Erosion Rates of Volcanic-ash Derived Soils in the Blue Mountains of Eastern Oregon, USA: A Comparison Across Sales in Space and Time.

We examined present day rates of erosion in the Blue Mountains of eastern Oregon to quantify background erosion rates to provide standards for assessing possible accelerated rates of erosion resulting from wild fire or from land-management activities such as prescribed fire. The Skookum Creek watersheds, where stream discharge and sediment yield have been recorded continuously since the watersheds were gauged in 1992, provided a watershed-scale estimate of erosion rates. We installed hillslope erosion plots on north- and south- facing slopes within the watersheds in 2002 and collected data for three years to estimate short-term, hillslope- scale erosion rates. We also collected soil samples and analyzed them for 137Cs to get a 40-yr time- integrated estimate of hillslope erosion rates. Our results showed large differences between whole-watershed sediment yields and hillslope erosion rates measured from plots, suggesting that episodic processes dominated sediment production and transport and therefore controlled watershed-scale sediment budgets. At the hillslope-scale, short-term erosion resulted primarily from digging by small mammals and trampling by elk. Visual observations at the plots suggested that annual down-slope sediment movement was usually less than one meter. There were no significant difference among slope positions, but erosion rates were significantly higher on south-facing aspects and positively correlated to the amount of bare ground. In contrast, the 137Cs data suggested that erosion rates differed with slope position. Higher erosion rates were measured in toe- and mid-slope positions, with little erosion occurring on upper slopes and ridge tops. We examine these results in light of the present-day pattern of surface soils resulting from redistribution of volcanic ash from upper- slope to lower-slope positions and the effects of disturbance, including wildfire and the preferential grazing of riparian and lower-slope positions by domestic livestock.