95 YEARS OF RIVER AGGRADATION DATA AT MOUNT RAINIER NATIONAL PARK AND ITS IMPLICATIONS

The mountain rivers in Mount Rainier National Park (Washington, USA) are glacially-sourced, and drain steep terrain prone to frequent debris flows. The high sediment production often exceeds the transport capacity of rivers within the park which are thus prone to significant aggradation over time. Bed aggradation reduces a river's conveyance capacity and increases flood frequency. Being an older park, many roads and facilities are located in valley bottoms, and progressively more subject to flooding and debris flow damage. River aggradation was previously estimated to 0.5 to 1 foot/year (Riedel, 1997), but until now, there has been no measured, long term data on river filling.

To quantify the historic rate of river aggradation, longitudinal profiles that were surveyed in 1910 (USGS 1913) for the White and Nisqually rivers in the park were re-occupied in the summer of 2005. These longitudinal profiles were also compared to topographic map-derived profiles to establish intermediate rates of river aggradation between 1910 and 2005.

Estimates of river aggradation are essential to maintain Park infrastructure and visitor safety, particularly given that sediment production may be increasing. Recent increases in river bed aggradation may be in part due to increases in sediment production resulting from accelerated glacial retreat associated with the warming climate. This connection is being explored by other park personal. River bed response data will also contribute to the understanding of the post-glaciation evolution of river morphology, alluvial landforms and forest development (Abbe and others, 2003).