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Sedimentology of a freshwater tidal system, Pitt River-Pitt Lake, British Columbia Ashley, Gail Mowry

Abstract

Pitt River, 30 km inland from. Vancouver, British Columbia at the southern margin of the Coast Mountains, links Fraser River estuary and Pitt Lake. Salt water seldom extends to within 10 km of Fraser - Pitt confluence; nevertheless, tides modulate Fraser flow and cause Pitt River to fluctuate 2 m and Pitt Lake as much as 1.2 m. There is an upstream movement of sediment in Pitt River from Fraser River, evidenced by identical mineralogy of Pitt River and Fraser River sediments, a decrease in grain size from the Fraser to Pitt Lake, and a predominance of flood-oriented bedforms in the river channel. A delta of 12 km2 area has accumulated at the lower (draining) end of the lake. The purposes of the study were to: (1) examine aspects of the hydrodynamics of Pitt River and Pitt Lake as a tidal system; (2) evaluate the effect of bidirectional flow on river and delta morphology; (3) determine processes of sediment movement in the river and of-sediment dispersal on the delta; and (4) estimate present sedimentation rate on the delta. Water Survey of Canada stage data from 3 locations in the system, used in conjunction with velocity measurements (profiles and tethered meter), revealed large seasonal and tidal variations in discharge. Calculations indicate that flood basal shear stress peaks early in the flow, whereas ebb currents have a lower basal, shear stress which peaks late in the flow. Thus, sediment moves farther forward on a flood flow than it moves back on the succeeding ebb. Studies of the river channel using hydrographic charts revealed regular meanders (^M = 6100. m) and evenly spaced riffles and pools which are scaled to the strongest flow (winter flood current, Qe). Meander point bars are accreting on the "upstream" side indicating deposition by the flood-oriented flow. The three dimensional geometry of the large-scale bedforms which cover the sandy thalweg of both river and delta channel was determined by echo sounding and side-scan sonar. Three distinct sizes (height/spacing = 0.8 m/10-15m; 1.5m/25-30 m; 3 m/50-60 m) of large-scale bedforms (sand waves) were found; their linear-relationship of height vs. spacing (XD) on log-log plot suggests a common genesis. The size appears to be related to channel geometry, not to depth of flow. Largest forms are found in reaches which shallow in the direction of water movement and smallest forms occur on relatively flat topography. The following tentative relationship is suggested for sandy meandering rivers: ^M/^B = Qe. Pitt delta morphology was studied with aerial photos and depth soundings. Its shape is considered an excellent example of sediment diffusion and deposition from a simple jet into a low energy lacustrine environment. Analysis of 190 sediment samples from river, delta, and lake bottom shows the sediment to be polymodal. Graphical partitioning of the cumulative probability plots reveals that sediments are composed of up to 4 log-normal distributions. Each distribution is interpreted as a population related to a process of sediment transport. Five subenvironments in the Pitt system are characterized by unique combinations . of these "process" populations. Cores in the delta topsets and lake bottom sediments reveal silt and clay rhythmites, interpreted as varves. The coarse layers are deposited during winter when discharge of Fraser River is low and tidally induced discharge in Pitt system is high. The fine layers are deposited during spring run-off when additional fines are added to the lake from the Pitt basin. 137Cs dating of sediments shows that as much as 1.8 cm/yr are accumulating in the active portions of the delta with an estimated 150 +/- 20 X 103 tonnes deposited annually.

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