DISTINGUISHING POST-FIRE DEBRIS FLOW EROSIONAL AND DEPOSITIONAL ZONES IN A BURNED WATERSHED

Wildfires alter soil hydrology and soil erosion potential in steep mountain watersheds. Soil infiltration rates often decrease after fire, leading to enhanced runoff, and soils become more susceptible to erosion after wildfire heating. Because of these soil changes, relatively common rain events can generate substantial overland flow that can easily entrain and redistribute sediment. Here we investigate the patterns of erosion and deposition in the Van Tassel watershed (southern California), a watershed burned in the Fish fire (June 20-July 23, 2016) where there were several debris flows. Using two airborne lidar datasets, the first obtained pre-fire (January 2016) and the second (August 2017) obtained after several debris flows, we can see distinct zones of erosion and deposition at the landscape-scale. The sediment yield is a function of the upstream contributing drainage area, resulting in a humped pattern with a distinct transition between erosion and depositional areas. Finally, we estimate stream power based on rainfall and topographic metrics to explore the underlying topographic/hydrologic drivers that influence locations of erosion versus deposition. This approach generalizes our observations at this site for future predictions at other sites.