THE ENVIRONMENTAL IMPLICATIONS OF AGGRADATION ON MAJOR BRAIDED RIVER CHANNELS AT MOUNT RAINIER NATIONAL PARK, WASHINGTON

The purpose of this study is to quantify historic rate of river bed filling, and to the extent possible, start to evaluate the factors that control the increasing sedimentation. Mount Rainier is the tallest and most glaciated of the Cascade Volcanoes, located in southwestern Washington State. Steep, braided river channels radiate outwards from the volcano in all directions, and transport sediment ranging from fine sediment to cobbles and very large boulders. As the gradient in the channels decreases downstream, sediment drops out of the river, and accumulates in the river bed. Over time, the river channel bed surface increases in height, or aggrades. River aggradation was previously estimated to 0.5 to 1 foot per year, but until now, there has been no measured, long term data on river filling.

Aggradation is a serious management and safety concern for Mount Rainier, as a great deal of park infrastructure is located in valley bottoms near, or in, major river channels. River flooding, debris flows and glacial outburst floods cause overtopping of natural stream banks, and, in some instances, levees built along the river, and these processes impact roads and buildings, as well as park visitors. These concerns are compounded with the prospect of increased sediment production due to glacial retreat associated with global warming. This aspect is being studied by other park researchers.

In response to this recently discovered threat, the park surveyed cross sections in the summers of 1997, 2005 and 2006 to determine the current rates of aggradation in the Nisqually and White Rivers, two major river channels that have the most ability to affect primary infrastructure in the park. These rates were also compared with data observed in historical topographic maps, as well as a long profile of the Nisqually and White Rivers, measured in 1910.

Data are imported into GIS to analyze trends in aggradation in the braided channels. We are also running floodplain inundation models to predict hazard zones 50 or 100 years into the future. With almost a century of historic data, we are able to predict where problem areas are located in the park with certainty, and develop solutions to protect infrastructure in a resource-sensitive manner. Additionally, river bed response data will also contribute to the understanding of the post-glaciation evolution of river morphology, alluvial landforms and forest development.