ABSTRACT: Sedimentology of a tidal lake, Pitt Lake, British Columbia, Canada

CGRG Bibliography of Canadian Geomorphology
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Abstract

Pitt Lake is situated in a relict fjord within the Coast Mountains of British Columbia, 30 km inland from the port of Vancouver. The lake is freshwater, but is directly connected to the ocean via Pitt River and Fraser estuary. Although the Fraser estuary and the entire Pitt system is tidal, salt water seldom reaches to within 10 km of the Fraser-Pitt confluence. Flow reverses in response to tides, causing Pitt River to fluctuate 2 m and Pitt Lake as much as 1.2 m per tidal cycle. 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 km^2 area has accumulated at the lower (draining) end of the lake. Morphology of the delta is considered to be an excellent example of sediment diffusion and deposition from a simple jet into a low energy lacustrine environment. 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 and the tidal effect is significantly reduced because of increased run-off. 137-Cs 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 + or - 20 x 10^3 tonnes deposited annually.

Bibliography of Canadian Geomorphology