Glacier Peak and the Chocolate Factory: Recurring debris flows from the eastern flank of Glacier Peak stratovolcano, North Cascades, Washington State, USA

Alpine mass wasting events can have wide ranging impacts that extend past their headwater origins reaching down to lowland population centers. 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 the volcano. Despite its known importance to the regional sediment budget, the specific geomorphic drivers of the anomalous sediment load on the Suiattle have received relatively little attention in the literature. In this study, I build on previous work to explore the magnitude, timing, triggering mechanisms, and the spatial distribution of sediment loading events in the Suiattle River Basin. My historical analysis shows that major debris flow activity initiated in the late-1930s, with a total of nine historic debris flows since then (RI = 9.3 years). One previously unreported circa late-1940s debris flow was identified from reanalysis of dendrochronology (Slaughter, 2004) and historical aerial imagery. From topographic differencing, I placed a minimum bound of ~4.9 M m3 (±0.6 M m3) on the material incised from the most recent valley filling debris flow deposits. Historical accounts suggest that major debris flows happen at the hottest times of the year in the absence of precipitation, with two eyewitness accounts of debris flows triggered by glacial outburst floods. Historical photos, remote sensing, and field measurements of terrace heights suggest that incision into historic debris flow deposits occurs soon after deposition and tapers after the first few years. To examine smaller more recent debris flows, I created a framework to automatically extract debris flow timing, duration, and magnitude from USGS turbidity and discharge data over the period 2011 to 2020. I identified 28 individual debris flow events that occurred in every year in the record. To evaluate triggering mechanisms, I calculated prior day maximum temperature anomalies for all non-debris flow days and for days when a debris flow started. Debris flow start days were shown to be statistically warmer than non-debris flow days (mean of -0.21 °C and 2.48 °C, respectively; ks test, dm = 0.314, p = 0.007). This suggests that minor debris flows are triggered by high temperatures and, like the historical major debris flows, points to glacier outburst floods as the primary initiation process. I estimate suspended sediment loads attributable to minor debris flows, anomalous sediment flushing events following debris flows, and suspended sediment loads outside of these categories. Together debris flows and flushing account for ~21% of the mean annual load on the Suiattle. At Glacier Peak, Chocolate Glacier is unique. Its high propensity for glacier outburst floods makes it the dominant source of debris flows and suspended sediment, vastly outweighing contributions from other glaciers on the mountain. The frequency and magnitude of debris flows from Chocolate Glacier bare similarities to South Tahoma Glacier at Mount Rainier. Combined, my findings show that debris flows deliver large quantities of sediment to the mainstem river at both annual and decadal timescales. This work is a step toward understanding how sediment supplied from alpine mass wasting events shapes downstream geomorphic processes. My findings have implications for how ongoing climate change may alter cascading hazards in these systems