Aggradation and Avulsions: A Case Study of a Carbon River Floodplain, Mount Rainier National Park, Washington

The Carbon River flows from the Carbon glacier, a major source of unconsolidated sediment and debris on the northwest face of Mount Rainier. Increased river flow from storms and glacial retreat mobilizes and transports large volumes of sediment into the Carbon River, which fundamentally changes the geomorphic functioning of the fluvial system. The resulting channel aggradation and braiding make the river susceptible to channel shifts and active channel migrations. Throughout the last several decades, the Carbon River\u27s responses to changing inputs of water and sediment have damaged surrounding ecosystems and park infrastructure. Few studies have examined the fundamental drivers of avulsions on the Carbon River. Analyses of historic aerial photographs, LIDAR and DEM imagery, hydrograph data, and field measurements suggest that the most substantial geomorphic change since 1950 has occurred in conjunction with high mean daily discharges. My study area, the Crescent floodplain, in the upper alluvial reaches activated a new channel and changed most substantially between 2006 and 2009, when three of the largest mean daily discharges in history were recorded. Aggradation of alluvial sediment in the main stem of the Carbon River led to the avulsion into the Crescent floodplain during peak flow events. Its location at the outside of a bend and the nearby slope-side tributaries made the Crescent floodplain especially susceptible to an avulsion. Although the active channel of the Carbon River\u27s main stem currently flows along the opposite bank, the defined channel and minimal riparian buffer in the Carbon floodplain make it extremely vulnerable to a future reactivation if peak flows increase in frequency and magnitude as a result of climate change