LITHOLOGIC CONTROLS ON THE LONG-TERM DOWNSTREAM FATE OF A SEDIMENT PULSE, SUIATTLE RIVER, WA

In 1938 a debris flow emanating from the northeast flank of Glacier Peak deposited nearly 5 million cubic meters of sediment in the upper reaches of the Suiattle River in the Washington Cascades. Since then, at least 1 million cubic meters of this material has been excavated from the deposit and transported downstream. We use this event to explore the long-term downstream effects of a large sediment pulse. In an era of major dam removals, as well as concerns about increased mass wasting hazards due to climate change, studies of sediment pulse transport and attenuation have both scientific and societal relevance. How much disruption will we see downstream? What pulse characteristics determine the downstream response? Here, we present field and remote sensing data to quantify the magnitude of the downstream response to the 1938 event and understand the role of sediment lithology and grain size on pulse dynamics. Beyond the headwaters of the Suiattle, the long-term downstream effects of the 1938 event appear to be muted. Since the 1930s, a USGS streamgage site 80 km downstream from the deposit has seen less than 0.3 m vertical change in bed elevation. Downstream measurements of bed material grain size and lithology suggest that rapid abrasion of friable volcanic bedload, abundant in the 1938 deposit, is a contributor to the attenuation of the sediment signal. These field observations are supported by tumbling experiments, in which pumaceous volcanics loose mass rapidly during transport, while the abrasion of non-vesicular volcanics is negligible. The 1938 deposit is predominately composed of gravel and cobbles. However, we hypothesize that the friable nature of this coarse material may lead to far field, long-term effects that are dominantly felt as an increase in suspended sediment rather than coarse bed material aggradation. Further work is needed to model the morphodynamics of the pulse, and to connect these findings to suspended sediment dynamics.