ICE DAM BREAKOUT AT BERING GLACIER, ALASKA

A normal 25-30 year surge cycle causes temporary modifications of the peripheral drainage system that includes a major ice dam breakout from lakes on the eastern sector into Vitus Lake. Mid August 1989 ice front retreat from Bentwood Narrows uncovered a new low threshold, thus causing an initial gradual lowering of Tsivat and Tsiu Lakes followed within days by a breakout. Standing waves in a 100 meter-wide channel cut headward into an ice knickpoint of Niagara proportion. Within approximately two weeks Tsiu and Tsivat Lakes dropped 17 m thus releasing approximately 170 million cubic meters of water and causing the drainage to shift from Abandoned River.

With the onset of the 1993 surge, Bentwood Narrows was once again closed and lake levels rose to reoccupy Abandoned River. Unlike conditions during the previous surge (1965-67), a large, continuous debris band oriented parallel to the ice front was draped across Bentwood Narrow in 1995. Rapid subsequent retreat (50-75 m/y) and ice thinning (10 m/y) monitored by annual surveys during 9 years following the1993-95 surge were notably retarded by the insulating effects of the debris band, thus forming a resistant “plug” within the Narrows. Measurements prior to 2006 indicate lake levels up-gradient of the plug progressively increased through the summer melt season. However, the 2006 records show that lake level receded in late June, suggesting a gradual, yet progressive leakage of the plug and its potential failure before the end of the summer melt season.