| Summary: | Sediment from Lillooet River basin, 3000 km² of alpine and subalpine landscape about 10% of which is glacierized, is largely deposited in Lillooet Lake.
Dissolved sediment concentration in the streams flowing into Lillooet Lake is generally less than 100 mg l⁻¹. Suspended sediment concentrations vary from 50 mg l⁻¹ to over 3000 mg l⁻¹, related closely to water discharge. Coarse grained sediment which moves as bed load is deposited as foreset beds at the delta lip. Suspended sediments are distributed through the lake (a) by dispersion and interflow above the thermocline at low to moderate inflow rates and (b) by interflow and underflow at higher inflows. The thermocline effectively limits vertical sediment movement
during much of the melt season though during high flows the thermal structure of the lake may be temporarily destroyed by underflow and powerful interflow.
The delta surface consists of rapidly shifting channels and sand bars. Bed load movement is sporadic and changing lake levels lead alternately to accumulation in backwater when the lake level rises or flushout as the lake level falls. Coarse sediment from bed load, deposited on the proximal, slope is redistributed by infrequent major slump events. Mounds of slumped material cover the foreset slope to its base at 120 m depth. Interflow and underflow give rise to a distinctive sedimentary sequence of varved silts and clays interspaced with turbidites. Thickness of the varves is related to characteristics of the inflowing water and fine grained sediment. Continuous records of water temperature near the lake bottom indicate that frequent laminae within varves are due to intermittent underflow of suspended fluvial sediment. Autumn floods result in the deposition from suspension in the lake water of an anomalously thick, fine grained layer during the following winter. Infrequent much smaller winter inflow peaks result in underflow and the deposition of a lighter toned, coarser grained lamina in the winter deposit. Both are useful stratigraphic markers.
Continuous seismic profile records indicate that as much as 200 m of sediment has been deposited in the lake. Calculation based on present rates of accumulation of 20 - 50 mm yr⁻¹, depending on distance from the delta, indicates that reflecting horizons in the sediment represent increased sediment input associated with volcanic activity 2400 years B.P. and, less reliably, to Pleistocene glaciation. Annual
long term (1913-1969) clastic sediment yield at Lillooet Lake is calculated as 1.1 x
10⁶m³yr⁻¹ (specific denudation 0.28 mm yr⁻¹). The advance rate of Lillooet
Delta indicates that yield may have trippled after 1950 associated with river
engineering, logging and agriculture. === Arts, Faculty of === Geography, Department of === Graduate
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