A Five Year Record of Sedimentation in the Los Alamos Reservoir, New Mexico, Following the Cerro Grande Fire

Synopsis: The effects of the 17,400 ha Cerro Grande fire patch in New Mexico on erosion and sedimentation processes were analyzed by this study, located in the Jemez Mountains upstream of the Los Alamos Reservoir, New Mexico. This study provides a unique data set demonstrating post-fire erosion rates relative to pre-fire conditions. The fire affected a large fraction of the watershed, with 32% of the basin experiencing a moderate to high severity burn, including some of the steepest mountainous portions of the basin. Average sediment deposition was 150 m3/year prior to the fire, equivalent to an average basin-wide denudation rate of 0.009 mm/year. The year after the fire, over 21,800 m3 of sediment accumulated in one year, equivalent to an average basin-wide denudation rate of 1.3 mm/year or 140 times higher than the average pre-fire rate. Assuming that all the sediment was derived from the high and moderate severity burn area, the average denudation rate for those areas was 4.0 mm/year or 450 times higher than the pre-fire rate. Erosion and sediment delivery into the reservoir were highest in the first year after the fire, accounting for about half the total sedimentation in the first five years, despite the occurrence of below average summer precipitation and relatively low-intensity storms (<1 year return interval). Sedimentation rates gradually dropped off in the second, third, fourth, and fifth years after the fire (Figure 1). Prior to the fire, the reservoir received very minor amounts of sediment and had never been dredged, preserving a 57-year record of sedimentation.