SEDIMENT BUDGET FOR A SMALL CANYON DOWNSTREAM OF THE CERRO GRANDE WILDFIRE, NEW MEXICO

The Pajarito Plateau in north-central New Mexico is a volcanic mesa dissected by a series of deeply incised alluvial canyons which head on the steep, forested eastern flank of the Jemez Mountains. Before May 2000, most of the sediment and runoff in these canyons were generated by summer thunderstorm rainfall on the sparsely vegetated Pajarito Plateau (which produces primarily medium sand and coarser sediment), as rainfall on the mountain front was generally intercepted by the dense forest canopy or infiltrated into deep soils. In May 2000, the Cerro Grande fire burned nearly 175 km2 of primarily ponderosa and mixed conifer forest in the Jemez Mountains and Pajarito Plateau, with the most severely burned areas in the steeper, montane portions of the watersheds. As a result of the loss of forest canopy and ground cover, and the formation of hydrophobic soils, the primary locus of runoff and sediment generation in the Pajarito Plateau watersheds shifted westward to the mountain front. In the canyons draining the Pajarito Plateau, large floods and fine-grained (fine sand, silt, and clay) sediment produced on the steep, soil-mantled slopes of the Jemez Mountains supplemented the moderate flash floods and coarser sediment normally derived on the Pajarito Plateau. Pueblo Canyon (drainage area 21.7 km2) was one of the most severely burned watersheds, with the upper 5 km2 affected by high severity burn and nearly 100% tree mortality. Pre- and post-fire field data from Pueblo Canyon and nearby Los Alamos Canyon, including runoff and sediment transport data from flash floods, repeated cross section surveys, reservoir sedimentation surveys, and floodplain stratigraphic information were used to quantify coarse and fine sediment input, export, and exchange in Pueblo Canyon after the Cerro Grande wildfire. Prior to the fire, the sediment budget of Pueblo Canyon was influenced by decadal fluctuations in sand supply and exchanges with the channel bed, resulting in downstream-propagating aggradation/degradation cycles. Since the fire, runoff and sediment production from the Jemez Mountains temporarily increased by over two orders of magnitude, supplying large quantities of sediment and ash in a series of flood events, resulting in widespread sediment exchange with the floodplain and local channel aggradation and degradation.