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

Abstract

[1] 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 km 2 ), paired-watershed forest-harvest experiments and six large (60-600 km 2 ) 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.Citation: Jones, J. A., and R. M. Perkins (2010), Extreme flood sensitivity to snow and forest harvest, western Cascades,

Legacy effects on sediments in river corridors

“…The most extreme example of this is urbanization, which increases impervious area and efficiently concentrates surface runoff in a storm sewer network that typically empties directly into river channels (Garcia-Fresca and Sharp, 2005). Other, more subtle changes in land cover can also increase water yield, such as reducing the areal extent or density of forest within a catchment (Tonina et al, 2008;Jones and Perkins, 2010).…”

Forest influences on snow accumulation and snowmelt at the Hubbard Brook Experimental Forest, New Hampshire, USA

“…The role of forests in intercepting snow and mediating snowmelt shown here have important implications for the maintenance of soil moisture and spring flows in this region, where recent trends show a gradual but substantial reduction in forest cover associated with development and biomass harvesting (Foster et al ., ; Stein et al ., ). The relatively large differences in SWE between forested (mean SWE = 11.7 cm) and cleared (mean SWE = 7.2 cm) plots on south‐facing slopes during the snowmelt period suggests that reductions in forest cover may result in larger spring snowmelt flooding and reduced summer soil moisture, as winter snow packs melt more rapidly in forest openings on sun‐exposed slopes; effects that have been documented in watershed studies at Hubbard Brook (Hornbeck, ) and other experimental forest sites (Jones and Perkins, ; Verry et al ., ). These effects may be of particular concern as the longevity of the snowpack is compromised and the timing of snowmelt shifts because of changes in climate that have been observed in the past and are projected to continue into the future (Campbell et al ., ; Huntington et al ., ).…”

Field evaluation of a new method for estimation of liquid water content and snow water equivalent of wet snowpacks with GPR

“…In the Arctic regions, up to 80% of the river water flow emanates from snow melt (König & Sturm 1998); in the Austrian Alps, this fraction amounts to 60% (Escher-Vetter et al 2009). Moreover, some of the most severe precipitation-based floods result from rain-on-snow events (Jones & Perkins 2010).…”