CONNECTING HEADWATER DEBRIS FLOWS TO SUSPENDED SEDIMENT EXPORT IN THE LOWER SUIATTLE RIVER, GLACIER PEAK, WASHINGTON STATE

Debris flows propagating from steep headwater canyons on Pacific Northwest (PNW) volcanoes have far reaching impacts extending past their alpine origins down to lowland population centers. The Suiattle River Basin, which drains the eastern flank of Glacier Peak stratovolcano in the North Cascades of Washington State, has a history of debris flows throughout the 20^th and 21^st centuries that are associated with significant changes to river morphology and damage to infrastructure. Additionally, the Suiattle is known to have an anomalously high suspended sediment load compared to other rivers draining stratovolcanoes in the region. The geomorphic explanation for the persistently turbid water on the Suiattle remains poorly constrained: To what extent are debris flows responsible for the anomalous suspended sediment load on the Suiattle? In this study, we leverage historical accounts, field observations, dendrochronology, and remote sensing to constrain the timing, magnitude, and fate of debris flow sediment in the basin. Historic high magnitude debris flows were identified in the Upper Suiattle Basin as evidenced by terraces observed in the field. A minimum bound on material eroded in the Upper Basin from mid-20^th century catastrophic debris flows was estimated at ~14% of the mean annual suspended sediment load (t/yr). In the lower river, modern lower magnitude and higher frequency suspended sediment pulses are recorded at a USGS streamgage (12189500), where spikes in turbidity, independent of discharge, are interpreted as debris flow events. Over the period of record (2011-09-23 to 2021-04-13), we found that ~11% of the mean annual suspended sediment load (t/yr) was attributable to modern debris flows. Initial results point to smaller contemporary debris flows being associated with dry days and high daily temperature anomalies. Ongoing analysis is focused on identifying processes driving debris flows and their connection to magnitude frequency relationships in the study area. This work is a step toward understanding how sediment supplied from alpine mass wasting events shapes downstream geomorphic processes, with implications for how climate change may alter cascading hazards in these systems.