New Mexico Geological Society Annual Spring Meeting — Abstracts

The Cerro Grande fire (May 2000) burned approximately 174 km² of forest in and around Los Alamos, New Mexico. Post-fire runoff was greatly enhanced due to the loss of forest litter and the development of hydrophobic soils. The upper 4.9 km² of the 21.7 km² drainage basin of Pueblo Canyon had high severity burn with 100% tree mortality. Pueblo Canyon has subsequently experienced multiple post-fire floods, the largest of which occurred on 2 July 2001 with an estimated peak discharge of 40 m³/s. Re-surveying of 41 cross sections established between November 1996 and June 200 along 10 km of canyon downstream of the burn area allows quantification of the geomorphic response to these floods. Channel response varied greatly, ranging from net incision of 0.75 m to net aggradation of 1.1 m. Channel widening was also documented at some sections. Extrapolation between sections yields as estimated net deposition of approximately 6000 m³ of sediment along the 10 km of channel. The predominant result of overbank flooding was net deposition on the floodplains, although local scour also occurred. Up to 0.45 m of deposition was measured with an estimated total floodplain deposition in the study area of 6,500-8,000 m³. Available data suggests that the net deposition following the fire may not be significantly different from pre-fire flood deposition. Data from the cross sections are being used to test and refine a sediment transport model, which will be used to estimate the effects of continued post-fire flooding in Pueblo Canyon and other canyons affected by the Cerro Grande fire.