ORIGIN AND BEHAVIOR OF THE 1938 CHOCOLATE CREEK DEBRIS FLOW FROM CHOCOLATE GLACIER, GLACIER PEAK VOLCANO, NORTH CASCADES, WASHINGTON

Chocolate Glacier is the main ice stream from the summit of Glacier Peak volcano. In 1938, distal stagnant ice resulting from long-term glacial recession collapsed, catastrophically releasing an outburst flood that rapidly bulked (entrained) sufficient sediment to transform to a debris flow. The debris flow inundated forests and formed terraces with several meters of deposits along Chocolate Creek, extending 4.6 km to the confluence with the Suiattle River. Forests of dead, standing trees extend at least 10 km downstream and evidence of flooding extends >70 km.

The flow is an example of an important hydrologic hazard at volcanoes in the Cascade Range. Two analogs that also resulted from long-term glacial recession are recognized: 1) The distal 1.6 km of Deming Glacier at Mount Baker collapsed in 1927, probably triggered by a local earthquake. 2) The distal 1.6 km of Kautz Glacier at Mount Rainier collapsed in 1947, triggered by intense rainfall. Both collapses yielded debris flows causing significant damage to transportation networks (highway and a railroad). Other glacial outburst floods at Cascade Range volcanoes are triggered by intervals of either hot weather or significant rainfall. Our continuing analysis of the Chocolate Glacier flow will determine its trigger, the processes of transformation both proximally to and distally from debris flow, and the characteristics of the glacier and its setting, with the goal of indicating what other glaciers at Cascade Range volcanoes could be at risk of collapse in the future.