WAVE-LIKE MOTION OF COARSE BED MATERIALS IN HIGH-GRADIENT PRO-GLACIAL BRAIDED RIVERS AT MOUNT RAINIER, WASHINGTON, USA

Field observations suggest large-clast (>50 mm) sediment transport as discrete sediment waves in Mount Rainier National Park, WA. Mount Rainier is a 4,392 m volcano in southwest Washington State. Proglacial braided rivers radiate from the mountain, all of which exhibit episodes of aggradation and incision but with a long-term trend of river filling. Median grain sizes in Park rivers range from 50-150 mm and include significant components of sand, large cobbles and boulders. Since 2005, the majority of coarse grained sediment inputs and transports in the park have occurred as discrete waves during fall floods. For example, since the 1960s, repeated debris flow activity in Tahoma Creek on the park’s southwest side has emplaced multiple large packages of sediment, which are being transported downstream, resulting in active channel widening. In some places, the channel has widened over 150 m, decimating old-grown riparian forests and inhibiting Park access. It is common for the bed of this and other Park rivers to locally be several meters higher than surrounding old-growth riparian floodplains. In some places, the floodplain disequilibrium is in excess of 5 meters. Overflow flood channels form when packages of sediment deposit in a reach, then are left perched as the sediment wave moves downstream. Infrastructure damage from aggrading rivers is worsening during recent peak flood events. This is likely due to more sediment coming on line in the form of debris flows and mass-wasting events, a consequence of retreating glaciers and climate change. While a great wealth of research exists regarding sand and gravel-bedded rivers with dispersing and translating sediment waves, little research exists regarding the possibility of similar wave-like motion being detected in rivers with high gradients (>2%) and very coarse median grain sizes. Observations at Mount Rainier suggest this motion could be wave-like and initial findings from the recent field work, as well as observations from the last decade, will be presented to suggest a new form of large-clast sediment transport in steep, braided rivers.