Effects of Thinning and a Wildfire on Sediment Production Rates, Channel Morpholgy, and Water Quality in the Upper South Platte River Watershed

The Upper South Platter River watershed is the primary source of Denver's water supply. There has been increasing concern over the potential adverse effects of large wildfires on runoff, erosion, and water quality, and this has led to extensive forest thinning projects to reduce the potential for high-severity wildfires. The initial goal of this project was to evaluate the effects of thinning at the hillslope scale using sediment fences and the small catchment scale by installing flumes and periodically monitoring water quality and channel morphology. The hillslope measurements began in mid-2001, and the flumes were installed in early summer 2002. In 2001 only 3 of the 40 hillslopes produced sediment. In June 2002, the Hayman fire burned half of the study sites. Mean percent ground cover decreased from 90% to 6% in the sites burned at high severity, and the soils were strongly water repellent from the surface to a depth of 6 or 9 cm. The first two storms of 11 and 17 mm caused extensive rilling in the formerly unchannelled swales and extensive downstream gullying. Erosion rates averaged 7.5 Mg ha-1 in the dry summer of 2002 and 10.7 Mg ha-1 in the wet summer of 2003. Thinning in the remaining unburned swales decreased the mean vegetative cover from 15% to 7% and increased the mean percent bare soil from 8% to 16%. However, 75% ground cover meant that no sediment was generated from any of the thinned or the control hillslopes from 2002 through 2006. At the small watershed scale, the observed high water marks were as much as 1.4 m above the estimated bankfull stage. The Saloon Gulch flume was completely buried by the post-fire deposition, and the nearly continuous deposition in the Brush Creek flume precluded accurate flow measurements. Potassium, magnesium, calcium, chloride and nitrate concentrations approximately doubled after the fire. Turbidity increased substantially, while the pH remained unchanged. These results clearly show the dramatic difference between the effects of thinning and high severity fires on runoff, erosion, and sediment delivery at both the hillslope and watershed scales.