Frequent Mass Movements From Glacial and Lahar Terraces, Controlled by Both Hillslope Characteristics and Fluvial Erosion, are an Important Sediment Source to Puget Sound Rivers

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AbstractMass movements from glacial and lahar terraces in the middle and lower reaches of rivers draining the Washington Cascade Range to Puget Sound may represent a substantial but poorly quantified portion of those rivers' sediment supply and pose significant mass movement hazards. We used repeat LiDAR elevation data, aerial imagery, and well logs to quantify and characterize terrace sediment delivery in nine major watersheds over a median period of 12 years. In the 1,946 river kilometers for which repeat LiDAR was available (71% of the 2,736 total river kilometers flanked by terraces), 167 mass movements eroded 853,600 ± 19,400 m3/yr. Analysis of mass movement frequency and volume indicates that terrace sediment delivery is dominated by small, frequent mass movements, as opposed to large, infrequent ones like the 2014 Oso landslide. This sediment source is low in river networks, well connected to streams, and has a substantial coarse‐grained and durable component, all of which increase its significance to sedimentation in developed, lowland reaches. However, rates of terrace sediment delivery vary among basins and between adjacent terraces, which are stratigraphically laterally heterogeneous. While lateral fluvial erosion is usually necessary to initiate terrace mass movements, valley bottom geometry and terrace stratigraphy poorly predict erosion volume, which is better predicted by hillslope geometry and mass movement style. Effective management of sedimentation and mass movement hazard should acknowledge the importance of terrace sediment delivery and the variability among and within watersheds in sediment delivery, sediment characteristics, and failure mechanisms.