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 andprocesses that extends from the atmosphere to the watershed. In the Cascade Mountainsof the US Pacific Northwest, the links in this chain include extremely high precipitationcells that are embedded in coherent streams of subtropical moisture, very steep sideslopes of active stratovolcanoes mantled in copious volumes of loose debris, and theover-steepened channel heads and courses left by rapidly retreating glaciers. Theconsequences of this suite of process linkages include extremely destructive debris flows,hyper-concentrated flows and bedload floods that are capable of stripping and buryinglower elevation old-growth forests and destroying infrastructure, while transporting largequantities of sediment to larger rivers downstream. Such an event occurred duringNovember 2006, and resulted in catastrophic debris flows from all major volcanoes in thePacific Northwest.We report on a coupled set of studies intended to describe the above linkages on Mt.Rainier, Washington, and evaluate the potential impact of climate warming on thesecomplex processes. Components include an evaluation of the frequency and dynamics offlood-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 ofdebris 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 orintensity of driving storm events; 2) changing the frequency or extent of precursoryevents, such as rain or snowfall; or 3) forcing glacial retreat thereby changing the spatialdistribution of potential initiating sites.Synoptic reconstruction of meteorological conditions accompanying debris-flowinitiating storms reveals that debris flows occur during both atmospheric river (AR) andnon-AR events. Atmospheric rivers are focused extropical excursions of very moist airmasses driven by the jet stream forcing into temperate latitudes; they typically involveboth large amounts of precipitation and very high freezing levels due to warmtemperatures. Non-AR events are moist air masses that typically are associated withzonal flow or large regional fronts. Recent debris flows on Mt. Rainier have resulted fromboth types of event, although the largest debris flow episodes have occurred during ARevents. Evidence is inconclusive as to whether there is a clear climate change signal in309 .the data. LiDAR and field evidence supports that recent debris flows initiated in recentlydeglaciated areas (last 20 years) on the steep upper slopes of Mt. Rainier. Initiation siteswere just downslope from active glaciers, areas of stagnant ice, or debris-mantled ice.We speculate that presence of impermeable ice layers may concentrate runoff duringextreme precipitation events, resulting in a ‘firehose’ effect on debris-mantled slopes justbelow the ice.Debris flows that initiate on the upper flanks of the volcano run out over manykilometers, transforming into debris-laden floods as slopes diminish. Recent debris flowshave deposited aprons and fans of very coarse material in channels, resulting in extensiveaggradation with in the park boundaries. Further deposition occurs where channelsdraining the volcanic edifice enter larger rivers. There is no consistent pattern of bedaggradation within these larger channels, however, suggesting that most of the sedimentderived from recent debris flow events is being stored closer to the base of the volcano.These studies have implications for montane regions in other areas of the world, wherechanges in temperature and precipitation patterns, coupled with geomorphic changes onthe mountains themselves, may be changing the risk from episodic debris flows andsimilar events.