GEOMORPHIC EFFECTS OF THE RODEO-CHEDISKI WILDFIRE, ARIZONA

The human-ignited Rodeo-Chediski wildfire (RCW) swept over 468,000 acres of the Mogollon Rim in east central Arizona during the summer of 2002. This fire complex is the largest recorded wildfire in Arizona history; as such, it has far-reaching geomorphic consequences that affect human habitations within and adjacent to burned watersheds. These responses include enhanced flooding, sediment-laden stream flows, rilling, and sediment bulking (debris flow initiation process). The confluence of Black Canyon and Buckskin Wash within the Heber, AZ, city limits is susceptible to destructive behavior related to those responses. Much of the contributing watershed (>183 km2) was moderately to severely burned. The litter and duff layers were burned in most cases (enhancing runoff), and ash is still present in some places (acting as a lubricant). The lithology comprises Paleozoic to Mesozoic limestones and sandstones and Tertiary Rim Gravels, all of which are capable of producing debris flows with boulder-sized clasts. Flooding and sediment-laden stream flows were observed during the first monsoon season after the fire. Rilling was not observed. Instead, poorly sorted, matrix-supported deposits are present on the upper slopes of first-order drainages. This sediment-laden runoff intercepted by trees implies that sediment bulking is occurring—a prerequisite for debris flow occurrence. However, no debris flows have been observed. This may result from weak monsoon precipitation and/or the high stem density (1-3 m-2) in the watershed. This stem density (burned and unburned trees) forms an obstacle course for the sediment and gravel mix, which tends to accumulate around the trees. Because of the severity of the RCW, the clear manifestation of surface processes, and the apparent hazard to structures downstream, the Black Canyon/Buckskin Wash watershed is an excellent place to study the long term geomorphic response to wildfires. Thus, we have established a long-term monitoring scheme to determine seasonal erosion rates, the cumulative effect of monsoon seasons, and the results of root decay using repeat stereo photography, geomorphic mapping, and topographic survey.