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Extreme flood sensitivity to snow and forest harvest, western Cascades, Oregon, United States

We examined the effects of snow, event size, basin size, and forest harvest on floods using >1000 peak discharge events from 1953 to 2006 from three small (<1 km2), paired‐watershed forest‐harvest experiments and six large (60–600 km2) basins spanning the transient (400–800 m) and seasonal (>800 m) snow zones in the western Cascades of Oregon. Retrospectively classified rain‐on‐snow events delivered 75% more water to soils than rain events. Peak discharges of >10 year rain‐on‐snow events were almost twice as high as rain peaks in large basins but only slightly higher in small basins. Peak discharges of >1 year rain‐on‐snow events increased slightly (10%–20%) after logging in small basins, but small basin peaks do not account for the magnitudes of large basin rain‐on‐snow peak discharges during >1 year floods. In extreme floods, despite very high infiltration capacity, high soil porosity, and steep hillslope gradients, prolonged precipitation and synchronous snowmelt produce rapid, synchronized hydrograph responses to small variations in maximum precipitation intensity. At the large basin scale, forest harvest may increase the area of snowpack and simultaneous snowmelt, especially in elevation zones normally dominated by rain and transient snow, thereby increasing large basin peaks without producing very large percent increases in small basin peaks. Further work is needed to describe water flow paths in melting snowpack, snow cover and the area experiencing snowmelt, synoptic peak discharges, and routing of flood peaks through the stream network during extreme rain‐on‐snow floods. The evolving structure of the forest on the landscape is a potentially very important factor influencing extreme rain‐on‐snow floods.