Sabino Canyon, Coronado National Forest, Tucson, Arizona | A major storm system stalled out over southern Arizona in the final week of July 2006. The resulting extreme rainfall event(s) recurs roughly every 400 to 1,200 years. Hundreds of small volume debris flows formed including this one that blocked the Sabino Canyon Tram Road with cobbles and boulders of Oracle Granite shed from on high. For a short, excellent account of the events of July 2006 see Phil Pearthree's and Ann Youberg's Recent Debris Flows and Floods in Southern Arizona. (Photo by Lee Allison)
http://www.azgs.az.gov/arizona_geology/archived_issues/Fall_2006.pdf