QGG ARTHUR D. HOWARD AWARD: THE MAGNITUDE, TIMING, AND CONNECTIVITY OF DEBRIS FLOW SEDIMENT TO THE SUIATTLE RIVER, NORTH CASCADES, WASHINGTON STATE, AS RECORDED BY TREE RINGS AND TERRACE MORPHOLOGY

Sediment connectivity between alpine headwaters and populated lowlands shape the downstream cascade of hazards that may be exacerbated by climate change. The Suiattle River, which drains the eastern flank of Glacier Peak in the North Cascades of Washington State, is a dominant contributor of suspended sediment in the region. Normalized for drainage area, the Suiattle River supplies more suspended sediment than nearly any other river in the region and more than twice as much as the White Chuck River, which drains the western flank of Glacier Peak. The geomorphic explanation for this anomalous sediment load remains poorly constrained. Partially buried Douglas fir ‘ghost forests’ along the headwater tributaries of the Suiattle record a history of intermittent landscape stability that has been disturbed multiple times throughout the 19^th and 20^th centuries by violent, valley filling debris flows. More recently, pulses of suspended sediment from lower magnitude debris flows have occurred at ~5 to 10-year intervals. Glacial sediment production has been proposed as another possible source explaining the persistently turbid water. To what extent are each of these sources responsible for the anomalous sediment load in the Suiattle? As a first step to answer this question, here we leverage historical accounts, field observations, dendrochronology, and remote sensing to constrain the magnitude, timing, and fate of the 19^th and 20^th century debris flow sediment. Cross-correlation of ‘ghost’ tree rings with known master chronologies provides precise kill dates reflecting when debris flows occurred and subsequently drowned trees. Rings from living trees found on forested debris flow terraces provide minimum ages of geomorphic surfaces. Raster differencing of a LiDAR DEM and an interpolated ‘pre-erosional’ valley fill surface provides a minimum bound on sediment volume eroded from catastrophic mid-20^th century debris flows. We predict that these debris flow deposits can explain the anomalously high suspended sediment loads observed downstream. This work is a step towards understanding how sediment supplied from alpine mass wasting events shapes downstream geomorphic and ecological processes, with implications for how climate change may alter cascading hazards in these systems.