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Open File Report 2007-1108

Debris Flows and Record Floods from Extreme Mesoscale Convective 

 Thunderstorms over the Santa Catalina Mountains, Arizona

by Christopher S. Magirl 1

, Craig Shoemaker 2, Robert H. Webb 1, Mike Schaffner 2, Peter G. Griffiths 1, and Erik Pytlak 2

1

US Geological Survey; 520 North Park; Tucson, Arizona 85719; magirl@usgs.gov

2

National Weather Service, Tucson, Arizona; craig.shoemaker@noaa.gov

ABSTRACT 

Ample geologic evidence indicates early Holocene and Pleistocene debris flows from the south side of the Santa Catalina 

Mountains north of Tucson, Arizona, but few records document historical events. On July 31, 2006, an unusual set of atmospheric conditions aligned to produce record floods and an unprecedented number of debris flows in the Santa Catalinas. 

During the week prior to the event, an upper-level area of low pressure centered near Albuquerque, New Mexico generated 

widespread heavy rainfall in southern Arizona. After midnight on July 31, a strong complex of thunderstorms developed 

over central Arizona in a deformation zone that formed on the back side of the upper-level low. High atmospheric moisture 

(2.00” of precipitable water) coupled with cooling aloft spawned a mesoscale thunderstorm complex that moved southeast 

into the Tucson basin. A 15-20 knot low-level southwesterly wind developed with a significant upslope component over 

the south face of the Santa Catalina Mountains advecting moist and unstable air into the merging storms. National Weather Service radar indicated that a swath of 3-6” of rainfall occurred over the lower and middle elevations of the southern 

Santa Catalina Mountains. This intense rain falling on saturated soil triggered over 250 hillslope failures and debris flows 

throughout the mountain range. Sabino Canyon, a heavily used recreation area administered by the U.S. Forest Service, 

was the epicenter of mass wasting, where at least 18 debris fl ows removed structures, destroyed the roadway in multiple 

locations, and closed public access for months. The debris flows were followed by streamflow fl oods which eclipsed the 

record discharge in the 75-year gaging record of Sabino Creek. In five canyons adjacent to Sabino Canyon, debris flows approached or exited the mountain front, compromising flood conveyance structures and flooding some homes. 

1) Atmospheric Physics of the Storm 

The heaviest rainfall began shortly after midnight on July 31 and lasted 6-8 hrs. An unusually moist atmosphere coupled with cooling aloft generated two mesoscale thunderstorm complexes that moved southeast into the Tucson basin 

(Figure 1). Simultaneously, a relatively strong, low-level southwesterly inflow jet (15-20 knots) developed with an

orographic upslope component over the south face of the Santa Catalina Mountains (Figure 2). The first of the mesoscale thunderstorms had a cold cloud-top structure (-72° C) and passed through the region from 2:00-4:00 AM (MST). 

A second mesoscale thunderstorm with a warm cloud-top structure (-60° C) developed near dawn and persisted until 

about 8:00 AM (MST), bringing heavy, sustained rainfall. The low-level, southwesterly jet provided lifting and fed 

additional moisture directly into the southwest-trending canyons on the southern side of the mountains. This atmospheric combination sustained convective activity for several hours over a number of mountain watersheds. 

2) Precipitation 

Rainfall amounts estimated from NWS radar (WSR-88D) data for the morning of July

31, the fifth consecutive day of widespread rainfall, showed totals up to 6” on the south 

side of the Santa Catalina Mountains (Figure 3). The peak 6-hour rainfall at the Molino 

Basin gage, operated by Pima County Regional Flood Control District, 5 mi east of Sabino Canyon (3,840 ft elevation), was 3.82” on July 31; the storm total was 4.64” and 

the 3-day total was 8.30”. Based on NOAA precipitation statistics [Bonnin et al., 2003], 

the 6-hour and 3-day recurrence intervals at this station were 175 and 1200 years, respectively (Figure 4). Similar multiday rainfall recurrence intervals occurred at the Sabino Dam gage; recurrence intervals on Mount Lemmon were more modest (Figure 4). 

Figure 6. Roadway damaged by debris flow in Sabino Canyon Recreation Area. The 

largest boulders on edge of channel were deposited by a prehistoric debris flow. 

4) Hillslope Failures 

72 72

!2):/.! Many of the hillslope failures initiated in steep colluvium or beneath bedrock cliffs near ridgelines, mobilizing debris flows in 

steep chutes leading to the valley axes (Figures 6-8). At least two 

modes of failure were observed: (1) failures occurring in a relatively small area at the base of a cliff or steep slope leading into a steep 

chute, and (2) a broad swath of failed colluvium creating a series of 

closely spaced rills and gullies coalescing into a single chute. More 

than 250 slope failures occurred, concentrated at elevations between 4,000 - 6,000 ft, on the southern slope of the Santa Catalina 

Mountains (Figure 9). 

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Figure 1. GOES water-vapor imagery of Arizona in the early morning of July 31 showing the humid air mass over the 

Southwest and the low pressure over New Mexico. 

Figure 2. Rawinsonde sounding from the Tucson 

International Airport from July 31 showing the lowlevel southwesterly jet and high precipitable water.
	


HOUR 
DAY 
DAY 
DAY 
DAY
 

0RECIPITATION%VENT
 

Figure 3. Cumulative precipitation from July 31 based 

on WSR-88D data with select ALERT raingage data. 

Figure 4. Calculated return period of rainfall for 

three sites on the Santa Catalina Mountains. 

3) Flooding Figure 8. Hillslope scars on western side of Sabino Canyon. 

Rattlesnake Canyon is in the background. 

A U.S. Geological Survey streamflow gaging

station on Sabino Creek (USGS 09484000)

showed the July 31 flood had four separate

peaks, the largest having a provisional discharge estimate of 15,700 cfs from 35.5 mi2

(Figure 5). This flow represents the largest 

flood in the 75-year gaging record (1932-

2006), and followed flood peaks of 6,390 and

7,930 cfs on July 29 and 30, respectively. 

Flood-frequency analysis, based on gage data

excluding the 2006 peak, determined the recurrence interval of the July 31 flood on Sabino Creek to be 100+ years. Including this

flood in the analysis lowered the recurrence 

interval to about a 90-year event. 

Figure 5. Provisional streamflow data in Sabino Creek 

and rainfall at Molino Basin over the last four day in 

July 2006. 

Figure 9. Over 250 hillslope failures occured in the Santa Catalina 

Mountains, most between 4,000 - 6,000 ft elevation. The extent of

Figure 7. Hillslope scar and debris-flow chute in Sabino Canyon. the 2003 Aspen Fire is indicated with the orange line. 

5) Debris Flows 

Within Sabino Canyon, most debris flows moved short distances 

down chutes and stopped upon reaching the flood waters in Sabino 

Creek (Figure 6). Although many chutes were scoured to bedrock, 

some chutes – particularly below broad hillslope failures – had considerable exposure of barren colluvium or regolith, increasing the 

likelihood of additional slope failures in the future. In five adjacent 

canyons, debris flows coalesced in the main channel and traveled 

several kilometers to the mountain front (Figure 10). In Rattlesnake 

Canyon at least 24 slope failures coalesced into a debris flow that 

traveled 2.5 mi, depositing large boulders (Figure 11) and destroying 

a roadway crossing. Where the debris flow entered Sabino Creek, 

about 20 ft of vertical aggradation occurred over a width of about 

100-150 ft (Figure 12). Bear Canyon experienced at least 41 slope 

failures that coalesced into a valley axis debris flow that reached the 

mountain front, clearing out a dense corridor of riparian vegetation. 

In Soldier Canyon at least 35 slope failures coalesced into multiple 

pulses of debris flow that exited the mountain front. Where this debris flow encountered a low bridge, deposition choked the stream 

channel with coarse sediment and forced recessional flow into a 

broad area across the alluvial fan (Figure 13). Several homes on this 

fan were flooded with sheet flow as a result of channel aggradation. 

In an unnamed canyon between Bear and Soldier Canyons, at least 7 

hillslope failures coalesced into a debris fl ow that traveled onto the 

alluvial fan nearly 0.5 mi from the mountain front. A roadway was 

washed out and several homes were flooded. 

Figure 10. Debris flows traveled several miles down streambeds approaching or exiting the mountain front in five canyons. Figure 11. Deposit of a debris-flow snout in Rattlesnake Canyon. 

Figure 12. Net vertical aggradation of the Sabino Creek channel below its confluence with Rattlesnake Canyon was about 20 ft. Figure 13. The debris flow in Soldier Canyon filled the existing channel near the Mt. Lemmon Short Road bridge and caused sheet flow 

6) The Aspen Fire 

In 2003, the Aspen Fire, the largest historical wildfire in the Santa Catalina Mountains, burned 85,000 acres of forest and shrubland on a 

mountain range that varies in elevation from 3,000-9,157 ft (Figure 9). Fire intensity was highest near the top of the range. On July 31, three 

years after the fire, 85% of the slope failures were either in unburned or low-intensity fire areas. Coupled with the conclusions of Cannon and 

Gartner [2005] that enhanced debris-flow potential on hillslopes impacted by forest fires decreases to pre-fire levels two years after the event,

preliminary analyses suggest that debris-flow occurrence was not strongly related to the Aspen Fire. Instead, an extreme precipitation event,

with a return period ranging from hundreds to about one thousand years, was responsible for the flooding and debris flows.
	

Acknowledgements 

Funding and support for this study was provided by Pima County Reginal Flood Control District. Supplemental funding was also provided by the 

U.S. Geological Survey and the National Weather Service. Rainfall data were provided by Pima County Regional Flood Control District. George 

Ezell at Southwest Helicopter flew the helicopter surveys needed to document slope failures. 

References 

Bonnin, G.M, D. Todd, B. Lin, T. Parzybok, M. Tekta, and D. Riley (2003) Precipitation-Frequency Atlas of the United States NOAA Atlas 14, Vol. 

1, Version 3, NOAA, National Weather Service, Silver Springs, Maryland. 

Cannon, S.H., and J.E. Gartner (2005) Wildfire-related debris flow from a hazards perspective, in Hungr, O. and M. Jacob (eds.), Debris-Flow Hazards and Related Phenomena, Springer-Praxis Books in Geophysical Sciences, 321-344.