A DECADE OF RAPID SEDIMENTATION IN PROGLACIAL, ICE-CONTACT LAKES, BERING GLACIER, ALASKA

Repetitious bathymetric surveys of Tsivat and Tsiu Lakes reveal a decade of rapid modification in lake basin shape, size, and morphology. Following the 1993-95 surge, multiple ice front vents conveyed a six-fold increase in suspended fine sediment from subglacial tunnels directly into the lakes. Bathymetric maps of both lake basins from 1990 through 2000 indicate changes in water depth, which when referenced to a stable datum, serve as a proxy for progressive sediment accumulation, thus yielding estimated rates of sedimentation. Rates prior to the surge (1990-1993) ranged from 0.6 to 1.2 m/year, whereas greater turbidity during the surge was accompanied by increased accumulation at rates of 2.2 to 3.1 m/year. Sediment settling was supplemented first by a surge-related outburst (July, 1994) that prograded a 1.5 km² sandur filling one-third of Tsivat Lake basin, followed in 1997 by rapid delta growth from vents on the Tsivat ice front. The sum of bathymetrically-determined accumulation in Tsivat Lake from 1991 to 1996 was nearly 75,000,000 m³. By June, 1998, an additional 49,000,000 m³ was added, thus filling the remaining basin. The impact of diminished, post-surge turbidity was partially offset by sediment bypassing and 26,000,000 m³ delta growth into Tsiu Lake from 1997-2000. Although an estimated 33,000,000 m³ of suspended sediment escaped the system during the decade of measurement, nearly 194,000,000 m³ remains within these lake basins.