GLACIAL LEGACY AND VALLEY RELIEF CONTROL ON LANDSLIDE MOBILITY IN NW WASHINGTON

In 2014, the catastrophic SR 530 debris avalanche-flow claimed the lives of 43 people and buried a small community near the town of Oso in NW Washington. The so-called Oso landslide is notable not only for the magnitude of damage it caused, but also for its high mobility (Iverson et al., 2015). The ability for landslides to fluidize and run out over long distances can depend strongly upon the lithology and material properties of the hillslope before failure (e.g., Iverson et al., 2000; 2015), and central questions remain as to where Oso-like failures have occurred in the past, where they may occur in the future, and what the necessary ingredients are in the landscape to create such a highly mobile slide. Toward this end, we use LiDAR data for NW Washington and characterize the mobility of ~150 landslides across a range of lithologic and geomorphic settings. The highest mobility landslides, as characterized by a higher ratio of runout length (L) to fall height (H), occur in Pleistocene glacial outwash and ice-marginal (e.g., lacustrine) deposits that spatially cluster with the maximum extent of the Cordilleran ice sheet (median L/H of ~6, maximum L/H of ~12). Landslides in consolidated till deposits, in contrast, have a median L/H closer to ~4 and a maximum measured L/H of ~6. High mobility slides are located in valleys where rivers have transiently incised gorges and carved wide valley floors in response to the retreat of the Cordilleran ice sheet. Runout in excess of 2 km can occur in terrace deposits less than 200 m thick, sufficient distance to traverse most of the valley floors within this region. Landslide runout within ice marginal sediments increases with relief until a height of ~200 m, where we then observe a large scatter in mobility. The observed difference in L/H values suggests that material property differences in consolidated tills and ice-marginal sediments exert a control on slide mobility. For example, laboratory data from Easterbrook (1964) show that ice-marginal drift deposits have an average porosity 1.5 times greater than tills, which may put them closer to a critical threshold for fluidization (e.g., Iverson et al., 2015). Together these findings suggest that landslide mobility in NW Washington is closely tied to the sedimentology of the ice-marginal environment and its geomorphic boundary conditions.