Floods and Debris Flows in the Catalina Front Range and Southern Arizona, July 31, 2006

Chris Magirl

US Geological Survey, Tucson, Arizona

Ample geologic evidence indicates early Holocene and Pleistocene debris flows from the south side of the Santa Catalina Mountains north of Tucson, Arizona, but few records document historical events. On July 31, 2006, an unusual set of atmospheric conditions aligned to produce record floods and an unprecedented number of debris flows in the Santa Catalinas. During the week prior to the event, an upper-level area of low pressure centered near Albuquerque, New Mexico generated widespread heavy rainfall in southern Arizona. After midnight on July 31, a strong and widespread complex of thunderstorms developed over the Mogollon Rim in central Arizona in a deformation zone that formed on the back side of the upper-level low. High atmospheric moisture (50 mm of precipitable water) coupled with cooling aloft spawned a mesoscale thunderstorm complex that moved southeast into the Tucson basin. These thunderstorms interacted with a low- to mid-level zone of atmospheric instability to create an initial wave of rainfall across the Tucson metropolitan area in the early morning hours. A second wave of thunderstorms and heavy rain developed over the Santa Catalina Mountain near dawn. A 15-20 knot low-level southwesterly wind developed with a significant upslope component over the south face of the Santa Catalina Mountains advecting moist and unstable air into the merging storms. NEXRAD radar indicated a swath of 75-150 mm of rainfall occurred over the lower and middle elevations of the southern Santa Catalina Mountains. This intense rain falling on saturated soil triggered multiple debris flows throughout the front range. Sabino Canyon was the epicenter of mass wasting where at least 18 debris flows removed structures, destroyed the roadway in multiple locations, and closed public access for months. To the west in Rattlesnake Canyon, a similar number of debris flows coalesced to travel several kilometers down the channel washing out the roadway, debouching into Sabino Creek, and contributing to a net 5-7 m of deposition within Sabino Creek. The debris flows were followed by streamflow floods which eclipsed the record discharge in the 75-year gaging record of Sabino Creek. The floods reworked most of the debris-flow deposits in or adjacent to the channel or buried them beneath coarse, sandy deposits.