SEDIMENT FROM MOUNT RAINIER, WASHINGTON, USA: GENERATION, TRANSPORT, AND IMPACT ON DOWNSTREAM RIVERS

Heavy sediment loads in rivers draining Mount Rainier, a 4,392-m glaciated stratovolcano in western Washington State, USA, are derived from four dominant geologic processes acting in concert. At the top of the mountain, frequent rock falls deposit large volumes of sediment on glaciers. In turn, these glaciers carry sediment downslope through glacial transport. At lower elevations of the mountain near the toes of glaciers, debris flows caused by either heavy winter rainfall or rapid summer melting efficiently deliver copious amounts of sediment to fluvial reaches downstream. Large winter floods then transport fluvial sediment, notably to river reaches within the populated Puget Lowland that leads to localized aggradation. This aggradation results in reduced channel conveyance with concomitant increased flood risk. Aggradation rates in flood-prone river reaches downstream of Mount Rainier can be as large as 4.5 centimeters per year, but sedimentation magnitude depends on the river reach, amount of sediment mobilized from large flood events, and local river-management actions. We determined that sediment yields from Mount Rainier rivers range from 350 to 2,000 tonnes/km²/yr, roughly an order of magnitude larger than sediment yields typical for other Cascade Range rivers. An estimated 70 to 94% of the total sediment load in lowland river reaches draining Mount Rainier originates directly from the volcano. Better understanding of the sources and fates of Mount Rainier fluvial sediment loads can aid in informing management decisions from Mount Rainier National Park to flood-prone communities far downstream from the mountain.