Daniel Scott, Brian D. Collins

Mass 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 portion of those rivers’ sediment supply and pose significant mass movement hazards. However, the quantitative importance of this sediment source is unknown, and the magnitudes, spatial distribution, styles, and controls of these mass movements have only been characterized over limited spatial and temporal extents. 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 1946 river kilometers for which repeat LiDAR was available (71% of the 2736 total river kilometers flanked by terraces), mass movements eroded 853,300 ± 19,500 m3/yr. This sediment source is low in river networks, well connected to streams, has a substantial coarse-grained and durable component, and is dominated by frequent, relatively small mass movements, all of which increase its significance to sedimentation in developed, lowland reaches. However, rates of terrace sediment delivery vary between basins and between adjacent terraces, which are stratigraphically laterally heterogeneous. While lateral fluvial erosion is usually necessary to initiate terrace mass movements, fluvial characteristics 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 between and within watersheds in sediment influx, sediment characteristics, and failure mechanisms.