Proglacial sediment trapping in recently formed Silt Lake, Upper Lillooet Valley, Coast Mountains, British Columbia

The sedimentology of proglacial Silt Lake was assessed by lake sediment coring and monitoring of lacustrine processes during a late-summer period of high glacier melt to characterize sediment delivery from the heavily glacierized catchment and investigate the sediment trapping dynamics of this upland lake. A complete varve chronology was established for a distal basin of the lake which was exposed by Lillooet Glacier retreat between 1947 and 1962. The varve record showed decreasing sedimentation rates in the basin while the glacier retreated, and as the lake became free of ice contact in the early 1970s. Although recession has continued over recent decades, and glacier proximity to the lake has, therefore, continued decreasing, lacustrine sedimentation rates are now accelerating due to changing basin morphometry caused by delta progradation. Over shorter time scales, lake sedimentation patterns respond to changing runoff conditions, including late-summer glacier melt intensity, intra-annual flooding events, diumal runoff fluctuations, and within-lake turbidity currents. Turbidity currents included quasi-regular flows during high diurnal discharges and an episodic flushing of temporarily stored sediment from the sandur or delta at a time of low stage. Suspended sediment yield to Silt Lake is estimated to exceed 10³ Mg km^-2 a^-1, a magnitude that surpasses previous local and regional yield estimates for the glacierized headwaters of the Lillooet River valley. Since Silt Lake currently traps a significant prooportion of that upland sediment supply, and the trapping efficiency of the basin has been variable at decadal time scales, the formation and continued development of Lilt Lake has likely had a significant influence on downstream sediment delivery. Lacustrine sediment-based proxies of longterm hydroclimatic variability being developed in glacially distal settings should include provisions for dynamic sediment trapping effects in upstream water bodies that often form in the active proglacial environment.