DEBRIS-FLOW ACTIVITY ASSOCIATED WITH EXTREME PRECIPITATION EVENTS, SANTA CATALINA MOUNTAINS, SOUTHERN ARIZONA

Geomorphic responses to extreme events are recorded in numerous debris-flow deposits emanating from all major canyons along the front range of the Santa Catalina Mountains north of Tucson, Arizona. Documented historical debris flows in the Santa Catalina Mountains have been generated by dissipating tropical storms (1983), prolonged winter storms (1993), intense convective summer storms (1997; 1999), and post-wildfire summer storms (2002; 2003). All of these debris flows were limited to steep mountain tributaries, however, and none of them came close to exiting the mountain front. Hence the potential geologic hazards associated with debris flows were underappreciated prior to July, 2006, when southern Arizona experienced five consecutive days of early morning storms generated from monsoonal moisture mixing with a persistent low-pressure system centered over northwestern New Mexico. These increasingly wet storms culminated on July 31, 2006, when 5 to 25 cm of rain fell on the already moistened hillslopes of the Santa Catalina forerange and generated numerous hillslope failures and debris flows. Large debris flows that exited, or nearly exited, from five canyons along the front range of the Santa Catalina Mountains caused significant damage to infrastructure and houses.

In light of these events, a multi-agency effort is ongoing to document evidence of debris flows at the canyon mouths and estimate recurrence intervals of debris-flow producing storms to quantify the potential geologic hazards. As part of this larger effort, we are conducting detailed mapping (1:6,000) of debris-flow deposits at the mouths of 11 major and 4 minor canyons along the front range. Debris-flow deposits are differentiated based on geomorphic and stratigraphic relationships, and on soil development. Excluding deposits from 2006, we have found evidence for at least 5 separate, probable Holocene, debris-flow deposits at individual canyon mouths. Urban development on Holocene fans near the mountain front has made it difficult to document the down-fan extent of debris-flow deposits. The extent of potential hazards into developed areas is being evaluated through a combination of field mapping and analysis of pre-development aerial photographs. These data will be used for modeling potential debris-flow runout distances.