THE 1938 CHOCOLATE GLACIER DEBRIS FLOW, GLACIER PEAK VOLCANO, NORTH CASCADES, WASHINGTON

Stratigraphy and sedimentology of debris flow terraces in the Chocolate Creek and Suiattle River drainages reveal that deposits from a single debris flow in 1938 spread over a 20 km reach downstream from the present terminus of Chocolate Glacier, the main ice stream from the summit of Glacier Peak volcano. Terrace thickness is over 2 m in places and partially bury dead, standing trees. Sediment analysis from terraces yield <3-5 wt.% clay sized sediment, indicating a non-cohesive debris flow. Climate records reveal the most probable triggering event – 11 cm of precipitation on October 10-12, 1938.

Climate records from Olga, WA show a mean surface temperature increase of 0.6°C between 1892 and 1938. Previous research on Chocolate Glacier indicates glacial recession from 1890 to 1946, with a noted rapid recession from 1920-1940. Similar periods of long-term glacial recession of other Cascade glaciers have created voluminous, stagnant glacial termini that have the potential to collapse and catastrophically release outburst floods. This process has been recognized on two other Cascade glaciers. 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). Smaller, more frequent glacial outburst floods at Cascade Range volcanoes are commonly triggered by either prolonged hot weather or intense rainfall. The 1938 Chocolate Glacier debris flow is an example of a more significant hydrologic hazard – a large debris flow formed by collapse of stagnant ice, most probably also with a meteorologic trigger.