FROM ATMOSPHERIC RIVERS TO RIVERS OF DEBRIS: COUPLING EXTREME PRECIPITATION EVENTS, GLACIAL RETREAT, DEBRIS FLOWS, AND CHANNEL CHANGES ON MOUNT RAINIER, WASHINGTON

Extreme floods in rivers can be viewed as the terminal link in a chain of causality and processes that extends from the atmosphere to the watershed. In the Cascade Mountains of the U.S. Pacific Northwest, the links in this chain include extremely high precipitation cells that are embedded in coherent streams of subtropical moisture, very steep side slopes of active stratovolcanoes mantled in copious volumes of loose debris, and he over-steepened channel heads and courses left by rapidly retreating glaciers. The consequences of this suite of process linkages include extremely destructive debris flows, hyper concentrated flows, and bedload floods that are capable of stripping and burying lower elevation old-growth forests and destroying infrastructure while transporting large quantities of sediment to larger rivers downstream. We report on a coupled set of studies intended to describe these linkages on Mt. Rainier, Washington, and evaluate the potential impact of climate warming on these complex processes. Components include an evaluation of the frequency and dynamics of flood generating storms, the location of and controls on debris flow initiation and runout, spatial patterns of disturbance to riparian forests, and historical trends in frequency of debris flows and floods, with an eye towards exploring the role of changing climate. Climate warming can potentially affect these linkages by: 1) changing the frequency or intensity of driving storm events; 2) changing the frequency or extent of precursory events, such as rain or snowfall; or 3) forcing glacial retreat thereby changing the spatial distribution of potential initiating sites; We explore the evidence for each of these mechanisms and the implications for the future of rivers draining large volcanoes in the Pacific Northwest.