Ancient and modern debris flows of the Santa Catalina Mountains, Tucson, Arizona
Ancient and modern debris-flow deposits along the front-range of the Santa Catalina Mountains will provide the basis for comparisons of ancient debris-flow deposits from the Pleistocene-Holocene climatic transition with the debris-flow deposits from an extreme multiday storm that occurred in 2006, and for discussions of hazard assessments for extreme events, the changing depositional environments in fixed-channel conveyance flood-control channels, and how geologic evidence informs land-use decisions for hazard mitigation or avoidance.

ABSTRACT:
The field trip:
In 2006, southeastern Arizona experienced an extreme multiday storm that culminated on July 31st with 435 slope failures in the Santa Catalina Mountains north of Tucson. These failures coalesced into debris flows in eight front-range canyons, and five of these reached the mountain front near or within housing developments. On this field trip we will make three stops along the front range of the Santa Catalina Mountains to compare and contrast ancient debris-flow deposits from the Pleistocene-Holocene climatic transition with the debris-flow deposits from the 2006 storm.

Our first stop will be at Finger Rock Canyon where we will take a short hike to look at Pleistocene-Holocene debris-flow deposits near the head of the Finger Rock Canyon alluvial fan. From there we will head over to Sabino Canyon where we will explore the confluence of Rattlesnake and Sabino Canyons, where a debris flow from Rattlesnake Canyon in 2006 raised the bed of Sabino Creek by about 15 feet. Our last stop will be in Soldier Canyon on the Mt. Lemmon Short Road to discuss the 2006 debris flows that blocked the Mt. Lemmon Short Road bridge, forcing the recessional floods through previously abandoned areas of the fan and into houses. These stops will encourage discussions of hazard assessments for extreme events, the changing depositional environments in fixed-channel conveyance flood-control channels, and how geologic evidence informs land-use decisions for hazard mitigation or avoidance.

Hiking at each stop will be short but please come prepared with water and a hat as it will be warm.

Organized by:
Arizona Geological Survey, Ann Youberg and Phil Pearthree
US Geological Survey, Robert Webb

Itinerary:

1. 7:00 am. Depart from the symposium hotel.
2. 7:30 am. Start hiking from the Finger Rock trailhead to explore Pleistocene-Holocene debris-flow deposits on the Finger Rock fan.
3. 8:30 am. Leave Finger Rock for Sabino. We will drive into Sabino Canyon and park at the confluence with Rattlesnake Canyon. There are rest rooms at this stop. We’ll walk a short way up Rattlesnake to look at debris-flow levees and down Sabino Canyon a short way to look at the deposits from Rattlesnake.
4. 10:00 am. Leave Sabino Canyon for Soldier Canyon.  We’ll walk a short distance along the channel to look at the Mt. Lemmon Short Road bridge and adjacent areas to discuss the debris-flow deposition and subsequent flooding.
5. 11:30 am. Depart from Soldier Canyon for the symposium hotel. Return to the hotel by lunch time.