Geologic versus wildfire controls on hillslope processes and debris flow initiation in the Green River canyons of Dinosaur National Monument

As in many areas of high relief, debris flows are an important process linkage between hillslopes and the Green River in the canyons of the eastern Uinta Mountains, yet the physical conditions that lead to debris flow initiation are unknown. A recent episode of enhanced debris-flow and wildfire activity provided an opportunity to examine the geomorphic impact of fire and the processes by which weathered bedrock is transported to the Green River. Field investigations and analysis of elevation and precipitation data were undertaken in 15 catchments with recent debris flows to determine how surficial geology, wildfire, topography, bedrock strength, and meteorology influence hillslope processes. The recent debris flows were triggered by intense summer rainstorms. The dominant debris flow initiation mechanism, the firehose effect, occurred when overland flow generated on bedrock hillslopes cascaded down steep cliffs onto colluvium, causing failure. Sixty percent of the debris flows occurred in unburned catchments. However, 15% of the burned catchments in the study area produced debris flows over the study period, whereas only 7% of the unburned catchments did. Thus, fire was not the primary driver of debris flows, but fire-related events did contribute to the increased debris flow activity. The geomorphic impact of wildfire in the eastern Uinta Mountains is not as great as in transport-limited settings with regolith-mantled hillslopes. The strong rocks and dry climate of the study area cause hillslopes to be very steep and weathering-limited, with high runoff ratios and a dearth of regolith. As a result, there is little vegetation, and thus we hypothesize that burning does little to change hillslope processes. The suite of hillslope processes in the eastern Uintas are like those documented in the similarly dry Grand Canyon, but differ from other locations in the western U.S. where wildfire is a primary control on debris flow processes.