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USGS Wildland Fire Research Page 3 of6 Scientists from the Landslides Hazards Program of the USGS are studying the linkage ~etween wildfires and debris flows at Storm King Mountain and sites in several other ~tates. During an intense wildfire, all vegetation may be destroyed; also the organic material in the soil may be burned away or may decompose into water-repellent substances that prevent water from percolating into the soil. As a result, even normal rainfall may result in unusual erosion or flooding from a burned area; heavy rain can produce destructive debris flows, as happened at Storm King Mountain. In 1997, wildfires charred many areas of southern California, leaving them bare before the 1997-98 winter's heavy EI Nino rainfall. A map delineating perimeters of 25 of the 1997 southern California wildfires that burned more than 300 acres can be found at http://geohazards.cr.usgs.gov/htmlfiles/landslides/scfires/scfiresloc.html. Ten of these burns produced debris flows after the first major winter storm, and flooding dominated in eight areas. Only four showed little or no erosion or runoff. The relative importance of topography, vegetation conditions, and the amount and pattern of rainfall are being investigated at each of the sites to develop a model for debris-flow susceptibility from recently burned areas. Effects of Fire on Water Supplies Water supplies are also affected by fire: the loss of ground- surface cover, such as needles and small branches, and the a chemical transformation of burned soils make watersheds _more susceptible to erosion from rainstorms. In May 1996, an 11,900-acre (4,820 hectares) fire burned most of the Buffalo Creek and Spring Creek watersheds; these small watersheds feed into the Strontia Springs Reservoir, which supplies more than 75 percent of the municipal water for the cities of Denver and Aurora (fig. 5), Two months after the fire, a severe thunderstorm caused flooding from the burned area, killing two people. In addition, the Denver Water Department immediately experienced a deterioration of water quality from floating burned debris and high levels of manganese. Two years after the fire, phosphate levels in the water remain high, and the Denver Water Department is concerned about loss of reservoir capacity and impaired water quality. tii.ti:, ~o;.I.l.'rr,"' ~..~. j i.Jjtlt '\(, . + '::.~:r.;:.o.Ylttp. . . .'N'" 01"', ;:, - USGS researchers are studying the post-fire hydrogeology of Buffalo and Spring Creeks to understand the processes of runoff generation, sediment production, and the transport of sediment out of the watersheds and into the reservoir. Although the burned hillslopes are showing signs of revegetation, 2 years later, sediment is still being eroded. Between September 1996 and August 1998, about 260,000 cubic yards of coarse sediment were trapped in a delta at the upper end of the reservoir. The postfire accumulation rate is about 10 times that calculated by the engineers who designed the reservoir. Furthermore, this was not an unprecedented event: stratigraphic studies in the Buffalo Creek watershed suggest that there may have been about seven similar fire-flood events en the last 2,000 years. The Buffalo Creek Fire has produced disastrous consequences for Denver and surrounding communities. However, the postfire evaluation is expected to help land managers along the Colorado Front Range better anticipate the effects of wildfire on other watersheds and other reservoirs. http://www,usgs.gov/themes/Wildfire/fire.html 8/15/02