The Hydrologic and Geomorphic Impacts of the 2010 Fourmile Canyon Fire, Boulder Creek Watershed, CO

A 2010 wildfire burned 23% of Fourmile Canyon near Boulder, Colorado. Cover loss combined with soil hydrophobicity to produce extreme runoff events including a 70-year flood. This and other floods resulted from rain events with return periods of 1-5 years. Changes are observed in postfire hydrology including hydrograph shape and rainfall-runoff response. Sediment yield increased after the fire delivering approximately 39,400 t of sediment. The return of vegetation and rewetting of hydrophobic soils may quickly reduce hillslope runoff and erosion. The ash radionuclide and geochemical signature is measured in fresh deposits from the valley floor of Fourmile Canyon. The ash signature in these deposits is also found in grain-size results. Geochemical analysis of trace metals documents the effect of historical mining. Channel storage increases sediment residence time in the watershed and increased hillslope erosion after wildfire may be a primary contributor to long-term geomorphic change.