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A  flood comprised largely of coarse particles - sands, cobbles and boulders - and containing only about 20% water; a viscous flood with the consistency of a wet concrete slurry. 

Debris flows occur on steep slopes where loose unconsolidated Earth materials – soils, vegetation, and rock material (from clays to boulders) – are set in motion frequently during heavy rains or snowmelt.  They travel downslope rapidly, easily attaining velocities of 20+ miles per hours in steep mountain canyons.  Debris flows are capable of great damage and loss of life.

Debris Flow Deposits

Small-volume debris flows, such as the 2006 flows, are typically confined to stream beds where they leave telltale deposits of their passing.  As they travel downstream they frequently produce narrow levees of a chaotic  mixture of boulders and cobbles.  As the flow moves further down channel, it clogs the stream channel with boulder-bar deposits of unsorted cobbles, boulders and interstitial sands.   At their snout - at the far end of their run-out - they build broad, u-shaped aggregates of boulderly debris.

Mapping debris flows
Youberg and her team used a time-sequence of aerial photographs – from 1960, 2002, and 2007 – along with detailed field observations to map debris flows.  In the field, they walked each canyon to the likely downstream limit of debris‐flow deposits.  They recorded their observations using GPS receivers. 

They used topographic relationships, surface appearance of boulders, and soil characteristics to distinguish between recent and older debris-flow deposits.   At times, the mapping was hampered by construction of homes and roads that obscured or otherwise modified the deposits.   






Santa Catalina Mtn. Debris Flows - July2006

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.
Figure 1. Debris Flow Chute
and Deposit
Figure 2. Mapped Debris Flow Deposit Locations
FREE Preview of Sabino & Bear Canyon Debris Flow Map

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.  

Figure 3. Stacked debris flow deposits in Soldier Canyon
...said Phil Pearthree, section chief of AZGS Environmental Geology (Fig. 3).  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.”

Want a Copy of this Report? | The text with digital versions of 11 map sheets on CD-ROM, is available for $20.00 from the State Map and Bookstore at 416 W. Congress, Tucson, AZ (520.770.3500), and at Explore Arizona: Outdoor Information Center, One North Central, Phoenix (602.417.9300). Printed copies of individual maps are also available; contact the Arizona Geological Survey for prices. Download Press Release Here .

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).

Related Links: Arizona Geology, v. 36, no. 3: “Recent Debris Flows and Floods in Southern Arizona” , USGS Debris-Flow Hazards in U.S. , FEMA – Landslides & Debris Flows , California Geological Survey – Debris flows , A narrated video of debris flows
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