A five-year record of sedimentation in the Los Alamos Reservoir, New Mexico, following the Cerro Grande Fire

Abstract

in significant increases in runoff and erosion relative to pre-fire conditions. This study documents five years of sedimentation in the Los Alamos Reservoir, providing a unique data set for evaluating changes in post-fire erosion rates and watershed recovery in a montane watershed. The reservoir was built in 1943 for water storage and supply, with an upstream drainage basin area of about 16.5 km2, elevations of 2320 to 3180 m, and largely supported a mixed conifer forest prior to the fire. Thirty two percent of the basin experienced moderate to high burn severity, including some of the steepest parts of the basin; 32 % experienced low burn severity and 36 % was unburned. Draining of the reservoir in June 2000 allowed for detailed surveying of the top of pre- and post-fire sediments. The initial total station survey, in combination with a detailed 1943 as-built drawing, indicated ~1600 m3 of sediment was deposited during 1 post-fire event and ~8600 m3 in the previous 57 years. Average sediment deposition was ~150 m3/yr prior to the fire, equivalent to an average basin-wide denudation rate of soil of ~ 0.009 mm/yr. When the total station survey was repeated in June 2001, a large delta consisting of sand and gravel had formed at the head of the reservoir. The remainder of the reservoir basin contained predominantly fine to very fine sand, silt, and ash. These combined facies constituted ~21,800 m3 of sediment accumulated in one year, equivalent to an average basin-wide denudation rate of ~1.3 mm/yr or ~140 times the average pre-fire rate. Assuming that all of the sediment deposited during the first year after the fire was derived from the high- and moderate-severity burn areas (5.4 km2),