Debris flows have left a 20,000 year history in the canyon mouths of the Santa Catalina foothills in Tucson and will occur again.
A new report by AZGS geologist Ann Youberg is being released in the morning with maps of debris flow deposits in 15 canyons along the mountain front. The report includes 11 digial 1:6,000 scale maps on CD.
The work was done in cooperation with the U.S. Geological Survey for the Pima County Regional Flood Control District to assess the potential for more debris flows following the spectacular events in Sabino Canyon and other areas in July-August, 2006. In recent years, the foothills have seen a boom in new housing in and around canyon mouths. Many, it appears, are built on or adjacent to prehistoric debris flows.
Ann and her colleagues concluded that the 2006 events are rare occurrences in individual canyons – on average they probably occur once in a thousand years or longer. However, invasive plant species are making the foothills more susceptible to wildfire and increased runoff on steep slopes. That could lead to larger and more frequent debris flows.
Reference: "Geologic mapping of the debris flow deposits in the Santa Catalina Mountains, Pima County, Arizona," 2008, A. Youberg, M.L. Cline, J.P. Cook, P.A. Pearthree, and R.H Webb, AZGS Open File Report-08-06.