WILDFIRE AND DEBRIS FLOW INITIATION IN THE CANYONS OF THE EASTERN UINTA MOUNTAINS-DOES A LINK EXIST?

Although wildfire-induced changes in watershed condition have been demonstrated to increase debris flow activity for some locations in the Rocky Mountains, such a linkage may not exist in the canyons of the eastern Uinta Mountains. Since 1997, 13 debris flow and stream flow sedimentation events have occurred in tributaries of the Green River canyons of Dinosaur National Monument in Colorado and Utah. Six of these events occurred in catchments that burned in wildfires during the previous year; however, 7 events occurred in unburned catchments. Thus, wildfire is not a necessary precursor to debris flows in this area. Debris flows have occurred in areas where a range of flat-lying Precambrian and Paleozoic sedimentary rocks are exposed. These include thick sandstone and limestone cliff-forming units and intervening shales that form slopes that are locally covered with colluvium. Forest cover is primarily pinion-juniper that undergoes severe burns when subjected to wildfire, with complete combustion of organic soil litter. Debris flows are generated during localized, high-intensity summer thunderstorms having 24-hour rainfall totals of 1-3 cm. The debris flow initiation and sediment transport processes are the same in burned and unburned catchments and can be generalized as follows: Overland flow generated by rainfall in the upper catchment enters high-gradient, bedrock channels or cascades down the canyon walls through hollows within the cliff forming units. The channelized overland flow inundates colluvium or talus stored on bedrock benches, in hollows, or in channels at the base of cliffs. The colluvium fails, initiating debris flows that continue to entrain material through channel scouring and bank failure as they travel downstream. This initiation mechanism is different from sediment bulking, a post-fire debris flow mechanism cited in other parts of the Rocky Mountains, because it does not involve entrainment of fine-grained sediment by overland flow. The lack of a clear post-fire hydrologic response may be because hillslopes in the study area are primarily weathering-limited, with thin regolith and low infiltration capacity. Though wildfire may further reduce infiltration capacity and increase runoff, burned catchments respond similarly to unburned catchments during high intensity storms.