Geomorphic and sedimentological signature of a two-phase outburst flood from moraine-dammed Queen Bess Lake, British Columbia, Canada

On 12 August 1997, the lower part of Diadem Glacier in the southern Coast Mountains of British Columbia fell into Queen Bess Lake and produced a train of large waves. The waves overtopped the broad end moraine at the east end of the lake and flooded the valley of the west fork of Nostetuko River. The displacement waves also incised the outflow channel across the moraine. Stratigraphic and sedimentologic evidence supports the conclusion that the flood had two phases, one related to wave overtopping and a second to breach formation. Empirical equations were used to calculate the peak discharge of the flood at various points along the west fork of the Nostetuko valley and to describe the attenuation of the flood wave. The velocity of the flood was also calculated to determine the time it took for the flood to reach the main fork of Nostetuko River. The highest peak discharges were achieved in the upper reach of the valley during the displacement phase of the flood. Peak discharge declined rapidly just below the moraine dam, with little change thereafter for approximately 7 km. Empirical formulae and boulder measurements indicate a rise in peak discharge in the lower part of the west fork valley. We suggest that flow in the upper part of the valley records the passage of two separate flood peaks and that the rise in discharge in the lower part of the valley is due to amalgamation of the wave and breach peaks. Hydraulic ponding in confined reaches of the valley extended the duration of the flood. In addition, erosion of vegetation and sediment in the channel and valley sides may also have exerted an influence on the duration and nature of flooding. Sediments were deposited both upstream and downstream of channel constrictions and on a large fan extending out into the trunk Nostetuko River valley. This study extends our understanding of the variety and complexity of outburst floods from naturally dammed lakes. It also shows that simple empirical and other models for estimating peak discharges of outburst floods are likely to yield erroneous results. Copyright (C) 2005 John Wiley Sons, Ltd.