COARSE SEDIMENT AGGRADATION EXACERBATED BY IN-CHANNEL WOOD ACCUMULATIONS ALONG HEADWATER REACHES OF THE CARBON AND NISQUALLY RIVERS WITHIN MOUNT RAINIER NATIONAL PARK, WA

Regionally deglaciating climate coupled with sediment loading of historically stable single-thread rivers that drain retreating glaciers on the flanks of Mount Rainier have driven extreme fluvial network evolution on subdecadal timescales. Rapid mainstem widening, streambank erosion, braidplain development, and channel aggradation is a transient sediment storage signal morphodynamically prograding downstream through the Carbon, Nisqually, and other rivers, from glacier meltwater channels towards lower gradients that approach Mount Rainier National Park (MRNP) boundaries. Coarse glaciogenic sediment is delivered to high-gradient (>10%) headwater systems through high ratio sediment:water flows and is stored along intermediate channel corridors where gradients decrease towards deposition (~5-10%) and further at MRNP boundaries (~1-2%). The upper Carbon R had been historically aggrading and widening until a full avulsion (ca. 2009) relocated the mainstem to an adjacent lower-elevation late seral stage conifer forested floodplain (i.e., 200-400+ old growth) and initiated incision into the previously accumulating headwater sediments. The Carbon R immediately downstream of the 2009 avulsion has since rapidly aggraded, widened, and eroded laterally into adjacent floodplains downstream towards the MRNP boundary. Modern aggradation along downstream reaches of the Carbon R is contrary to historically stable surveys before 2009. Large wood recruitment and accumulation along high-gradient reaches rapidly exacerbates deposition and transient storage of coarse sediment, observed to force 1-5 meters of deposition along upper reaches of both the Carbon R and Nisqually R. Preliminary carbon dating along the Carbon R suggests a pattern of punctuated coarse sediment delivery to upper reaches of the system that have periodically inundated, eroded, and ultimately buried mature forested mountain floodplains at the sampling site repeatedly for many centuries before current aggradation and erosion undermines the modern old growth above. Aggrading channels, despite widening, have diminished flow capacity and flooding within MRNP during historically normal river discharges is strongly associated with the downstream-prograding evolution that is driven by in-channel sediment and wood storage.