From Atmospheric Rivers to Rivers of Debris

Extreme floods are 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, links in this chain include extremely high precipitation embedded in streams of subtropical moisture; steep slopes of active stratovolcanoes mantled in large volumes of debris, and over-steepened channels left by rapidly retreating glaciers. The consequences of these process linkages include extremely destructive debris flows and floods that are capable of stripping lower elevation old-growth forests, and destroying infrastructure. We describe these linkages on Mt. Rainier, Washington, and evaluate the potential impact of climate warming on these complex processes. We evaluate the frequency and dynamics of flood generating storms, controls on debris flow initiation and runout, spatial patterns of disturbance to riparian forests, and historical trends in frequency of debris flows and floods. Climate warming can potentially affect these linkages by: 1) changing the frequency or intensity of driving storms; 2) changing the frequency or extent of precursory rain or snowfall; or 3) forcing glacial retreat thereby changing the spatial distribution of initiating sites; We explore the implications for the future of rivers draining large volcanoes and other types of mountains in the Pacific Northwest.