SEDIMENT SOURCE AND HYDROCLIMATIC INFERENCES FROM GLACIAL LAKE-SEDIMENTS - THE POSTGLACIAL SEDIMENTARY RECORD OF LILLOOET LAKE, BRITISH-COLUMBIA

Lillooet River drains 3850 km2 of partly glacier-covered terrain before entering Lillooet Lake in the southern Coast Mountains of British Columbia. The elongated lake covers an area of 21 km 2 and forms a deep basin with water depths to 137 m. Acoustic profiling of the subbottom and sampling of the surface sediments reveals that total sediment thickness varies from 30 + m in the north half of the lake near Lillooet delta and then declines to less than 16 m in the south. Up-valley ice retreat approximately 11 000 years BP resulted in conformable sediments in the main lake indicative of turbidity currents off Lillooet delta which infilled and flattened the underlying surface. A sill which separates the main and south basins prevents the down-lake progression of turbidity currents resulting in conformable deposits indicative of rain-out (i.e. settling) processes only. A major acoustic reflector at about 6-9 m below the modern sediment surface is associated with a well-dated volcanic eruption and sediment yield event in the upper basin at 2400 years BP. Modem glaciolacustrine deposition forms varves which accumulate at a rate of up to 28 mm year-1 in the north and decline to less than 0.9 mm year-1 in the south. The occurrence of two sediment-runoff regimes, one average and the other extreme leads to distinct differences in varve sedimentology and varve thickness. De-coupling the two signals using sedimentary evidence alone cannot be done consistently so a sediment yield-runoff relation for the lake (r2 = 41%) contains considerable 'noise'. An annual sediment accumulation chronology covering the last 125 years shows a much higher frequency of 'extreme' runoff-sediment yield events during the post-1940 interval. This parallels a documented change in climate of the region after 1945 and suggests that a longer varve chronology would provide a good, high-resolution, proxy record of hydroclimatic variations.