Summer monsoon July 2006 & resulting Debris Flows
For five days in late July 2006, the mountains of southern Arizona received unusually heavy rainfall. A final burst of precipitation on the morning of July 31st produced at least 433 hillslope failures in the Santa Catalina Mountains of Pima County (Fig. 1). In the drainages near Sabino Canyon, these masses of unconsolidated soil, rock and vegetation coalesced into debris-flows that traveled to the mouths of several canyons.
Fortunately, the 2006 debris-flows did not result in any injuries. But there was considerable damage to some canyon roads, outbuildings and hiking trails. At Gibbon, Soldier, and Bird Canyons debris flows nearly reached or spilled out of canyon mouths. The 2006 debris-flows were much larger than any that had occurred historically in the Santa Catalina Mountains, raising new concerns about the potential for damage to roads and homes near many canyon mouths in the Catalina Foothills.
New Mapping Shows 20,000-year History of Debris Flow Activity
In addressing homeowners concerns, the Pima County Regional Flood Control District contracted the Arizona Geological Survey (AZGS) and the US Geological Survey (USGS) to map and date debris-flow deposits in 15 canyons along the southern face of the Santa Catalina Mountains (Fig. 2). The high resolution, 1:6,000-scale maps are constructed on a color aerial photographic base. (At 1:6,000-scale, one inch on the map equals 500 feet on the ground.)
The results of that research are now available in a report by Youberg and others (2008), “Geologic mapping of the debris-flow deposits in the Santa Catalina Mountains, Pima County, Arizona”. The report describes deposits of large boulders in and around canyon mouths in the upper Catalina Foothills that were laid down by prehistoric debris flows.
Debris flows have occurred before and will occur again at the canyon mouths of the Catalina Foothills.
~ Phil Pearthree, section chief of AZGS Environmental Geology
Using field relationships, soil conditions, and radiometric age dates of surface exposures on boulders, Youberg and her team showed that numerous debris-flows occurred during the past 10,000-20,000 years at the mouths of each of the canyons.
And those same field relationships show that the 2006 events are rare occurrences in individual canyons – on average they probably occur once in a thousand years or longer. According to Pearthree, “… they have been rare events and the areas that are likely to be affected by them are quite limited”.
Pearthree warns, however, “increasing fire frequency on the steep slopes of the Santa Catalina Mountains due to invasive species like bufflegrass may result in greater runoff, and possibly increased debris flow frequency, in the coming decades.”
AZGS' Geologic mapping of debris flows in the Santa Catalina Mountains report - Online
The text with digital versions of 11 map sheets is online at the Arizona Geological Survey's Online Document Repository.
Full Report Citation: OFR-08-06, 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. The authors are affiliated with the Arizona Geological Survey, with the exception of R.H. Webb (US Geological Survey).