THE IMPORTANCE OF EVENT SEQUENCING ON THE GEOMORPHIC IMPACT OF WILDFIRE IN THE CENTRAL GREAT BASIN

Research in a variety of climatic and geomorphic settings indicates that the geomorphic impact of wildfire is variable. We compare the geomorphic responses of two different drainage basins in the Toiyabe Mountains to lightening-generated wildfires. Crow Canyon is a 2 km2, drainage basin with approximately 600 m of relief, that is underlain by quartz monzonite and granodiorite. Wall Canyon is a 17.6 km2, drainage basin with approximately 800 m of relief, that is underlain primarily by shale, limestone, phyllite and schist. Both basins were covered by a Pinyon-Juniper canopy with an understory of sage, rabbit brush, and desert grasses. Crow Canyon was completely burned in August 1981 and the headward 62% of Wall Canyon was burned in August 2000. In each basin, the burn intensity was categorized as “intense” (complete destruction of all ground and tree foliage, leaving only charred trunks and branches). Despite similarities in relief, location, and burn intensity, the geomorphic impacts have been quite different. The axial drainage system in Crow Canyon was severely degraded following the 1981 wildfire. Maximum channel incision measured at 28 cross-profiles ranged from 0.6 m to 3.9 m with an average of 1.9 m. Most of the channel incision occurred in the first two years after the fire which were characterized by precipitation as much as twice the annual average. In contrast to Crow Canyon, there has been no observable or measurable geomorphic impact in the two years following the Wall Canyon fire. The two years, 2001 and 2002 have been typified by normal to below normal, rather than extreme precipitation. In the two years since the fire, grasses and underbrush have been re-establishing on the hillslopes, and may mitigate significant geomorphic impact by the time the next significant hydrologic event occurs. The disparity in response to wildfire in these two watersheds suggests that the geomorphic impact of wildfire in the arid Great Basin is highly dependent upon event sequencing.