GLACIAL OUTBURST FLOOD: 15-YEAR CYCLE OF THE ICE-CONTACT TSIU LAKE, BERING GLACIER, ALASKA

During the 1993-95 surge event, the eastern Bering Piedmont Lobe advanced into the basin of the ice-contact Tsiu Lake. The surge dammed the low elevation narrows connecting Tsiu Lake and the larger Vitus Lake, and caused Tsiu Lake to drain through a higher elevation outlet via the Abandoned River channel. Between 1995 and 2006, the ice steadily retreated across the foreland and lake basin, filling the basin with ice- marginal and subglacial sediments.

In August 2006, Tsiu Lake drained over a 7-day period, in which the lake level dropped 17 m, beheading the Abandoned River channel. Although it seemed likely that an ice-contact channel would develop to carry the released water directly to Vitus Lake, instead, drainage proceeded through englacial conduits. Englacial storage of the lake water was evidenced by the appearance of water-filled lacunas on the surface of Bering Glacier and the 1 m decrease in the level of Vitus Lake. Englacial drainage was observed along the adjacent ice front.

By the summer of 2007, the 300 m-wide band of lacunas along the margin had drained and the roof of the main ice tunnel collapsed, exposing a meandering ice-walled canyon. The drained Tsiu lake basin exposed shoreline features, deltaic sediments covering the basin floor, and a drowned forest (likely 2,000 years old) buried in lake sediments. The 2010-11 surge of Bering Glacier has once again dammed the lower elevation outlet and reformed Tsiu Lake, which currently drains through the Abandoned River channel.