HYDROLOGIC AND GEOMORPHIC IMPACTS OF THE 2010 FOURMILE CANYON FIRE, BOULDER CREEK WATERSHED, COLORADO

In September 2010 an intense wildfire burned 23% of Fourmile Canyon, a high relief, high gradient basin west of Boulder, Colorado. Cover loss through the combustion of litter and vegetation combined with heat-induced soil hydrophobicity produced extreme runoff events and unprecedented peak discharges in the watershed. In the summer after the fire, Fourmile Creek experienced the largest flood events in a 20-year record, with one flood more than doubling previously recorded peaks. These elevated discharges were the result of high frequency rain events with recurrence intervals of 1-5 years. Hydrographs display a rapid response between rainfall initiation and peak discharge. Sediment yield following the wildfire increased through the contribution from burned hillslopes. Previous studies and field evidence suggest that two years after the fire the return of vegetation and rewetting of hydrophobic soils may quickly reduce hillslope erosion. Concentrations of ¹³⁷Cs and ²¹⁰Pb are higher in burnt sediment and ash deposited on hillslopes and floodplains than pre-fire sediment. Overbank deposits near the channel may display a return to pre-fire concentrations of these radionuclides in the two summers of rain, flooding, and sediment transport since the fire. Additionally, geochemical analysis of overbank flood deposits display the effect of historical mining on Fourmile sediment chemistry, such as elevated concentrations of gold. Channel storage may increase sediment residence time in the watershed, as the reduction in large flood events will limit the stream’s ability to transport sediment downstream. The potential for increased wildfire occurrence, perhaps linked to climate change, may result in increased fire damage in the populated Front Range. Associated floods are capable of structural damage and increased sediment transport negatively impacts water quality, especially in heavily mined areas such as Fourmile Canyon. Sediment yields following wildfire are many times greater than background delivery and may be a primary contributor to long-term erosion in fire-prone landscapes.