AEG Student Field trip – Impacts and Deposits of 2006 Debris-Flows in Sabino Canyon, Santa Catalina Mountains, Arizona

It was a brisk Sunday morning at the Sabino Canyon Recreation Area Visitor Center when Ann Youberg addressed the University of Arizona's student chapter of the Association of Environmental and Engineering Geologists (AEG) on the nature of debris flows in the Santa Catalina Mountains. Joining the AEG crew were faculty from Pima Community College's Earth Science Department and a geologist from the Coronado National Forest.

Ann began with an overview of the impact of wildfires on vegetation and the subsequent increased likelihood of debris flows during monsoon storms. Ann, Joe Cook and Phil Pearthree (AZGS Environmental Geology) then led the AEG crew through Sabino Canyon examining deposits and impacts of the 2006 debris flows. Jim Washburne, Sahra Associate Director for Education, was there, too, to discuss infestations and effects of non-native vegetation on the local ecosystem.

At our first stop below the confluence of Sabino Creek and Rattlesnake Creek, we compared photographs of the post-debris flow 2006 channel with the current channel geometry. The photos showed more than 20 ft of local channel aggradation - an admixture of boulders, coarse gravel, and sand. In the intervening 4.5 years, much of the fine-grained and matrix material was washed downstream, returning the channel floor to near its pre-2006 debris flow event level. Locally, the channel remains choked with large boulders, weighing 100s of pounds to tons.

AEG student leader John Lyons-Baral said, "the levees, scars, and scouring gave everyone a sense of the enormity of what had happened here." He noted, too, that, "those who returned down the canyon on the Phoneline Trail were also able to cross below perched rocks and some debris flow matrix of the Ocho Grande slide and see many other large landslide paths on the opposite canyon wall."

Copies of the field trip guide are available in PDF format at the AZGS Document Repository; Geology and Geological Hazards Field Trip of Sabino Canyon: results of the July 2006 Storms. For additional information on the July 2006 debris flow event, see "Recent Debris Flows and Floods in Southern Arizona" by Phil Pearthree and Ann Youberg.

Background. The 2006 debris flow event in the Santa Catalina Mountains motivated a two-prong study involving the US Geological Survey (USGS) and the AZGS. The USGS focused on the 2006 debris flows and floods, while the AZGS examined and mapped older Holocene and Pleistocene debris flows of 15 canyons on the south slopes of the Catalinas.

References:

Magirl C.S. and 5 others, 2007, Debris Flows and Record Floods from Extreme Mesoscale Convective Thunderstorms over the Santa Catalina Mountains, Arizona. USGS Open File Report 2007-1108.

Pearthree, P.A. and Youberg, A., 2006, Recent Debris Flows and Floods in Southern Arizona. Arizona Geology, v. 36, no. 3, Fall 2006.

Youberg, A., Cline, M.L., Cook, J.P., Pearthree, P.A., Webb, R.H., 2008, Geologic Mapping of Debris-flow Deposits in the Santa Catalina Mountains, Pima County, Arizona. DM-DF-01 v.1.0. (Report and 11 map sheets, 1:6000 scale, online at http://repository.azgs.az.gov/uri_gin/azgs/dlio/118)

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