n his study area. Retreat of the i c e i n t h i s century has l e f t remnants of these glaciers occupying 6 percent of the basin. 2.2.3.  Watershed form  The r e l i e f range of the Wasp basin i s from-575 m to 2,hk0  m.  The  steep-sided, trough-shaped main v a l l e y (see frontispiece) r e f l e c t s modification by the Fraser and preceding g l a c i a t i o n s .  The slopes, constructed  largely of r e s i s t a n t granodiorite and quartz d i o r i t e , maintain an average gradient of 3 ^ ° ^" with the steeper slopes predominating below t r e e l i n e . The more subdued upper elevations are broken by cirque forms, serrate ridges and steep headwalls, products of l o c a l l y intense alpine g l a c i a t i o n . 2.3.  Climate There i s a t r a n s i t i o n across the L i l l o o e t watershed from maritime  influences along the southwestern  margins to a more continental climate t o -  wards the northeast (Slaymaker and Zeman, 1 9 7 5 ) .  The l o c a l and regional  v a r i a b i l i t y i s poorly represented by the network of climate stations.  Long-  -term records are available only f o r the r e l a t i v e l y low elevation s i t e s at A l t a Lake ( 6 7 0 m) to the southwest and Pemberton Meadows above L i l l o o e t Lake along the L i l l o o e t River.  ( 2 3 0 m) 26 km  In addition, a snow course  i s maintained at Tenquille Lake on the eastern side of the L i l l o o e t River at 1,700 m. elevation (B.C. Ministry of the Environment). Although the recorded amounts are unrepresentative of the region the d i s t r i b u t i o n of p r e c i p i t a t i o n through the year i s i l l u s t r a t e d by the Pemberton Meadows station. conditions which p r e v a i l .  Figure 3 shows the winter wet and summer dry In the study area the occurrence and storage of  p r e c i p i t a t i o n as snow varies with elevation from November through A p r i l at lower -elevations.\to*October "through July a t higher elevations. accumulation'  Mean snow  i n the basin, where 50 percent of the area i s above 1,775 ni,  i s expected to be at least as great as that recorded at Tenquille Lake. 1. This value i s based on a randomly selected set of measurements made on a 1 : 5 0 , 0 0 0 scale (100 foot contour i n t e r v a l ) topographic map. 2. This record was discontinued i n 1966 and the station moved to Pemberton.  tomberton  Meadows, 230 20  15  10 (J 5 s  0  o.  P  -5  10  180 |  160  |  snow water equivalent  140  T 120 J. |  00  g  80 60  40  ao 0  Jan  Feb  Mar  Apr  May  |un  Jul Aug  Sep  Oct  Nov  Dec  Figure 3- P r e c i p i t a t i o n and temperature data f o r Pemberton Meadows, B.C., 1 9 ^ 1 - 1 9 6 6 The normal environmental lapse rate of 6.U°C/l,000 m may be applied to the seasonal temperature data of Figure 3 to give a f i r s t indication of temperature change with elevation. The hydrometeorologic presented 2.k.  observations made during the study period are  i n section k.  Vegetation The vegetation composition  i n the Wasp basin shows a continuous  dation i n species through the range of elevation represented.  gra-  The tree  species and major shrub species are i d e n t i f i e d f o r three generalized zones to describe t h i s gradient only, and not to suggest the occurrence  of par-  t i c u l a r communities. The slopes below 1,200 m elevation are forested by menziesii  (Douglas F i r ) , Tsuga  p l i a a t a (Western Red Cedar).  heteroTphytla  Pseudotsuga  (Western Hemlock) and Thuja  Stands of Pinus  aontorta  latifolia  (Lodgepole  Pine) are present i n a 1930 burn area on the northeast slopes. shrub species are Salix  Vaaoinium Abies  spp.  lasiocarpa  Salix  horridum,  Rubus  (Alpine F i r ) , Tsuga  amabilis  (Yellow Cypress).  spp. and Paahistima  heterophylla,  Tsuga  myrsinites.  Alnus  sinuata  albiflorum,  mevtensiana  The major shrub species are Vaaoinium  spp.,  Above the 1,500 m elevation to tree-  (Whitebark Pine) and Tsuga  are Rhododendron  and  Chamaeoyparis  (Amabilis F i r ) and  l i n e at 1,600 m to 1,900 m the tree species are Abies albioaulis  parviflorus  In a zone of 1,200 m to 1,500 m tree species present are  (Mountain Hemlock), Abies nootkatensis  spp. , Oplopanax  The major  Vaaoinium  mevtensiana. spp.,  and  lasiocarpa,  Pinus  The major shrub species Paahistima  myrsinites.  (Sitka A l d e r ) , the r e s i l i e n t ' tree cover of the snow avalanche  tracks, i s found through the entire elevation range c i t e d above.  Above  t r e e l i n e the vegetation cover, where present, i s p r i n c i p a l l y herbaceous (forbs  and  grasses) with Cassiope  mertensiana  and Phyllodoae  empetriformis  the common shrubs. 2.5-  Land use Clear-cutting of a lower, west-facing slope of the Wasp basin commenced  i n 1973 and continued i n 197^ and 1975 with annual removal of Uo, hectares respectively.  33 and U2  The clear-cut extends over an elevation range of  7^5 m to 1,235 m and occupies 3-5 percent of the t o t a l basin area.  The  area was not burned after timber removal and had been replanted at the time of writing. A high-lead system was used over most'of the area with some timber removal by skidder (a large-wheeled, a r t i c u l a t e d vehicle) i n areas of r e l a t i v e l y low gradient slopes.  High lead operations involve an overhead cable  system which "yards" logs to a central landing for loading.  Of the systems  used l o c a l l y t h i s one minimizes the extent of road coverage.  Surface d i s -  turbance r e s u l t s from logs being dragged along the yarding track, a n c i l l a r y vehicle operation and the r e q u i s i t e road construction.  12  3.  Methods  3.1.  Introduction Throughout the s i x month study p r e c i p i t a t i o n inputs and streamflows  were monitored to - characterize the hydrometeorologic variations within and between snowmelt, summer low flow and f a l l storm periods.  These variations  were related to suspended sediment flux at the stream gauging stations. Mapping of sediment, sources was conducted and the important sediment  trans-  fers and sinks described to i n f e r t h e i r r e l a t i v e importance to the watershed scale sediment y i e l d s during the observed events.  Congruent with the  scale of the study a largely descriptive approach was taken to d i f f e r e n t i a t e responses of the source areas. 3.2.  Precipitation In the Wasp Creek basin r a i n gauges were established to provide an  index of storm magnitude.  Canadian Atmospheric Environment Service standard  r a i n gauges were maintained at the 790 m and 1,220 m elevation on the clear-cut ted  slope and at the lower s i t e a tipping bucket recording gauge was operaduring the f a l l storm period.  Information from these gauges was aug-  mented by standard gauge data measured 7 km. to the. cant .along the.Ryan River at the 275 m elevation.  Standard gauge catches were measured following i n -  dividual r a i n f a l l events and the timing of the events was given by the t i p ping bucket gauge record. To test the n u l l hypothesis of a higher catch at the 1,220 m gauge than at the 790 m gauge a Student's t test was performed for the eleven paired observations.  The test revealed the two sets of data to be equal at the  95 percent significance l e v e l .  The mean of the catches at each gauge was  used as the index of storm magnitude u n t i l the onset of snow i n October. After mid-October, with snow accumulating at low elevations of the Wasp basin, the Ryan River station data were used to estimate p r e c i p i t a t i o n i n the study area.  Gauge catches at t h i s station were regressed against the  mean catches i n the study area for nine storm events to define a predictive relation. 3.3.  Stage and discharge  3.3.1.  Measurement  Manual s t a f f gauges were established i n three Wasp Creek t r i b u t a r y channels draining the clear-cut area and periodic measurements of stage were taken.  At two stations along Wasp Creek (designated upper and lower)  stage was measured continuously by Ott XX f l o a t type water l e v e l recorders. Discharge of Wasp Creek was measured by d i l u t i o n gauging (Church and  K e l l e r h a l s , 1970) through a range of stages spanning those observed during the study period.  These data were regressed i n simple arithmetic and  logarithmic form and the strongest r e l a t i o n s h i p for each station was selected. 3-3.2.  Discharge record length and synthesis  By application of the stage-discharge relations the stage records were converted to discharge hydrographs.  At the lower Wasp Creek station the  discharge record i s then continuous from May 8 to November 1 5 . At the upper Wasp station a continuous record i s available from July 1 to October 2 1 . This record was extended for the periods June 21 to July 1 and October 21 to November 2 by i n t e r p o l a t i o n from point observations.  For synthesis of  the November storm event reference was made l ) to the observed lag times between the stations of the hydrograph peaks, 2) to the timing of the flood r i s e at the lower s t a t i o n , and 3) to the single discharge measurement made about two hours before the estimated peak flow.  The discharge record for  both stations i s shown i n Figure A . l . 3.3.3.  Hydrograph separation  A common but a r b i t r a r y method of separation o f direct storm runoff from baseflow was used.  The trend of the pre-storm flow was extrapolated  to the time of the peak flow and from that point a l i n e was drawn to a point on the hydrograph a prescribed number of days (N) after the peak. Linsley et a l ( 1 9 7 5 ) c i t e a "rule of thumb" that was used as follows: N = 0.8A ' 0  (1)  2  2  where: 3.3A.  A = drainage basin area (km ).  Flood frequency analysis  Long-term discharge gauging stations have not been maintained on either Wasp-Creek or Ryan River.  The frequency of occurrence of the observed peak  flows cannot therefore be determined d i r e c t l y .  Reference was made to the  hydrologic records of s i x other rivers of the L i l l o o e t River system (Water Survey, of Canada, 1 9 7 7 ) .  These records span d i f f e r i n g periods, are of  varying length, and, at the time of study, had been'discontinued except f o r that of the L i l l o o e t River.  Nonetheless, the flood frequency analyses are  i n d i c a t i v e of the c h a r a c t e r i s t i c hydrologic response of each r i v e r . For comparative purposes the annual maximum d a i l y flows were divided by basin area to give recurrence intervals for the maximum d a i l y discharge intensities. 3'h. Sediment source mapping 3.1+.1. Undisturbed area  The f i r s t reconnaissance of the Wasp watershed was conducted "by helicopter.  Mapping of the s u r f i c i a l materials and channel network was  then done at the 1 : 2 5 , 0 0 0 scale "by interpretation of a e r i a l photos with subsequent f i e l d checking carried out on foot. 1:50,000  This map has been reduced to  for i n c l u s i o n i n t h i s report.  During.the f i e l d season the most recent a e r i a l photos were 1 : 1 ^ , 5 0 0 scale"'" black and white photos taken i n 1 9 7 3 .  In September, 1975 an a e r i a l  photo run was conducted by the'Canada Centre.for Remote.Sensing to provide colour infra-red photography  at the 1 : 1 ^ , 5 0 0 . and 1 : 3 7 5 0 0 " s c a l e s ; these 1  5  photos were available for analysis subsequent to the f i e l d programme. The metric 1:50,000.. scale topographic, map drafted from the N.T.S. 1:.50,000 scale (100  appearing herein was re-  foot contour i n t e r v a l ) map.  A  1:37»500 colour infra-red photograph i s presented with an overlay showing the channel network.  Site references are to quadrats defined by the grid  established, on the overlay. 3.k.2.  Clear-cut area Road access to the clear-cut area.permitted more detailed observations  to be made of sediment sources than were possible for the undisturbed area. Mapping of sediment  sources was carried out primarily by ground reconnais-  sance with reference to the available maps and a e r i a l photos.  Slope f a i l u r e  and channel forms were surveyed by theodolite and resurveyed i n the event of s i g n i f i c a n t change.  A Brunton compass was used for surveys over d i f f i c u l t  terrain. A l : 1 5 , 8 U O scale (100 foot contour i n t e r v a l ) topographic map,  sup-  p l i e d by the B.C. Forest Service, covered the entire clear-cut area.  This  map was redrafted to metric contours and enlarged to a 1 : 1 0 , 0 0 0 scale. Site references for the clear-cut area are to the grid established on t h i s map. 3.5-  Sediment y i e l d  3.5-1.  Network  Suspended sediment samples were collected at the three manual s t a f f gauge sites within the clear-cut area and at the continuous stage recording stations along Wasp Creek above and below the clear-cut area. 3-5.2.  Sample c o l l e c t i o n  Since the channel sections at the Wasp gauging stations could not be waded sediment  1.  samples were taken at a single point within the cross-section  Approximate scales at the mean watershed elevation of 1,775 m.  of flow.  This method does not integrate variations i n the v e r t i c a l and  horizontal.  I t was assumed that the highly active turbulent exchange of  the steep, tumbling flow regime stream reaches would afford s a t i s f a c t o r y dispersion of suspended sediment through the cross-section.  Samples were  taken either by hand or by a probe connected to an Instrumentation Specialties Co. model 1391 automatic water sampler. 3.5-3.  Sample analysis  The method of sample analysis entailed:  s e t t l i n g of samples; decan-  tation of a dissolved sediment f r a c t i o n ; and evaporation of each f r a c t i o n in porcelain dishes.  Table B . l . sets out the steps of the procedure.  The  method has the advantage of s i m p l i c i t y and of y i e l d i n g measures of the d i s solved, c l a s t i c and v o l a t i l e fractions of the sample.  However, i t i n t r o -  duces s i g n i f i c a n t error for very low sediment concentrations.  Weighing  error and weight changes of the porcelain dish during treatments were the s i g n i f i c a n t problems.  The magnitude of these error sources was evaluated  by determination o f the weight changes of four dishes subject to a series of treatments  designed to r e p l i c a t e the range of conditions during sample  These tests indicated that 95 percent of the observations w i l l  analysis.  f a l l within ± 3 mg/l of the true value for 100 ml samples.  Of the 260  samples analysed 16 registered concentrations less than 3 mg/l and of thes data two were between - 3 and 0 mg/l and one was -U.5 mg/l.  Indirect e v i -  dence for the accuracy of the method i s given by the consistent trends of the s e r i a l data and'by' the:-.'strength of the sediment rating curves. 3.5-1+.  Sediment y i e l d computation  . Sample analysis gives a measure of the concentration of suspended inorganic sediments passing the gauging station at the time of sampling. The product of concentration and discharge gives an estimate of the sediment transport rate or sediment load which i s expressed as a mass passing the gauging station during a specified'; time (tonnes per hour for t h i s study).  The several methods used to determine sediment y i e l d during a  given period are described below. a)  Interpolation method  The most direct and dependable method i s to define a sediment dischar graph by interpolation from the calculated rates of sediment transport. The area beneath t h i s l i n e represents the t o t a l sediment discharge during the period.  The frequency of sampling was adequate to employ t h i s method  for a l l but one of the storm runoff events at the lower Wasp station and for the single summer storm runoff event only at the upper Wasp station.  b)  Sediment rating curve method  This method involves the regression of the calculated rates of sediment transport against stream discharge to produce a sediment r a t i n g curve f o r the reference period.  The rating curve i s used to predict a sediment tran-  sport rate at each of the observed flow l e v e l s .  These rates are m u l t i p l i e d  by the t o t a l time occupied by the corresponding flow l e v e l (as shown by a flow duration curve) and the products summed to give t o t a l sediment y i e l d . While t h i s method i s widely used there are several problems inherent to i t s application i n t h i s study.  Rating curves may misrepresent  sediment  y i e l d s i n situations where sediment concentrations are not s a t i s f a c t o r i l y explained by runoff variations alone.  Such i s the case i n Wasp Creek where  slugs'of sediment may: be'discharge! caused by episodic a c t i v i t y such as :<;•-."" s'16pe."fai-Iure;..'oroby-vehicle operation within streams during high flow, non-storm periods.  A second problem i s that a systematic change of watershed  conditions through the period, such as. s o i l moisture or snow cover v a r i a t i o n , might effect a concomitant s h i f t i n sediment concentration at a given flow.  The sediment concentration-discharge plot for the three largest  runoff events, at the lower Wasp station (Figure k) illustrates x  a considerable  between storm v a r i a b i l i t y of concentration at a given discharge.  Rating  curves for i n d i v i d u a l storms may also be unsatisfactory. During each storm runoff event monitored at the lower Wasp station the peak sediment concent r a t i o n preceded the stream discharge peak. storm response exemplifies t h i s behaviour  The graph of the October 17  (Figure 5 ) .  The proximity of the  gauging station to the clear-cut area,, the p r i n c i p a l , source of sediment, causes the peak sediment concentration to precede the peak flow by several hours.  In t h i s case the lags between the hyetograph centroid and the peaks  of concentration and discharge were 8 . 5 and 12.5 hours respectively. Since sediment concentration i s normally found to increase d i r e c t l y with discharge the sediment transport rate (T) increases more rapidly than discharge (Q).  Sediment r a t i n g curves are therefore frequently defined as  simple functions of the form: logT = alogQ - b where a and b are numerical constants  (2)  (Leopold'and Maddock, 1 9 5 3 ) .  A limi-  t a t i o n here i s that Q, being used to derive T, appears on both sides of the equation thereby giving spurious strength to the r e l a t i o n .  A sediment con-  centration-discharge r e l a t i o n would be required to describe.the true cov a r i a t i o n of the measured flow and sediment data.  The r a t i n g curve i s used  i n t h i s form for p r e d i c t i v e purposes only, not to describe the physical  10  ;  f 102 c o  ro  kC  CD O  •  c o o  • ••  c  CD  4  E  1  CO  101 •  T Storm date: August 27 October 17 November 17  J  IfjO  10  L  I I I I 111  J  10 Discharge (m^/s) 1  •  «J  ••  I I I I 111 10'  Figure h. Discharge-sediment concentration relationships for the three largest storm runoff events at lower Wasp Creek.  18  Figure 5. R a i n f a l l , discharge and sediment concentration graphs f o r the October IT storm, lower Wasp Creek.  relationship underlying the data. The rating curve method must c l e a r l y be applied with caution.  It i s  used for the computation of sediment y i e l d s during the snowmelt and storm periods on the upper Wasp.  During these periods the direct covariation of  flow and sediment transport rate appear,  reasonably consistent.  The 95  percent confidence l i m i t s on the mean have been defined to express the i n herent accuracy of the r e l a t i o n s . c)  Mean sediment concentration-discharge method  During periods when, sediment data showed no consistent v a r i a t i o n with, discharge or were inadequate i n number to allow construction of sediment discharge graphs a.third approach was taken to estimate sediment y i e l d . The mean daily flow was multiplied by the mean sediment concentration and thence by time to give an approximate sediment y i e l d for the period.  This  method introduces a bias to the results which may be substantial where flow and sediment concentration variations are large (Church,  1978).  The method  was used for the lower Wasp non-storm data and for the upper Wasp data during the baseflow period of the summer.  During the summer period while  flows were r e l a t i v e l y steady and concentrations very low this bias i s not significant.  However, for the lower Wasp Creek snowmelt period measurements  s i g n i f i c a n t inaccuracies could be present hence estimates- of sediment delivery based on slope observations were necessitated. d)  Upper Wasp Creek y i e l d estimate for the November storm  The sediment transport rate at the upper Wasp station was obtained at only one point for the November storm. the peak sediment concentration.  This datum i s thought to be close to  A sediment discharge hydrograph was de-  r i v e d from the synthesized flood hydrograph using the f a l l storm rating v : . curve. e)  Y i e l d from the clear-cut area  An estimate of the y i e l d from the clear-cut area alone i s made by f i r s t assuming that the sediment production rates for the undisturbed area of the lower Wasp basin may be approximated by the measured rates of the upper Wasp basin and that the residual represents the y i e l d from the clear-cut area.  Since Stream C, the major Wasp Creek t r i b u t a r y between the up-  per and lower stations was found to have very low sediment concentrations this assumption i s deemed conservative.  I f the sediment y i e l d at the  upper station i s subtracted from that at the lower station i t must also be assumed that the sediment i s maintained i n suspension between the two stations.  This second assumption i s thought to be v a l i d within the l i m i t s of  accuracy of the  calculation.  21  k. k.l.  Hydrometeorologic Introduction  observations  This section d e t a i l s the p r e c i p i t a t i o n , stream stage and discharge observations pertinent to the study.  The seasonal v a r i a t i o n , magnitude  and frequency of events and effects of watershed condition are discussed. k.2. k.2.1.  Precipitation Gauge relations The regression of the catches at the Ryan River and Wasp Creek  gauges produced the following predictive r e l a t i o n : M = 1.59R where:  - 5.65  (3)  M = mean catch of clear-cut gauges R = Ryan River gauge catch (mm) R= 2  (mm)  0.983  standard error = 6 . 6 9 . This equation shows consistently higher inputs at the Wasp gauges however, as Figure 6 i l l u s t r a t e s , no data were available for a large range of storm sizes.  The regression r e l a t i o n e i s produced i n large part by the b i v a r i a t e  mean of the lower data and the single o u t l i e r hence the true relation'through t h i s i n t e r v a l cannot be estimated with a high degree of confidence.  In  Figure 6 a regression r e l a t i o n f i t to the lower data alone i s shown for comparison.  Since the predictions of i n d i v i d u a l amounts required are be-  yond the range of t h i s l a t t e r r e l a t i o n equation 3 has been selected and the confidence l i m i t s on the observation defined. k.2.2.  Seasonal v a r i a t i o n Measurements taken at the Tenquille Lake snowcourse, about 15 km to  the northeast of the Wasp basin, are used to indicate the timing of snow accumulation  and ablation i n the study area.  Figure 7 shows the water  equivalent of the snowcover from March 1 to June 1, 1975 to be below the median amount through the 22"-yearcrecord and that on June 1 the exceedance p r o b a b i l i t y of the snow water equivalent was temperatures  about 67 percent.  were near normal for the winter period at Pemberton  Since r (B.C.  Department of Agriculture) the below average snowpack i s attributed primarily to low winter snowfall.  The snowpack recession i n the study area  and resultant flows are discussed i n section A 50 year p r e c i p i t a t i o n record was  k.3.2.  available from the Pemberton  Meadows station about 10 km east of the study area at an elevation of 230 m. ' This station was throughout  closed i n 1 9 6 6 , however, d a i l y measurements were taken  the study period at a standard gauge nearby.  Although these data  are not p e r f e c t l y comparable they suitably i l l u s t r a t e the general seasonal  Figure 6. Regression r e l a t i o n of Ryan River and Wasp Creek r a i n gauge catches.  2.00  r  1.75  h  exceedance probability:  * ~ c  5%  Figure 7- Tenquille Lake snow course data, 1,700 m  1.50  2" 1.25 .A $  A  - N ^  A  50%  A  •  1953-1974  A  A 1975  1.00  o  >  \  0.75 r95% 0.50  J  Feb  Mar  280  L  Apr  May  Jun  240  Figure 8. Pemberton Meadows p r e c i p i t a t i o n data, 230 m 200 1916 - 1965 1975  M  Missing data  160^| c  o u 120£  exceedance probability: 5% -  ^  80  -I 4 0  50% M  M 95%  Nil . May  |un  Jul  Aug  Sep  J  Oct  L  Nov  2k  "variation r e l a t i v e to the long-term record (Figure 8 ) .  Precipitation in  1975 was below normal i n May, July and September and well above normal i n October and November.  Estimates based on the Ryan River data suggest pre-  c i p i t a t i o n below normal i n June and above normal i n August. The rainstorm amounts measured during the study period are summarized in Table  1.  The f i r s t major storm occurred at the end of August and  delivered about 75 mm of r a i n f a l l . study area.  During September no r a i n f e l l i n the  October p r e c i p i t a t i o n was r e l a t i v e l y heavy; to October 20,  f i v e discrete storms supplied a t o t a l of 271 mm of r a i n to the Wasp gauges. This p r e c i p i t a t i o n occurred as r a i n at the gauges and snow at high elevations.  In the f i n a l 10 days of the month p r e c i p i t a t i o n occurred primarily  as snow throughout  By November the snowline was at the 750 m  the basin.  elevation and at 1,000 m on the clear-cut slope snow had accumulated to a depth of about 0.5 m.  The water equivalent of t h i s cover was not e s t i -  mated, however, the p r e c i p i t a t i o n amount predicted by solution of equation 3 i s kk mm at the clear-cut s i t e .  Between November 2 and 5 r a i n f e l l at  a l l elevations; the estimated amount for the clear-cut area given by equation 3 i s 180 mm. U.3.  Stage and discharge  I4.3.I.  St age-discharge  relations  The stage-discharge relations for the Wasp Creek stations are defined below and i l l u s t r a t e d i n Figures 9 and 10. r e l a t i o n i s the logarithmic form: logQ where:  = 1.3UlogS  For the lower Wasp station the - l.kk  (k)"  = discharge at lower Wasp (m^/s) S^ = stage at lower Wasp (cm) i  2  = 0.956  1  standard error (for log units) = O.O85. For the upper Wasp station the r e l a t i o n i s the semilogarithmic form: logQ  = 0.013S  (5)  -0.604 3  where:  Q = discharge at upper Wasp (m /s) S^ = stage at upper Wasp tern) i = 0.975 2  standard error (for log units) = 0.031. A problem of s e r i a l correlation of the residuals of equation 5 may be present, however, the data were considered i n s u f f i c i e n t to define a higher order r e l a t i o n . 1. For"transformed data the correlation index ( i ) (Ezekiel and Fox, 1959) i s used to show the strength with which the function explains the observed values.  25  Table 1. Wasp Creek Watershed Hydrometeorologic Data Summary  Storm date Precipitation  9  Oct. 14  U8  7.U'  22  Aug.  Oct.  27  3 .  Oct.  (mm)  76  lower Wasp Creek (LWC)  16.5  4.3  0.34  1.33  upper Wasp Creek (UWC)  13.1*  3.1  N/A  0.40  LWC - UWC  22.1  6.5  N/A  Storm runoff LWC Storm runoff UWC  1.2  1.4  Storm runoff LWC - UWC Storm runoff UWC  1.6  Storm runoff (mm)  Storm runoff Precipitation  Oct.  Oct.  17  31  Nov. 4  163  113  180  13.2  2.9  39  3.0  U/K  34  3.1  32  U/K  49  N/A  3.3  4.5  U/K  1.2  2.1  N/A  7.8  10.9  U/K  1.5  lower Wasp Creek  0.22  0.09  0.05  0.06  0.08  0.03  0.22  upper Wasp Creek  0.18  0.06  N/A  0.02  0.02  U/K  0.27  Basin area: lower Wasp Creek - 3 3 . 0 km ; upper Wasp Creek - 2 1 . 5 km N/A: not applicable; no detectable.change i n discharge. U/K: unknown; recorder inoperative  50  r  Stage (cm)  Figure 9.  Lower Wasp Creek discharge rating curve.  Figure 10.  Upper Wasp Creek discharge rating curve.  U.3.2.  Seasonal discharge v a r i a t i o n  Figure 11 presents the long-term mean of the mean daily discharge"and the 1975 mean daily discharge for the L i l l o o e t River gauging station Survey of Canada).  (Water  Congruent with the accumulated snowpack measured at  Tenquille Lake i n 1975 the snowmelt runoff i n the L i l l o o e t River was somewhat below the mean to the end of June.  The long-term record shows a  second peak i n July related to glacier and high elevation snowpack melt; i n July, 1975 above normal temperatures caused a r e l a t i v e l y high runoff. Flows then remained generally below normal u n t i l the early November r a i n -on-snow event which generated a flood that was the t h i r d largest of the 57 year record. The discharge record in the Wasp basin commenced May 8 (Figure A.'l.) at which time the snowline was at about the 750 m elevation.  From May 8  to May 27 the snowline receded to about the 1,200 m l e v e l and flows at the lower Wasp station were r e l a t i v e l y low at a median 1.8  m /s.  Between May  27 and the end of July discharge was derived increasingly from the higher and areally more extensive levels of the basin. Creek had a median of k.k  Flows i n the lower Wasp  m /s and during periods of high radiant energy  exhibited the d i s t i n c t i v e , n i v a l l y - c o n t r o l l e d daily rhythm of peaks and troughs  'ranging over about 1 m /s.  Discharge during t h i s period peaked i n  July with contributions from the small glaciers and the high elevation 3  Peak discharges were 7-5 and 1 0 . 2 m /s for the upper and lower  snowpack.  Wasp stations respectively. A summer low flow period i s distinguished for the months of August and September. The median flows i n upper and lower Wasp Creeks were 2 . 3 3  and 2 . 9 ' m /s respectively.  The single major storm which occurred during 3  and 5-9 m /s at the upper  this period caused instantaneous peak flows of h.l and lower stations respectively.  Throughout the October storm period baseflow was lower than during both the snowmelt and summer periods. for the 20 days of low flow was 1.7 3  The median flow i n lower Wasp Creek 3  m /s.  1  Instantaneous storm peaks  ranged to 2.8 and 3.7 m /s at the upper and lower stations respectively. These flows are s i g n i f i c a n t l y lower than the preceding snowmelt peaks. The November rain-on-snow flood was the only one to exceed the snowmelt peaks.  lower Wasp station. 1.  3  An instantaneous peak flow of 1 9 . 9 m /s was recorded at the  3  The estimated peak flow for the upper Wasp was 11 m /s.  The October record for upper Wasp Creek i s incomplete.  4.3-3.  Flood frequency analysis  The maximum daily discharge intensity observed on the Ryan River 3  estimated at 0.4 m /s/km  2  on November 4.  1  Figure 12  2  was  shows the peak flood  oh the L i l l o o e t River brie day l a t e r to be the t h i r d largest on record with 3  2  a discharge i n t e n s i t y of O.36 m /s/km  and a recurrence i n t e r v a l (with t h i s  datum included i n the analysis) of about 20 years.  The.relative s i m i l a r i t y  of these discharge i n t e n s i t i e s i s a f i r s t indication that the Ryan River may  respond more as the L i l l o o e t , Green and Birkenhead Rivers than as the  Soo River or Rutherford Creek.  The recurrence i n t e r v a l for the observed 3  Ryan River flood might then be approximated at 10 to 20 years.  There i s  i n s u f f i c i e n t evidence to assign a recurrence i n t e r v a l for the Wasp Creek peak because of the scale contrast; however, the flood frequency  curve for  t h i s smaller basin would probably have a higher intercept and slope. 4.3.4.  Hydrologic response  The hydrologic response of Wasp Creek to storm inputs i s shown i n Figure 13 and the data summarized i n Table 1 gauging stations.  for both the upper and lower  The r a i n f a l l -.arid- .runoff -.events are d i s t r i b u t e d over  three orders of magnitude and are therefore displayed on logarithmic paper. R u n o f f - r a i n f a l l r a t i o s are lowest for the October storms and highest for the larger summer storm and the November rain-on-snow event.  The  statis-  t i c a l relations underlying each set of data have not been defined for the observations are few and encompass widely varying environmental conditions. The r e l a t i v e p o s i t i o n i n g of the data i s best explained q u a l i t a t i v e l y . Two  factors have an over-riding influence upon the v a r i a b i l i t y of the  data of Figure 13 - storage changes of snow and land use e f f e c t s . The single major summer storm i n late August gives an indication of basin hydrologic response with the snow cover at a minimum extent.  The  higher runoff at the lower station i s considered an effect of the surface disturbance by logging a c t i v i t y .  This storm produced about 65 percent more  runoff i n the lower basin alone than the upper basin. 1. Based on a stage-discharge rating curve established for the gauging station at the 275 m elevation. 2. The flood frequency analyses of Figure 12 are constructed to i l l u s t r a t e the array of annual peaks and thus include storm runoff, glaciermelt and snowmelt peaks. The data are presented i n t h i s form for descriptive purposes and should not be used for p r e d i c t i o n (U.S. Water Resources Council, 1977).  3. This estimate i s corroborated by one l o c a l resident's r e c o l l e c t i o n s of Ryan.,River'stage, .fluctuations during the past 60 years (G. Ross, pers. comm.  Years of record 22 38 26 57  River  • • •  25  O V  24  •  Basin area (km ) 9 z  Birkenhead River at Mount Currie (08MG008)  595  Green River near Pemberton (08MG003) Green River near Rainbow (08MG004) Lillooet River near Pemberton (08MG005)  854 195 2,170  Rutherford Creek near Pemberton (08MG006) Soo River near Pemberton (08MG007)  V  V  N.B. 1. The flood records span differing periods and all but the Lillooet River gauging station had been closed by 1975. 2.  161  Snowmelt, glaciermelt and storm  runoff peaks are included in the analyses.  238  v v  7 ?  • •  OA  1 1.01  e 12.  •  o  O  O  Nov. 5, 1975  O fi  _L 1.1  »  I  1.3  I  L  1.5  J  2  L  2.33  J JL 3 4 5 Recurrence interval  I  L  7 (yr)  10  20  _L  30  JL  40  _L  60  ' ' 'J 80  100  Frequency of maximum d a i l y discharge i n t e n s i t i e s for r i v e r s of the L i l l o o e t River drainage syst  32  10  2  Storm 1.  conditions:  August 27 - summer storm  2. October 3 - snow above 1,400 m $. October 9 - snow above 1,100 m (no stormflow at upper Wasp station) 4. October 14 - snow above 1,100 m 5. October 17 - snow above 1.100 m 6. October 31 - snow above 900 m  7. November 4 -rain-on-snowat all aievations •  1  •  1  •  5  10'  • 2 • 2  6 •  10',0 _  lower  Wasp  upper Wasp  • 10"  J  10  u  I  I I I I I  I  10  1  Rainfall  Figure 13.  (mm)  I  I  Creek • Creek •  i  10'  Wasp Creek hydrologic response to storm events.  Throughout the October storm period the singular effect of the c l e a r -cut  area becomes less d i s t i n c t .  As the annotations on Figure 13 indicate  snow was f a l l i n g at upper elevations during t h i s period.  A portion of the  p r e c i p i t a t i o n input then was stored as snow and not immediately released as runoff.  The hypsometric curves of Figure 14 suggest that t h i s zone of storm  runoff production represented less than 25 percent of the lower basin alone and less than 5 percent of the upper basin.  The factors by which runoff  production from the lower basin exceeded that from the upper basin ranged from 2 . 1 to 1 0 . 9 through t h i s period (Table l ) . The large November storm supplied at least 180 mm of r a i n f a l l during a two day period at a l l elevations to a snowcover exceeding 0 . 5 m above the 1,000  m elevation.  Flows generated i n the lower Wasp area exceeded those  from the upper basin by about 50 percent. 2,600 r  Proportional area (%)  Figure 14.  Hypsometric analysis of Wasp Creek-basin.  4. 4. Conclusion The foregoing observations record a wide range of flows through three periods of snowmelt, summer low flow and f a l l storms.  Given the range of  watershed conditions and the scale and time period of the study a s t a t i s t i c a l modelling of the system's responses has not been undertaken.  This section  has provided a description of the conditions encountered as background to  the analysisoof •hasin sediment y i e l d . controls.of hydrologic  In this context the three important  response at the watershed scale have been i d e n t i f i e d  as storm magnitude, the variable d i s t r i b u t i o n the clear-cut area.  of snow, and the presence of  5. 5.1.  Sediment sources and a v a i l a b i l i t y Introduction The purposes of t h i s section are to describe the sources of c l a s t i c  sediment d i s t r i b u t e d throughout the Wasp Creek watershed, to assess  their  r e l a t i v e a v a i l a b i l i t y to the channel network and thence t h e i r contribution to basin sediment y i e l d .  The s u r f i c i a l materials are mapped and described  and morphologic evidence i s used to assess the character and d i s t r i b u t i o n of sediment transfer processes 5.2.  and sediment sinks within the basin.  Sediment sources Figure 15 i s an idealized cross-section of a Wasp Creek v a l l e y wall  showing the c h a r a c t e r i s t i c zonation and s t r a t i g r a p h i c sequence of the s u r f i c i a l materials.  The upper elevations of the basin are dominantly  exposed bedrock overlain i n places, by felsenmeer, colluvium, loess, and Holocene or Pleistocene t i l l . per north-facing slopes.  Small glacierized areas are present on up-  At lower elevations deposits of Pleistocene  till  are extensive, broken by coarse colluvium, alluvium and frequent bedrock outcrops.  The d i s t r i b u t i o n of the s u r f i c i a l materials i s shown i n Figure  16 and t h e i r texture, c h a r a c t e r i s t i c landforms and degree of vegetation cover described i n Table 2. The clear-cut area i s displayed i n Figure 17 and i t s surface conditions described i n Table 3.  The c h a r a c t e r i s t i c s of the t r i b u t a r y basins  draining the clear-cut are also tabulated.  Figure 15. Idealized cross-section of Wasp Creek v a l l e y wall and s t r a t i graphic column.  36  Table 2 Sediment Sources Surficial Material  glacial  Texture  % of Basin Area  bedrock  Characteristic Landforms  Degree o f Vegetation Cover  consolidated  s t e e p h e a d v a l l s ; low g r a d i e n t exposures  sparse  6  not  not  none  0.1  a n g u l a r c o b b l e s and b o u l ders  l e v e l t o gently accumulations  29 ice  o f Wasp Creek B a s i n  felsenmeer  applicable  applicable sloping  sparse  colluvium  2h  a n g u l a r c o b b l e s and b o u l ders ; minor f i n e sediment  t a l u s sheets and cones  sparse  Pleistocene till  31  poorly sorted, v e i l - i n durated; pebbles t o b o u l ders w i t h f i n e m a t r i x  g e n e r a l l y t h i n mantle on v a l l e y w a l l s ; decreasing depth w i t h e l e v a t i o n  well vegetated  Holocene till  8  poorly sorted; fines to boulders  lodgement, l a t e r a l and t e r m i n a l moraine  sparse  alluvium  1.8  poorly sorted, fines to boulders (cones); s i l t and sand over g r a v e l (valley flats)  cones o f s t e e p t r i b u t a r y c r e e k s ; narrow, fragment a r y v a l l e y f l a t s along main channel  well vegetated  silt  weathered veneer throughout b a s i n i n a s s o c i a t i o n with Pleistocene t i l l (not differentiated)  well vegetated  U / K  loess  Table  3  Surface Conditions of Clear-cut Surface c o n d i t i o n  Total Clear-cut  Surficial Material  t i l l ; minor bedrock, colluvium and a l l u v i u m  % o f Area C l e a r - c u t  100  Area  1.15  (km^)  E l e v a t i o n range  71*5-1,235  (m)  % S u r f a c e Exposure Road Length  B.8  (km)  Road G r a d i e n t  11.07  (degrees)  Basin A t i l l ; minor bedr o c k and iD O I I U vium 3* 1  1.16  7 *5-1,800 1  15  area Basin B t i l l ; minor bedr o c k and c o l l u vium 100  71*5-880  925-2,1*1*0  16  6.5  0.35  0-15  0-12  0-10  3l*-l*2  3h-k0  36-1)2  C u t - s l o p e angles  (degrees)  k0-60  kO-6o  g e n e r a l l y 1-2; up t o 6  1-2  1.9 0.123  (degrees)  (m)  1  0.01k  F i l l - s l o p e Angle  Cut-slope heights  Basin C t i l l ; minor rock, colluv: and a l l u v i u m  g e n e r a l l y 1*0-60; up t o 9 0 up t o 6  3.5 0.60  <3 3I4-I4O 1*0-60  1-2  37  Figure 16.  clear-cut  WASP C R E E K WATERSHED SURFICIAL GEOLOGY  divide treeline road —  index contour  1000—  contour interval 100m  B  bedrock  G  glacial ice  F  felsenmeer  C  colluvium  P  Pleistocene till  H  Holocene till  38  Figure IT. WASP CREEK CLEAR-CUT.  39  5.3.  Sediment transfers - undisturbed  area  In this section the most s i g n i f i c a n t processes of sediment transfer are discussed.  These are grouped as g l a c i a l , slope and f l u v i a l processes.  The  discussion i s based largely on an evaluation of the morphologic evidence for sediment movement.  The mechanics of the i n d i v i d u a l processes  are only con-  sidered for purposes of d e f i n i t i o n and basic description. 5.3.1.  G l a c i a l processes  The two most important  controls of g l a c i a l erosion are generally recog-  nized as being ice thickness and v e l o c i t y (Embleton and King, 1 9 6 8 ) .  In the  Wasp basin the extensive presence of recent t i l l deposits suggests that r e l a t i v e l y active g l a c i a l erosion has taken place.  However, the g l a c i e r s  have retreated considerably from t h e i r maximum Holocene advances and at present are t h i n , low r e l i e f forms which do not generally meet the ice thickness and v e l o c i t y c r i t e r i a for e f f e c t i v e erosion.  The controls and net contribu-  tions of the glaciers to sediment y i e l d are evaluated i n section 6 . 3 . Slope  5.3.2.  processes  Of the processes  operating on slopes there i s a continuum from the  s t r i c t l y c o l l u v i a l (or mass wastage) mode through to a f l u v i a l mode. Some writers,?• (e.g. Rapp, i 9 6 0 ) have grouped the " t r a n s i t i o n a l " processes debris flows and debris avalanches as f l u v i a l processes. all  such as  In this report  mass movements of sediment are grouped as slope processes. a)  Large scale mass movements  Examination of the erosional and depositional landforms has revealed no evidence  for large scale mass movements such as deep-seated debris or  rock slides and slumps. b)  Rockfalls  From bedrock slopes fragments released by frost shattering are delivered downslope by r o c k f a l l s with further size reduction caused by impacts during transport.  In alpine areas the widespread d i s t r i b u t i o n and scale of the c o l -  l u v i a l slopes attest to the importance of the process  (Photo; l ) . Below  t r e e l i n e the process i s active where steep bedrock exposures remain. c)  Debris avalanches  Debris avalanches involve planar f a i l u r e and rapid downslope movement of c l a s t i c and organic debris (Swanston, 1 9 6 9 ) . tent downslope a debris avalanche may line.  With increases i n water con-  generate a debris flow along a drainage  In the Wasp basin i t i s the shallow.j weathered t i l l horizons of the  steep v a l l e y walls below the 1,600 m elevation that are most subject to t h i s process.  The c h a r a c t e r i s t i c forms are spoon-shaped scars i n the i n i t i a t i o n  zone funnelled into l i n e a r tracks along drainage l i n e s or stream (Photo.2).  F a i l u r e of the oversteepened  occurs (e.g. s i t e 1 1 2 , Figure 1 8 ) .  channels  walls of the t r i b u t a r y creeks also  Smaller scale debris avalanches  without  a s i g n i f i c a n t downslope flow component are common on slopes of 3 5 ° to 4 5 ° . Debris avalanching i s less evident in the alpine zone congruent with the generally lower slope gradients. steep t i l l slopes are present of small masses may d)  The process i s nonetheless  (e.g. s i t e Gl6).  active where  On c o l l u v i a l slopes f a i l u r e  cause debris flowage downslope.  Debris flows  Debris flows are rapid movements of material of high water content 20  to 80 percent p a r t i c l e s coarser than sand (Bloom, 1 9 7 8 ) .  tinguished from mudflows by t h e i r coarser texture.  and  They are d i s -  On alpine c o l l u v i a l  slopes in the Wasp basin debris flow channels are present flanked by levees and terminating i n small, lobate cones at the change of slope to the v a l l e y f l o o r ( s i t e J 1 5 ; Photo. 3 ) .  A pre-condition for debris flowage i s the avai-  l a b i l i t y of fine-grained material; on the slope i l l u s t r a t e d t h i s material derives p r i n c i p a l l y from underlying Pleistocene t i l l .  Slopes constructed of  Holocene t i l l appear to be less susceptible to t r a n s f e r of material by t h i s mode. for  This i s attributed more to the r e l a t i v e l y small upslope area available  c o l l e c t i o n of the mass of water and sediment than to inherent s t a b i l i t y .  Below t r e e l i n e the slopes along the main v a l l e y transmit debris flows which originate on steep g l a c i a l t i l l slopes, form leveed channels debris accumulations  at the slope base.  The process may  in transit  and  be p r i m a r i l y res-  ponsible f o r the construction of the a l l u v i a l cones along the lower v a l l e y walls (Photo,  4).  Large debris flows were not observed i n operation i n the study area, however, one such event was witnessed along the Ryan River during the November rain-on-snow event.  Planar f a i l u r e of an undercut  till  slope at  1,200 m elevation contributed material to a steep t r i b u t a r y channel.  Along  the channel material was both deposited i n coarse-textured levees and  en-  trained by the dense, r a p i d l y flowing s l u r r y .  At the slope base, at 275  m,  3  successive lobes bearing boulders as large as 10 m  i n a matrix of f i n e r  sediment were c a r r i e d into the Ryan River over a period of several hours. Some of these were s u f f i c i e n t l y large to momentarily block the flow of the r i v e r which had a flow width of 16 m, a mean depth of 4 . 5 m and an estimated mean v e l o c i t y exceeding 2 m/s. Numerous run-out channels, levees and small 1. It might be noted here that the erosional consequences to the downstream channel of the Ryan River, which was i t s e l f i n f l o o d , were enormous. 1  Approximate scale at mean basin elevation: 1:37,500 Date of photography September 5, 1975  watershed divide intermittent stream channel channel flowing during September low flow period  Figure 1 8 . Colour i n f r a r e d a e r i a l photograph of study area with overlay showing drainage network.  lobes remained (Photo. 5). e)  Snow avalanches  In the Wasp basin snowfalls are heavy and i n l a t e winter and spring rapid temperature increases and high i n t e n s i t y r a i n f a l l may  occur.  conditions are i d e a l for the release, ofcwet' snowsavalanches-.  These  Slopes,  above and below t r e e l i n e were inspected at the time of spring melt during a three year, period.  Winter avalanche deposits were, r e l a t i v e l y free of sedi-  ment and the l a t e winter and spring deposits contained only small amounts of both f i n e and coarse material.  At the times of inspection the alpine slopes  were largely protected by snow hence sediment movement was  limited.  As the  snowcover diminishes transfer of debris by snow avalanching may be more significant.  These slopes display many of the c h a r a c t e r i s t i c features re-  s u l t i n g from avalanching including exposed fine sediments on upper slope surfaces, downslope size sorting, basal concavity and the cases, of channelization.  absencei?cin:i.most  Below t r e e l i n e minor deposits of debris were ob-  served which had been removed from the surfaces of the run-out slopes. These slopes are maintained  largely free of a l l trees but Alnus  sinuata,  however, t h i s cover offers considerable protection from erosion.  Experience  i n the Rocky Mountains has shown the geomorphic role of avalanches to be n e g l i g i b l e on vegetated slopes.(Luckman;- 1978).  ..  Unvegetated "avalanche  cones""^" are not present below t r e e l i n e i n the Wasp area. f)  Slow mass movements  Slow mass movement processes are i d e n t i f i e d which may  be r e l a t i v e l y  im-  portant to landscape evolution yet, by v i r t u e of t h e i r less v i s i b l e e f f e c t s , t h e i r s i g n i f i c a n c e i s more d i f f i c u l t to determine than that of the more rapid mass movements discussed above.  Reference i s made here to the most  e f f e c t i v e processes observed i n temperate mountain environments - s o i l creep, s o l i f l u c t i o n and frost  creep.  Inr'.the: Waspibas.in:*t.he>,ubiquit'6us recurved^.treeltrunks •.•andvibver-hangihg soilvmantles at convex breaks i n slope provide ample evidence of s o i l on the steeper slopes.  creep  The t i l l slopes appear to be subject to the most  rapid.-.creep", although the process may  be active as well on the lower angle  c o l l u v i a l slopes; however, as Gardner (1973) comments, the "true creep" of the talus mantle i s d i f f i c u l t to discern given the v a r i e t y of formative processes. Above t r e e l i n e s o l i f l u c t i o n i s the dominant creep process  (Carson  and  1. Luckman recommends reservation of this term for the deposits of this form and genesis which develop below t r e e l i n e .  Kirkby, 1 9 7 2 ) .  In the Wasp Creek alpine zone the c h a r a c t e r i s t i c forms  such as lobes and terraces (Washburn, 1 9 7 3 ) are developed to only a minor extent on slopes underlain by s i l t y s o i l s derived from Pleistocene and loess. 1957),  The coarse-grained  till  colluvium i s less frost-susceptible (Williams,  more r a p i d l y drained and therefore less subject to s o l i f l u c t i o n .  U p l i f t i n g of p a r t i c l e s by needle i c e growth at the s o i l surface or s o i l heave by deeper-seated f r o s t penetration may cause f r o s t creep of material v.o Needle i c e growth occurs at lower elevations i n spring and throughout the basin i n late summer and f a l l .  Since t h i s period of ac-  t i v i t y coincides with the periods of snowmelt'and storm runoff i t may be important to the loosening of fine materials f o r f l u v i a l transport.  Frost  penetration i n t h i s area of heavy snow cover i s confined to a shallow depth (Mackay and Mathews, 1974b) thus transfer by deep-seated f r o s t heave is  l i k e l y of l i m i t e d s i g n i f i c a n c e .  5.3.3.  Fluvial  processes  The network of channels detectable on the 1:14,500 scale photographs i s shown as an overlay i n Figure 1 8 .  The channels known to be flowing during  the September, 1975 low flow period are distinguished from the remaining scoured lengths v i s i b l e on the photographs7 depressions  The non-scoured, l i n e a r drainage  (discussed below) are not included i n the analysis; the r e s u l t i s  a conservative measure of drainage density. In alpine areas the network i s less developed due to the extensive sence of bedrock and coarse colluvium.  pre-  The 64 percent of the upper Wasp  basin which l i e s above t r e e l i n e has a mapped drainage density of only 0.10 2 km/km . The effectiveness of surface erosion on'the exposed alpine slopes has not been determined.  Inspection of the t i l l  slopes reveals that r i l l i n g  occurs where fine-grained sediments are present. The channels t r i b u t a r y to Wasp Creek below t r e e l i n e are confined to the more erodible g l a c i a l t i l l  and a l l u v i a l materials.  On the forested slopes 2  of the upper Wasp basin a higher drainage density of 0.25 km/km to transmit the supply of water and sediment.  has formed  Material i s conveyed i n a  range of modes from mass movements to the s t r i c t l y f l u v i a l mode. Non-channelized, l i n e a r drainage depressions till  slopes below t r e e l i n e .  -cut area.  are also developed i n the  Examples are v i s i b l e i n Figure 18 i n the c l e a r -  These features may have formed by channelization during the im-  mediate p o s t - g l a c i a l period, however, contemporary enlargement i s more l i k e l y by mass movement and solution.  They have been observed to transmit  flow only when t h e i r weathered s o i l mantles are saturated.  surface  Wasp Creek flows mainly over g l a c i a l t i l l which i t has s e l e c t i v e l y eroded to leave a coarse l a g boulder deposit r e s i s t a n t to degradation ( c f . Ponton,  1972b).  Over the greater length of channel l a t e r a l a c t i v i t y i s  r e s t r i c t e d by strongly vegetated, confining v a l l e y walls of colluvium, glacial t i l l ,  a l l u v i a l cone material or bedrock; Because of these controls  the longitudinal p r o f i l e i s one imposed by the glacially-conditioned v a l ley  slope (Figure 1 9 ) . The resultant flow regime., described as "tumbling  flow" by Peterson and Mohanty of  (i960),  has roughness elements of the scale  the mean channel depth and a conconmitant  high energy  dissipation  (Photo. 6 ) . - Along short lengths of channel l o c a l base-level controls have caused upstream aggradation allowing channel formation i n i t s own alluvium. Bed material loads along such reaches and i n pools along tumbling flow reaches are sand to cobble size. 2,000 r  6 5 4 3 Disuncc to confluence with Ryan River (km)  Figure 1 9 . Longitudinal p r o f i l e of Wasp Creek.  5.1+.  Sediment transfers - clear-cut area The focus i n t h i s section w i l l be upon the way i n which the sediment  transfer processes are altered by logging a c t i v i t i e s .  The observations of  slope processes during the snowmelt period have p a r t i c u l a r significance because the suspended sediment sampling programme i s unlikely to detect i s o lated slugs of sediment moved i n the manner described.  For the f a l l  storm  period the sediment y i e l d data give a more certain measure of the export of sediment from the clear-cut area. 5.1+.1.  Slope processes 1  a)  Debris avalanches  As elsewhere i n the Wasp basin drainage depressions and channels on the oversteepened t i l l slopes of the clear-cut area are most subject to debris avalanching.  The steepest slopes are i n the area that was logged  during 1 9 7 5 ; during the study two debris avalanches were recorded i n t h i s On May 16 a debris avalanche was i n i t i a t e d at an elevation of  area.  1,010 m on the slope below the road at s i t e F20 (Figure 1 7 ) . F a i l u r e occurred at a mean depth of 20 cm (measured normal to the f a i l u r e plane) 150 m^ of weathered t i l l materials were carried down-  on a 42° gradient.  slope by a combination avalanche-flow mode and deposited on the snow and ice  overlying Wasp Creek.  The depression within which f a i l u r e occurred  had a high sodtwater content imparted by the on-going snowmelt at the s i t e and by the deflection of snowmelt runoff from the road onto the slope.  A  second debris avalanche was i n i t i a t e d at 1^050 m elevation above the road (site G 1 9 ) .  The f a i l u r e took place at shallow depth i n a depression at an  angle of 40° and caused 125  of debris to move downslope p r i n c i p a l l y by  flowage to a lower gradient slope at the 925 m elevation.  This f a i l u r e  occurred during the early November rain-on-snow event and was not related to  road drainage.  Similarly steep slopes are found between the main logging  road and Wasp Creek at s i t e s E9 and E10.  This area may have sustained debri  avalanching p r i o r to 1 9 7 5 , however, the material would have been delivered d i r e c t l y to Wasp Creek and the erosional evidence obscured by l a t e r road work. 28°,  The remaining area of the clear-cut, standing at a mean angle of i s r e l a t i v e l y stable.  b)  Mass movements along roads  During the snowmelt period considerable slumpage of cut-slopes was observed along a 500 m section of road p a r a l l e l i n g Wasp Creek above 950 m elevation.  The slump block volumes were generally less than 10 m  3  with the  3  largest being 35 m • These f a i l u r e s most frequently took place where there was a concentration of surface or subsurface snowmelt drainage.  During  road maintenance the slumped? material'-and further- cutr-slope and road surface material was pushed over the f i l l - s l o p e . flow which ran out into Wasp Creek.  In one case t h i s i n i t i a t e d a debri  Below the 950 m l e v e l the road paral?-.::  l e l i n g the creek had been cleared by the time of study.  Cut-slopes are high  along t h i s segment (Photo. 7) hence at least similar volumes might be expected to have been moved. During the f a l l rains cut-slope slumpage was an order of magnitude less than that during snowmelt and subsequent road maintenance was not immediately required.  5.4.2.  F l u v i a l processes  The pre-logging channel network i s shown i n Figure IT.  Road construc-  t i o n e f f e c t i v e l y extends t h i s network by surface compaction  and, i n many  cases, removal of the weathered s o i l materials. To l i m i t road surface erosion runoff i s diverted onto non-channelized surfaces downslope causing i n f i l t r a t i o n of the water and deposition of the sediment.  This diversion  i s effected by various types of cross-ditches, road-side ditches, road obstructions and, on roads following contours, by i n c l i n a t i o n of the road width downslope: (or "oixtsloping" the road).  Direct storm runoff was con-  fined largely t o the road surfaces (including cut- and f i l l - s l o p e s ) and the established channels.  However, i n some cases runoff volumes were suf-  f i c i e n t to maintain flow along the non-channelized drainage l i n e s .  Inspec-  t i o n of these drainage l i n e s revealed flow and sediment transport in cases where runoff was derived from road surfaces. Where drainage was not from roads flow was observed during only the largest storm. s i g n i f i c a n t erosion of the drainage l i n e s themselves  In neither case was  observed.  The net effect of road construction f o r the entire clear-cut area was to extend the continuous drainage network by a factor of 2.5-  For the par-  t i a l l y clear-cut Basin A'.the::drainage-.densityrwas. dbubledr-from. 1.T''to 3.5 2 km/km.. Basin B, non-channelized before road construction, gained a 2 drainage density of 25 km/km . The drainage, density of Basin C was not s i g n i f i c a n t l y changed by road construction.  These measures of the l i n e a r  extension of the drainage system only p a r t i a l l y express the modification of the f l u v i a l system.  I t i s estimated that the extended network d i r e c t l y  drained 5 to 6 hectares of exposed mineral s o i l by surface and channel erosion.  This represents 4 to 5 percent of the clear-cut area.  A further  4 to 5 percent of the area i s s i m i l a r l y disturbed but was not connected to the continuous channel network. For a given length and design of road surface, erosion increases with increasing road gradient, sediment a v a i l a b i l i t y and runoff.  The most severe  road surface erosion observed was along the steep (10° to 15°) segments of the recently constructed road at s i t e s FIT to"F21.  During the snowmelt 3  period gullying of t h i s road caused removal of approximately 40 m  of sedi-  ment (Photo. 8). "Less readily measureable contributions from sheet erosion and r i l l i n g would be added to t h i s t o t a l .  Sediment entrained along t h i s  road was carried into either Wasp Creek or Stream C. During the snowmelt period then, f o r this one 500 m segment of road, a conservative figure 3  would be an export of 50 m  of sediment by f l u v i a l a c t i v i t y alone.  5.5.  Sediment sinks - undisturbed  area  There i s substantial storage of unconsolidated basin.  sediment i n the Wasp  The products of recent g l a c i a l and subaerial erosion are added to  an extensive mantle of Pleistocene t i l l .  Storage of c o l l u v i a l and g l a c i a l  sediments has been at a time scale of m i l l e n i a .  Transfers from one sink  to another may occur without making the material available to the channel network.  Deposits which experience  shorter terms of storage are those  which are connected to, or may be transported to the channel network and are s u f f i c i e n t l y fine-grained for f l u v i a l transport.  In the areas affected  by surface runoff, p a r t i c u l a r l y i n the alpine zone, coarse materials which r e s i s t transport occur extensively. Sediment entrained by f l u v i a l action may i n turn be deposited for long periods.  The t o t a l alpine area that i s lake-drained represents 27  percent of the basin.  The two largest of these lakes, draining 20 percent  of the basin, are e f f e c t i v e sinks for a l l but. the f i n e s t wash-load f r a c t i o n This i s p a r t i c u l a r l y s i g n i f i c a n t for these lakes drain the apparently most active g l a c i e r s .  The a l l u v i a l cones along the main Wasp v a l l e y also con-  s t i t u t e e f f e c t i v e sediment sinks although for p o t e n t i a l l y shorter periods. Although the material i n these cones may be re-entrained by f l u v i a l action, as for other deposits a f r a c t i o n i s too coarse for transport along the main channel.  Along Wasp Creek a l l u v i a l sediment i s stored along lower gradient  reaches i n both bed and banks which may be moved during the larger floods. 5.6.  Sediment sinks - clear-cut area As elsewhere i n the basin the storage time of g l a c i a l deposits i s long  where they are removed from f l u v i a l action and r e l a t i v e l y short along the drainage network.  Along the altered network sediment sinks are present  wherever the energy for transport i s d i s s i p i t a t e d .  Along roads such d i s -  sipation occurs at gradient changes and i r r e g u l a r i t i e s on road and f i l l -slope surfaces.  Flow diversion onto non-channelized areas w i l l cause sedi  ment to be deposited if. the distance to the next road or channel i s s u f f i c i e n t l y long.  Along channels sediment i s deposited along lower gradient  reaches, i n pools and by obstructions such as log debris.  The sinks within  the drainage network are, i n most cases, r e l a t i v e l y short term; successive storms transport material further downslope. are f i l l e d and, assuming a constant  With time the e f f e c t i v e sinks  sediment supply, shorter term sediment  storage and higher sediment y i e l d result* 5.7-  Sediment a v a i l a b i l i t y - undisturbed  area  An examination of the s u r f i c i a l deposits i n the undisturbed  area  reveals that the character of sediment sources and transfers above and below t r e e l i n e can be d i f f e r e n t i a t e d .  In Figure 20 the d i s t r i b u t i o n of s u r f i c i a l  materials and the c h a r a c t e r i s t i c landforms are shown for the undisturbed area of the Wasp for which sediment y i e l d data have been c o l l e c t e d .  Only 25  percent of the alpine zone i s overlain by g l a c i a l t i l l materials containing s i g n i f i c a n t proportions, of fine-grained sediment; the remaining area i s bedrock, g l a c i a l i c e and the generally coarse-grained colluvium and felsenmeer. This l i m i t e d extent of fine-grained materials and the low drainage density 2  of  0 . 1 0 km/km  processes.  indicates a dominance of mass wastage over f l u v i a l transfer  Much of the sediment moved i s returned to storage without con-  t r i b u t i n g material to the drainage network.  Below t r e e l i n e g l a c i a l  till  overlies 65 percent of the area, slopes are steeper, and a higher drainage density of 0 . 2 5 , km/km  has developed.  The most e f f e c t i v e transfers occur  along the steep t r i b u t a r y creeks which incise the t i l l .  A l l u v i a l cones are  constructed of material, thus transported and these cones i n turn provide a source of sediment available f o r removal from the basin.  Along Wasp Creek  the t i l l deposits, are eroded along some reaches, however, the c o l l u v i a l deposits are generally':too coarse.  Alluvium deposited by Wasp Creek i s  available for transport but has only a minor presence. A conceptual model has been constructed to summarize the system of c l a s t i c sediment transfer and to i l l u s t r a t e the most important (Figure 2 l ) . to  processes  I t i s concluded that, by virtue of i t s texture and a v a i l a b i l i t y  the f l u v i a l system, g l a c i a l t i l l  sources are most important and that at  the watershed scale Pleistocene t i l l sources below t r e e l i n e dominate. 5.8.  Sediment a v a i l a b i l i t y - clear-cut area The transfers adjudged to be r e l a t i v e l y important f o r the undisturbed  area (shown on Figure 21) are the same for the clear-cut area.  The change  has been of the r e l a t i v e magnitude, frequency and extent of operation of the processes.  To a drainage network which has been more than doubled i n extent  are connected exposed s o i l surfaces representing k to 5 percent of the c l clear-cut area.  As w e l l , t e r r a i n disturbance has increased, the susceptibir-.i  l i t y of slopes t o mass f a i l u r e .  The limited sediment storage within the  altered f l u v i a l system, and the p a r t i c u l a r juxtaposition of roads, channels and steep slopes have rendered the activated sediments readily available for  delivery t o Wasp Creek.  Figure 20.  V e r t i c a l zonation of sediment sources i n upper Wasp Creek watershed.  Figure 21.  Wasp Creek watershed c l a s t i c sediment transfer model.  BEDROCK primary weathering  glacial erosion Pleistocene till  ^  loess ^  felsenmeer mass wastage and/or fluvial transport  glaciai erosion  -•o  •(>  Holocene till  i  glacial eriosion  mass wastage  <  HoioceneorV Pleistocene y till X  mass wastage and/or fluvial transport  mass wastage  t  ~ r  Jluvial _ J transport r  fluvial transport  alluvium  lacustrine  lake drainage fluvial transport YIELD  |  | sediment source, sink y extra-watershed origin  <^ O  decision -•  transfer  mmm^  important transfer  <^^^  output  6.  Sediment y i e l d  6.1.  Introduction The  study p e r i o d was l i m i t e d t o s i x months hence t h e data a r e i n t e r -  preted t o d i s t i n g u i s h the effects of the p r e v a i l i n g hydrologic conditions and t h e d i f f e r i n g a v a i l a b i l i t y o f sediment b u t n o t t o e l u c i d a t e l o n g e r term geomorphic changes. 6.2.  Sediment y i e l d r e s u l t s  6.2.1.  Snowmelt p e r i o d - May t o J u l y  D u r i n g t h e snowmelt p e r i o d o f May t o t h e end o f J u l y no c o n s i s t e n t v a r i a t i o n o f sediment c o n c e n t r a t i o n w i t h d i s c h a r g e was i d e n t i f i e d f o r t h e lower Wasp Creek s t a t i o n .  The v a r i a b i l i t y o f t h e d a t a was caused p r i n c i -  p a l l y by t h e e f f e c t o f t h e on-going l o g g i n g o p e r a t i o n s , i n p a r t i c u l a r t h e r o a d maintenance and v e h i c l e t r a f f i c .  The mean c o n c e n t r a t i o n o f t h e IT  samples was n e v e r t h e l e s s r e l a t i v e l y l o w a t 21 mg/l ( s t d . dev.: 21 m g / l ) .  2 The  estimated  sediment y i e l d i s 600 t ( l 8 t/km ) f o r t h e p e r i o d  (Table  C.I.). For t h e upper Wasp Creek s t a t i o n t h e continuous commenced June 21. was  discharge  The mean c o n c e n t r a t i o n o f t h e t w e l v e  v e r y l o w a t 6 mg/l ( s t d . dev.: 7.h m g / l ) .  record  samples t a k e n  F o r these -samples--sediment -  c o n c e n t r a t i o n and t h e r e f o r e sediment t r a n s p o r t r a t e v a r i e d d i r e c t l y w i t h d i s c h a r g e a l l o w i n g d e f i n i t i o n o f t h e sediment r a t i n g curve i n F i g u r e The  22.  strongest simple r e l a t i o n t e s t e d describes a l o g a r i t h m i c f u n c t i o n o f  the form: l o g T = 3.591ogQu - 3.30 T = snowmelt p e r i o d sediment t r a n s p o r t r a t e (t/h) = d i s c h a r g e a t upper Wasp (m / s ) i = 0.97 s t a n d a r d e r r o r ( f o r l o g u n i t s ) = 0.352. s  where:  (6)  5  2  2  A p p l y i n g t h i s r e l a t i o n a y i e l d o f 65 t (3 t/km ) i s e s t i m a t e d period.  The c o r r e s p o n d i n g  for the  e s t i m a t e f o r t h e lower Wasp i s 230 t (about  7 t/km ). 2  6.2.2.  Summer p e r i o d - August and September  Throughout t h e summer p e r i o d Wasp Creek sediment c o n c e n t r a t i o n s were g e n e r a l l y v e r y low.  W i t h t h e e x c e p t i o n o f t h e s i n g l e storm e v e n t , con-  c e n t r a t i o n s e x c e e d i n g 5 mg/l were not d e t e c t e d and t h e sample means a t b o t h s t a t i o n s were i n t h e range o f 2 t o 3 mg/l.  S i n c e t h e d a t a a r e scant  and l i e g e n e r a l l y w i t h i n t h e d e t e c t i o n l i m i t s o f t h e a n a l y s i s no cant d i f f e r e n c e i n t h e means can be a s s i g n e d .  signifi-  F o r t h e upper and lower  52  53 stations the y i e l d s are estimated at 3*+ t (1.6 respectively.  2  2  t/km ) and h3 t (1.3  t/km )  The s p e c i f i c y i e l d i s somewhat higher for the upper "basin;  t h i s difference i s an a r t i f a c t of the higher discharge i n t e n s i t y and i s not known to be s i g n i f i c a n t . summer storm are h.h and 9-5  The sediment y i e l d estimates f o r the single t (0.2  and 0.3  t/km ) for the upper and lower  stations respectively.  6.2.3.  F a l l storm period - October and early November  At the upper Wasp station, with the exception of the November rain-on-r -snow event, sediment concentrations during the f a l l storm period were very low.  The f i v e samples taken near the peak flows of the October storms had  concentrations less than 10 mg/l through a flow range of about 1.5 m /s. of  to 2.5  The single sample taken during the November storm had a concentration  715 mg/l.  Taken together with the summer storm data 21 observations  were available, to define a sediment rating curve f o r storm runoff events (Figure 22).  The strongest simple r e l a t i o n was the logarithmic form:  . log T = U.091ogQ - 2.82 where:  T  (7)  u  r  = storm period sediment transport rate (t/h)  r  i = 0.999 2  standard error (for l o g units) = The  95 percent  0.191.  confidence i n t e r v a l on the slope of t h i s r e l a t i o n i s  ±0.55  thus i t i s not s i g n i f i c a n t l y different from the slope of equation 6. Considerable strength i s imparted to equation 7 "by the four order of magnitude range of the data.  The confidence l i m i t s show the band within which  the l i n e would l i e at the 95 percent l e v e l .  Howevermore data could en-  large these l i m i t s or show that such a simple "'form does not apply over the entire range.  Applying t h i s r e l a t i o n the y i e l d for the October period of  2 baseflow and stormflow together was estimated at"10 t (0.5  t/km ). The  2 estimate of the November storm y i e l d i s approximately 300 t (lh t/km ). At the lower Wasp station sediment concentrations during the f a l l storm events ranged to a maximum of 1,610 storm.  mg/l recorded during the November  The t o t a l sediment y i e l d from October 1 to November 6 was 955 t  2  2  (29 t/km ) of which an estimated QkO t (25 t/km ) or 88 percent was produced during the November storm. 6.3. Sediment discharge regimen In this section the regimen of sediment discharge at each gauging station i s discussed with respect to the control exerted by the variable hydrologic conditions and sediment a v a i l a b i l i t y through the study period.  6.3-1.  Upper Wasp Creek  Throughout b o t h snowmelt and storm p e r i o d s t h e h i g h e r f l o w s were most s i g n i f i c a n t i n terms o f t o t a l work done. ment d u r a t i o n curve ( F o s t e r , 1934)  ( S t r a u b , 1935)  For the snowmelt p e r i o d t h e  together with the flow d u r a t i o n  show t h a t 50 p e r c e n t  o f t o t a l y i e l d was  flows, t h a t o c c u r l e s s than 7 p e r c e n t  discharged  o f the time ( F i g u r e 23).  sedi-  curve during  For  the  storm p e r i o d the h i g h e r f l o w s and dependent sediment t r a n s p o r t r a t e s are not known w i t h s u f f i c i e n t a c c u r a c y  t o i n d i c a t e more t h a n t h a t the November  storm s u p p l i e d about 95 p e r c e n t o f the p e r i o d ' s y i e l d .  Storm r u n o f f  a more e f f e c t i v e agent o f e r o s i o n t h a n snowmelt r u n o f f a t a g i v e n  was  discharge  ( F i g u r e 2 2 ) ; however, t h e sediment produced d u r i n g t h e snowmelt p e r i o d o f l a t e June and J u l y exceeded t h a t o f the October storm p e r i o d . a greater frequency r u n o f f may  By v i r t u e o f  o f h i g h f l o w s and a l o n g e r p e r i o d o f a c t i v i t y  snowmelt  do t h e most work t h r o u g h the l o n g e r term.  F o r the snowmelt p e r i o d , g i v e n t h e r e l a t i v e l y low sediment  concentra-  t i o n s and t h e i r s y s t e m a t i c v a r i a t i o n w i t h d i s c h a r g e , f l u v i a l e r o s i o n o f glacial t i l l  and i t s r e d i s t r i b u t e d p r o d u c t s  i s c o n s i d e r e d most i m p o r t a n t .  D u r i n g storm e v e n t s sediment y i e l d was  t r o l l e d by t h e v a r i a b l e o c c u r r e n c e basin.  a l o n g t h e steep t r i b u t a r y  creeks con-  o f p r e c i p i t a t i o n as snow t h r o u g h o u t the  Snow a c c u m u l a t e d d u r i n g October above t h e 1,100  m t o 1,400  e l e v a t i o n s ; h y p s o m e t r i c a n a l y s i s shows t h a t sediment and r u n o f f would thus be r e s t r i c t e d t o the lower  5 percent  m  production  of the b a s i n area.  The  c h a r a c t e r o f sediment movement i n t h i s zone and t h e v e r y low sediment  F i gure 23. Upper Wasp Creek f l o w and sediment d i s c h a r g e d u r a t i o n f o r June 21 t o J u l y 31, 1975-  curves  y i e l d suggest only minor f l u v i a l erosion of the available material and l i t t l e mass movement a c t i v i t y .  For the summer storm and the rain-on-snow  event the contributing area of sediment and runoff extended to a l l elevations and y i e l d s were correspondingly higher.  The high suspended sediment  concentration of the rain-on-snow event may r e f l e c t the increasing importance of mass movement contributions to the channel network as storm s i z e , watershed saturation and contributing area increase. The summer low flow observations at the upper Wasp station are of interest with respect to g l a c i a l sediment supply.  Sediment production  from glaciers would be high during t h i s period of high rates of ablation and g l a c i a l erosion.  The very low sediment concentrations measured pro-  vide no evidence of significant g l a c i a l sediment y i e l d . 6.3.2.  Lower Wasp Creek  Comparison of the upper and lower Wasp Creek records provides an i n dication of the amount of sediment removed from the clear-cut area. For the snowmelt period, however, the effects of t e r r a i n disturbance are better i l l u s t r a t e d by the measurements made within the clear-cut area i t self.  The estimated y i e l d at the lower Wasp station i s higher (the con-  tributions from the upper Wasp being included) and of the same order as that of the mass of sediment observed to have been delivered from the c l e a r -cut area.  Although  i n agreement with the slope observations the':yield  data lack the resolution to further characterize the regimen of sediment discharge.  I t i s estimated that the s p e c i f i c y i e l d at the lower station  exceeded that at the upper station by a factor of two to three during snowmelt (Table C . l . ) . For the f i v e storms for which adequate data are available the sediment y i e l d s are p l o t t e d against storm runoff and r a i n f a l l (Figure 2k). sion r e l a t i o n i s not defined yet the trend suggests*-an  A regres-  exponential rate of  increase of y i e l d over a two order of magnitude range of r a i n f a l l and runoff. In general, for progressively larger storms the f l u v i a l processes at a s i t e become more e f f e c t i v e and sediment a v a i l a b i l i t y increases with the expanding drainage network.  A d d i t i o n a l l y , slopes are increasingly susceptible to  f a i l u r e as s o i l moisture contents increase. For the summer storm the low sediment production rate r e l a t i v e to storm size for the clear-cut area i s attributed to the low antecedent  mois : i_  ture conditions and therefore both the less extensive drainage network and the lower s u s c e p t i b i l i t y to slope f a i l u r e .  The s p e c i f i c y i e l d at the lower  station was greater than that at the upper station by a factor of 1.5; i n  Rainfall (mm)  1  10  10  2  1 I I III  1  <  1  SB  1  • 5  3  1 I I I I  Storm date: 1. August 27 2. October 3 3. October 14 4. October 17  5. November 4  4  «  •  2  3«  0 1 0  l 10°  1  rainfall-sediment yield datum • runoff-sediment yield datum •  • 3  I  • 4  I  1  I I 1 II I  10 Storm runoff (mm) 1  I  I  I I III I 10  2  Figure 24. Lower Wasp Creek sediment y i e l d response to storm runoff events.  I  t h i s case the r e l a t i v e effects of t e r r a i n disturbance are masked by sediment and runoff production from higher elevations of the basin. During October the snowline remained below the t r e e l i n e allowing comparison of s p e c i f i c y i e l d s from clear-cut and forested slopes. The rates 2 . 2 were 9-3 t/km for the forested area and at least 80 t/km for the clear-cut area assuming that only the portion below 1,250 m was producing runoff and sediment. During the November storm, although y i e l d s from the clear-cut area were very high, the r e l a t i v e effects of t e r r a i n disturbance at the watershed scale were less than during the smaller October storms.  As noted for the  summer storm the contributions from the lower, forested slopes could not be discerned against the background of sediment delivery from a l l elevations of the basin.  The lower Wasp s p e c i f i c y i e l d s exceeded those of the upper  Wasp by about two times. For the s i x month study period the sediment production rate on the 2 clear-cut slope was about 750 t/km . The rate f o r the undisturbed basin, 2 without adjustment for the variable contributing area, was 23 t/km . 6.3.3.  Wasp Creek tributary streams  Sediment samples were taken during storm periods i n Streams A, B and C draining the clear-cut slope (Table C.2.).  The sediment concentration data  confirm the singular importance of roads upon accelerated erosion.  Basins  A and B, which experienced substantial drainage network extension and mineral s o i l exposure as a r e s u l t of road construction, had sediment concentrations two to four orders of magnitude higher than those for Basin C which had v i r t u a l l y no drainage network a l t e r a t i o n .  In addition, the data  show Basin C to have had concentrations lower than those at the upper Wasp station; the lake above the clear-cut apparently acts as a sink for sediment delivered from the undisturbed slopes of the basin allowing the effects of the clear-cut area to be i s o l a t e d .  No increased sedimentation related to  clear-cutting and timber removal independent of road effects was observed. 6.4.  Conclusion In t h i s section the observed  sediment y i e l d s have been related to the  physical c h a r a c t e r i s t i c s of the basin and to the variable hydrologic conditions.  Rates of sediment production during snowmelt and storm runoff  periods are shown to be s i g n i f i c a n t l y higher on clear-cut than forested slopes.  7. 7.1.  Discussion and conclusions V e r t i c a l zonation of geomorphic processes A d i f f e r e n t i a t i o n of sediment sources and transfers i n the study area  suggests that the area below t r e e l i n e i s of dominant importance to basin sediment y i e l d .  In t h i s zone slopes are steeper, g l a c i a l t i l l i s extensive  and a higher drainage density has developed to convey material by f l u v i a l action and mass movement to the main channel.  In the alpine zone mass  movement a c t i v i t y i s widespread yet much of the material i s returned to storage without contributing sediment to the channel system. Evidence for the r e l a t i v e importance  of f l u v i a l processes i n this- zone i s less conclusive  surface erosion of the exposed s o i l surfaces may make f l u v i a l a c t i v i t y more s i g n i f i c a n t than i s suggested by the limited extent of the channel network. 7.2.  Seasonal analysis The sediment y i e l d data show that whereas the absolute quantities of  sediment passing each gauging station d i f f e r e d markedly the proportions i n passage during each season were r e l a t i v e l y s i m i l a r .  As much as UO percent  of the s i x month t o t a l sediment'yield, may have been discharged during the three month snowmelt period; y i e l d s of the two month summer period represented less than 10 percent of the t o t a l ; and the October to early November storm period accounted for the remaining proportion of at least 50 percent. Of this l a t t e r amount more than 85 percent (95 percent for the upper Wasp basin) was removed during the large rain-on-snow event. Throughout most of the study period, f o r both clear-cut and undisturbed areas, sediment activated by f l u v i a l processes was quantitatively most s i g nificant.  At the watershed scale the variable d i s t r i b u t i o n of snow was  an important control upon the contributing area of runoff and sediment. During the snowmelt period sediment concentrations were low and varied d i r e c t l y with melt runoff.  Higher concentrations at a given discharge were  caused by storm runoff.  Sediment y i e l d s from the area below the snowline  exhibited a direct v a r i a t i o n with storm magnitude. Sediment activated by g l a c i a l processes was not detected at either the Wasp Creek or Stream C gauging stations.  The fine sediment deriving from  t h i s source i s both l o s t to lacustrine and minor a l l u v i a l sinks and diluted by runoff from the 9^ percent of the basin area which i s presently ungla^ cierized. Slope processes at the watershed scale become increasingly important as s o i l moisture content and storm.magnitude increase.. Increasing s o i l moisture supplies the requisite external stress f o r slope f a i l u r e and an  expanded drainage network renders these sediments more available to basin sediment y i e l d .  Swanston  (1970)  and O'Loughlin  (1972)  report that a 24-  -hour r a i n f a l l of 150 mm w i l l saturate the weathered t i l l horizons i n steep drainage depressions.  Assuming a similar response i n the study area  saturation of these sites was achieved during the November storm and poss i b l y during one additional storm as a result of high antecedent moisture contents.  The slope f a i l u r e observed i n the clear-cut attests to the  i n s t a b i l i t y of these slopes under such conditions. Sediment movement through a watershed to eventual y i e l d i s t y p i c a l l y viewed as a continuum of slope to f l u v i a l transfers. glacial t i l l  The presence of  i n the study area modifies this concept somewhat; g l a c i a l  sediments are available to the f l u v i a l system without the necessity of slope mass movements.  Slope processes nevertheless supply material yet  channels incise the mantle of t i l l to contribute sediment d i r e c t l y to basin yield.  For the periods f o r which f l u v i a l processes are adjudged to be  dominant the observed y i e l d s may have derived either from r e d i s t r i b u t e d sediments or d i r e c t l y from g l a c i a l t i l l which has been'substantially stable since deposition. determination.  The data base does not permit a more d e f i n i t e  The assessments made of the r e l a t i v e importance of slope  and f l u v i a l processes to the activation of sediment are applicable only at the watershed scale f o r the sequence of events observed.. A further q u a l i f i cation i s that geompr.phic' processes take place i n response to progressive change i n the system hence not a l l at  physiographically similar s i t e s respond  the same time to a given external stress (Schumm,  1973).  I t i s clear  that evaluations of the geomorphic evolution of a system must be based on a long-term.  study.  The data reported here are suitable only to the assess-  ment of clear-cut effects and of the undisturbed basin's response over the observation period. 7.3.  Effects of t e r r a i n disturbance Specific sediment y i e l d s at the lower Wasp station exceeded those at  the station above the clear-cut by a factor greater than two during the s i x month study period. 2 km of  I f the excess s p e c i f i c y i e l d i s assigned to the 1.15 •  clear-cut then rates of sediment delivery from t h i s area exceeded those the undisturbed area by more than 30 times.  This difference was con-  t r o l l e d i n large measure by the variable sources of runoff at the watershed scale.  During the October storm period, when sediment delivery was from  lower elevations only, rates of erosion i n the clear-cut area were about eight times those of the forested slopes.  This magnitude of accelerated  erosion i s more representative of the effects of disturbance upon the forested slopes than the observations made while a l l levels of the basin were producing  sediment and i s within the range observed elsewhere i n  the P a c i f i c Mountain System (cf. Anderson,  Fredriksen,  1970;  1970).  In Oregon and northern C a l i f o r n i a Anderson found the e f f e c t s of roads to generally account f o r 80 percent of the increased y i e l d s .  In the  study area, however, due i n large part'to the absence of slash burn e f f e c t s , the accelerated erosion was caused almost e n t i r e l y by roads. t i o n extends the surface.drainage  Road construc-  network, under-cuts and steepens slopes,  and brings about higher concentrations  of runoff downslope; together these  changes cause higher rates of sediment transfer by cut-slope slumping and debris avalaiching below roads and by accelerated removal of exposed material by f l u v i a l action.  The pattern of roads caused the continuous drainage net-  work to be extended by 2 . 5 times and to e f f i c i e n t l y drain exposed surfaces covering k to 5 percent of the clear-cut area.  An a d d i t i o n a l , equivalent  proportion was exposed by road construction but was not connected d i r e c t l y to the channel system, thereby allowing activated sediments to be returned to storage on the slopes. The lower road traversing the area clear-cut during 1975 experienced the most severe mass movement and surface erosion.  This road had been recently  constructed and, as noted elsewhere (Fredriksen, t i a l l y high production of sediment. resulted i n increased erosion.  1965),  this causes an i n i -  Continued maintenance of the road also  A d d i t i o n a l l y , i n this area of the clear-cut  t i l l slopes are n a t u r a l l y oversteepened and susceptible to f a i l u r e when moisture contents are high.  One shallow debris avalanche during the snow-  melt period occurred below the road on a slope of 3 9 ° ; a second occurred during the rain-on-snow event on a k0° slope independent of road e f f e c t s . Observations i n other areas show the greatest frequency of f a i l u r e of weathered t i l l materials to be i n the range 3 6 ° to ^ 0 ° (Bishop and Stevens, 196h;  Swanston,  1969;  O'Loughlin,  1972).  Above the h0° gradient s o i l s are  increasingly thin and less subject to s i g n i f i c a n t mass movement a c t i v i t y . It  has been demonstrated that the effect of roads i n the clear-cut  area has been to reduce the storm size required to exceed the resistance to both slope f a i l u r e and surface erosion.  The higher frequency storms are  thereby rendered more important than i n undisturbed areas. however, has addressed only the short-term range of hydrologic conditions.  This study,  effects related to a limited  Over the longer term i t may be expected  that road surfe.ce erosion would supply decreasing quantities of sediment  and that, the slopes would adjust by f a i l u r e to the oversteepened condition and disrupted drainage.  (Gradual root decay, which may provide additional  stress (Bishop and Stevens, here.)  I96U;  Swanston,  1970),  has not been investigated  The duration and character of t h i s recovery would be dependent upon  the p a r t i c u l a r combination of hydrologic conditions experienced and the management practices 7.4.  adopted.  Conclusions The central conclusions of this, study may be summarized as follows: 1)  C o l l u v i a l deposits are extensive i n the study area yet, p a r t i c u -  l a r l y i n alpine areas, mass wastage appears- to effect mainly a r e d i s t r i b u t i o n of materials on slopes with l i m i t e d supply of sediment to the channel system.  Pleistocene and Holocene t i l l and minor a l l u v i a l cone sediments are  more available for export and this i s achieved most e f f e c t i v e l y by snow and debris avalanches, debris flows and f l u v i a l processes which transmit sediment along the steep t r i b u t a r y channels below t r e e l i n e to the main channel. Relatively l i t t l e a l l u v i a l material i s stored i n the banks of Wasp Creek itself. 2) of  During the snowmelt and the October storm periods the activation  sediment by f l u v i a l processes i s thought to have been most important at  the watershed scale.  Storm runoff was a more e f f e c t i v e agent of erosion  than snowmelt runoff at a given discharge, however the greater duration of high flows during the snowmelt period caused rates of sediment transport to be 2-3 times greater than during the October storm period. 3)  The estimated 10-year recurrence i n t e r v a l rain-on-snow event i n  November supplied 52 and 60 percent of the s i x month t o t a l y i e l d from the lower and upper basins respectively.  The high sediment transport rates  during this event suggested that delivery of sediment to the channel network by slope mass movements was 4)  signifcant.  The low sediment transport rates of the summer period indicate  that sediment produced i n the 6 percent g l a c i e r i z e d area has l i t t l e  im-  portance at the watershed scale.. 5)  Road construction caused the continuous drainage network to be ex-  tended by a factor of 2.5 and to drain d i r e c t l y exposed mineral s o i l covering 4-5 percent of the clear-cut area. of  Under-cutting of slopes and deflection  sediment and water over f i l l - s l o p e s accelerated sediment delivery to  Wasp Creek by both f l u v i a l and mass movement a c t i v i t y .  Debris avalanches  were i n i t i a t e d at gradients of 3 9 ° and 40° within the clear-cut area. 6)  For the s i x month study period sediment y i e l d s from the clear-cut  area were at least 30 times greater than y i e l d s from the area of the watershed.  undisturbed  This can be attributed p a r t i a l l y to the curtailment  of erosion by a snow cover at. elevations above the clear-cut.  During the  October storm period sediment y i e l d s from the slopes below t r e e l i n e could be isolated and rates of sediment production were about 8 times greater on the clear-cut than the forested slopes.  This accelerated y i e l d  was  caused almost e n t i r e l y by road e f f e c t s . 7 • 5 • Further work This study provides a description of sediment sources and t h e i r r e l a t i v e contributions to basin y i e l d which d i f f e r e n t i a t e s clear-cut and turbed areas.  To describe adequately the response of the  undis-  undisturbed  mountain watershed more d e t a i l i s required at the s i t e and basin scales and, as for any s a t i s f a c t o r y hydrologic record, a longer term of monitoring i s indicated.  I t i s f e l t that further work within a basin of more manageable  size would be valuable. that would allow: detailed monitoring  The study area should be of a scale and  a thorough description of the  character  s u r f i c i a l materials;  of the p r e c i p i t a t i o n inputs and; inspection of the  important sediment source areas during the course of an i n d i v i d u a l hydrologic event of perhaps a 2^-hour duration.  Accompanying studies of the  importance of bed load sediment transport are also required.  Bibliography Anderson, H.W. , 1 9 7 0 . Relative contributions of sediment from source areas and transport processes. In: Eroc. Symp. on Forest Land Use and Stream Environment, Oregon State Univ., C o r v a l l i s , p. 55-6.3. B.C. Department of Agriculture, no date. Climate of B r i t i s h Columbia: Tables of Temperature, P r e c i p i t a t i o n and Sunshine: Report for 1975 B.C. Ministry of the Environment, seasonal. 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Dyrness and R.L. Fredriksen, I 9 6 7 . Hydrologic and 'related.characteristics of three small watersheds i n the Oregon Cascades. P a c i f i c Northwest For. and Rg. Exp. Stn. , U.S. Dept. of A g r i c , Portland, 5h p. Ryder, J.M. ,.1972.. Pleistocene chronology and g l a c i a l geomorphology: studies i n southwestern B r i t i s h Columbia. In: Mountain Geomorphology, H.O. Slaymaker and H.J. McPherson (eds.), Tantalus Research Ltd., Vancouver, p. 6 3 - 7 2 Schumm, S.A., 1 9 7 3 . Geomorphic thresholds and the complex response of drainage . systems. In: F l u v i a l Geomorphology, M.. Morisawa (ed.), Publics, i n Geomorphology, State Univ. of New York, Binghamton, p. 2 9 9 310 Slaymaker, H.O., 1 9 7 ^ . Rates of operation of geomorphological processes in the Canadian C o r d i l l e r a . In: Geomorphologische Prozesse und Prozesskombinationen i n der Gegenwart, H. Poser (ed.), p. 3 1 9 - 3 3 2 Slaymaker, H.O., 1 9 7 7 - Estimation of sediment y i e l d i n temperate alpine environments. In: Erosion and S o l i d Matter Transport i n Inland Waters, proc, of the Paris symp., I.A.S.H. -AVI.S.H. Pub. No. 122 Slaymaker, H.O. and T. G a l l i e , 1979- Mountain watershed solute sources: implications for nivation. Canadian Assoc. of Geographers Abstracts, Annual Meeting 7 9 , C.N. Forward (ed.), Univ. of V i c t o r i a Slaymaker, H.O. and R.E. G i l b e r t , 1 9 7 2 . Geomorphic process and land use changes i n the Coast Mountains of B.C.: a case study. Symp. Int. de Geomorphologie, Liege, France, p. 2 6 9 - 2 7 9 Slaymaker, H.O. and H.J. McPherson, 1 9 7 7 . An overview of geomorphic processes i n the Canadian C o r d i l l e r a . Zeits. fur Geomorph. 2 1 ( 2 ) : \ n ' ~ " '  169-186  Slaymaker, H.O. and L.J. Zeman, 1 9 7 5 - Influences of a l t i t u d e and continent a l i t y on watershed hydrology i n the Coast Mountains of B r i t i s h Columbia. In Canadian Hydrology Symposium p r o c , National Research  Council, Assoc, Comm. on Hydrology, p. 224-232 Straub, L.G., 1935. S i l t investigation on the Missouri River basin. House Document 238, 73rd. Congress, 2nd. Session, Appendix XV, p. 1125-1140 Swanson, F.J. and C.T. Dyrness, 1975Impact of c l e a r - c u t t i n g and road construction on s o i l erosion by landslides in the western Cascade Range, Oregon. Geology 3(7): 393-396 Swanson, F.J. and D.N. Swanston, 1977. Complex mass-movement t e r r a i n in the western Cascade Range, Oregon. Geol. Soc. of America Reviews i n  Eng. Geol., 3: 113-124  Swanston, D.N., 1967. Geology and slope f a i l u r e i n Maybeso Valley, Prince of Wales Island, Alaska. Unpub. Ph.D. d i s s e r t a t i o n , Michigan State Univ. 206 p. Swanston, D.N., 1969. Mass wasting i n coastal Alaska. 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Geological Survey of Canada B u l l e t i n 196, 89 p. & maps U.S. Water Resources Council, 1977Guidelines for determining flood flow frequency. Hydrology Committee, B u l l e t i n 17A, Washington, D.C. Washburn, A.L., 1973. P e r i g l a c i a l Processes and Environments. Edward Arnold (Publishers) Ltd., London, 320 p. Water Survey of Canada, annual. Surface Water Data B r i t i s h Columbia, Fisheries and Environment Canada, Ottawa Water Survey of Canada, 1977. H i s t o r i c a l Streamflow Summary B r i t i s h Columbia to 1976. Fisheries and Environment Canada, Ottawa Williams, P.J., 1957Some investigations into s o l i f l u c t i o n features i n Norway. Geogr. Journal ,123: 42-55 Williams, R.C., 1964. Sedimentation i n three small forested drainage basins in.the Alsea River basin, Oregon. U.S.G.S. Circular 490, 16 p. Woo, M. and H.0. Slaymaker,.1975• Alpine streamflow response to variable snowpack thickness and extent. Geog. Ann. 3-4(57A: 201-212 Woodsworth, G.J., 1977. Geology Pemberton map-area (sheet 92J). Geological Survey of Canada, O.F. 482 Wooldridge, D.D., 196H. E f f e c t s of parent material and vegetation on properties related to - s o i l erosion i n central Washington. S o i l Science Soc. Amer. Proc. 28(3): 430-432 Zeman, L.J. and H.0. Slaymaker, 1975Hydrochemical analysis to d i s c r i m i nate variable runoff source areas i n an alpine basin. A r c t i c and Alpine Research 7(H): 3H1-351  66  P H O T O G R A P H S  67  68  Photograph k. View up Wasp Creek showing coarse colluvium on right and a l l u v i a l cone on l e f t .  Photograph 6. View up Wasp Creek above the upper Wasp gauging station.  Photograph 7. glacial t i l l .  Road cut-slope i n clear-cut area  exposing  Photograph 8.  Gullying of road surface during snowmelt period.  71  A P P E N D I C E S  Figure A . l .  Wasp Creek seasonal hydrographs.  Appendix B.  Sample Analysis Procedure Table B,l.'  Steps: 1.  store the 275 ml samples i n dark for 30 days before analysis;  2.  draw o f f the top 100 ml into a tared evaporation dish and weigh;  3.  withdraw a second 100 ml and dispose;  k.  agitate remaining f r a c t i o n of sample containing s e t t l e d suspended load  sediment and then draw o f f into a second tared evaporation dish and weigh; 5.  dry samples i n oven at 90°C u n t i l a l l water has evaporated;  6.  reweigh both dishes to determine weight of organic and dissolved and  suspended mineral sediment; 7-  burn o f f organic matter at 1+00°C for 1+5 minutes; and  8.  reweigh both dishes to calculate weight of dissolved and suspended load  mineral sediment.  Appendix C.  Suspended Sediment  Data  Table C . l . Suspended Sediment Y i e l d Data Summary  Specific Yield  Yield Period  upper Wasp tonnes  Snowmelt - May 8 to July 31  l65  Snowmelt - June 21 July 31  65  Summer low flows Aug. 1 to Sept. 30  3h  Summer storm Aug. 27 October baseflow and stormflow  a  k.k .10  (t/km2)  lower Wasp  study study perioo. tonnes perioo.  upper Wasp  lower Wasp  18  6oo.  37  7-7  13  230  lit  3.0  7.0  2.3  2.7  1.6  1.3  0.8  0.6  0.2  0.3  1.5  7.2  0.5  3.5  7.0  6.8 0.9 2.0  h3 9-5 115  —  8-5.  0.5  0.3  October Ik storm  —  2.6  0.2  0.1  October 17 storm  November k storm Total study period a. "b-  68 6.0b  —  300 order  - 500  upper. Wasp .  33  October 3 storm  October 31 storm  lower. Wasp  6o 100  3k0 1,608  k.2  •  100  2.1 0.2  O.k  52  2.3  Ik  26  1.9  23  kg  2.1  Estimate made to compare stations - flow record not continuous, Estimate based on Figure 2k.  Table C.2. Sediment Concentration Data for Wasp Creek Tributary Streams Stream  Date 11/5 27/8 3/10 3/10 5/10 5/10 5/10 5/10 5/10 9/10 14/10 16/10 16/10 16/10 27/8 27/8  15/9 3/10 3/10 5/10 5/10 5/10 5/10 5/10 9/10 14/10 16/10 16/10 16/10 27/8 27/8 15/9  3/10 5/10 9/10  16/10 16/10  17/10 4/11  Time  Stage (cm) Sediment ••^Concent r a t i o n (mg/l)  0920  1020 2020 0915 1115 1515  1715 1835 1305 1830 1130 1315 2250  0920 1515 1105 1020 2020 0915 1115 1515 1715 1835 1305 1830 1130 1315 2250 1200 1430 1305 1030 1615  1405 1250 2310 1435 1200  71.'0 68.5 67.5  68.0 66.5 67.5 68.0 68.0 68.0 66.0 66.0 63.5  22 500 31 235 16  -14 260  25 64 123 338 93 256 600  59.0 59-5 57.0 58.0  200 22,300 694 5,730 14,200 1,820 855 9,470 2,640  58.O  1,860  59-0 57-5 58.5 58.5 59.0  717 6,770 4,790  63.0 61.0  58.O  7,960  14,100 <1 6 2  78.0 60.0 67.O 62.0  63.0  <i <l <i <i 2 <1 6  

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