QUANTIFYING DEBRIS FLOW CONTRIBUTIONS TO BASIN-SCALE SEDIMENT SUPPLY, SUIATTLE RIVER, NORTH CASCADES WASHINGTON STATE

The Suiattle River, which drains Glacier Peak stratovolcano in the North Cascades Mountain Range of Washington State, is a major contributor of sediment to the Puget Sound. Normalized for drainage area, it contributes two times more suspended sediment than basins on the west side of Glacier Peak and two times more than similar basins in the region. This anomalous sediment load has been attributed to glacial sediment sources from Glacier Peak. However, streams draining glaciers within the basin remain understudied in terms of field investigation of headwater sediment sources. We build on the work of Slaughter (2004), combining historical accounts, field observations, and analysis of remote sensing data to better constrain the timing and magnitude of debris flows on the upper Suiattle River. We correlate and map cut-and-fill terraces using remote sensing data to highlight the geomorphic record of debris flows in the upper basin. Historical photographs and accounts suggest that the upper Suiattle valley went from forested to bare in the late 1930s implying that there was a sudden onset of sediment supply from this portion of the basin. Debris flow events reported throughout the 19^th, 20^th_, and 21^st centuries are associated with damage to infrastructure and abnormally elevated suspended sediment levels. Initial dendrochronology work confirms that a high magnitude debris flow is likely to have occurred during the 1930s, but the question remains as to whether subsequent events are simply reworking the 1930s deposit, or if they are providing additional sources of sediment. This study is an important step towards understanding how extreme sediment supply from alpine headwater basins may shape hazards to both human infrastructure and endangered salmonid habitat.