Arizona Geological Survey OPEN-FILE REPORT OFR 08-06

Arizona Geological Survey
INTRODUCTION
Background
Debris‐flow activity in the Santa Catalina Mountains in July 2006 dramatically illustrated the potential for
sizable debris flows to exit canyons along the mountain front and the potential for adverse impacts to
properties near canyon mouths. The purpose of this study was to map the extent of prehistoric debris‐flow
deposits at the mouths of selected Santa Catalina Mountain canyons in eastern Pima County. Arizona
Geological Survey (AZGS) geologists mapped the extent of latest Pleistocene to Holocene debris‐flow
deposits along channels at the mouths of 15 canyons. In late 2007 the scope of work was expanded to
include mapping of modern (2006) debris‐flow deposits in Soldier Canyon, and to delineate the
downstream extent of these deposits in other canyons. United State Geological Survey (USGS) geologists
collected wood and rock samples from selected paleodebris‐flow deposits to date deposit‐ages using
radiocarbon and cosmogenic dating techniques. This report summarizes our findings and describes the
mapped debris‐flow deposits depicted on accompanying map sheets.
Debris flows can be triggered by rare, extreme precipitation events when soils with high antecedent
moisture conditions receive prolonged, and sometimes intense precipitation resulting in failure of
saturated soil (Anderson and Sitar, 1995; Wieczorek and Glade, 2005). Evidence of geomorphic
responses to extreme precipitation events, such as remnant debris‐flow deposits, can be found
throughout southern Arizona. Potential geologic hazards associated with numerous, coincident debris
flows, however, were not fully appreciated prior to July 2006. During the last week of July, southern
Arizona experienced five consecutive days of early morning storms generated from monsoonal moisture
mixing with a persistent low‐pressure system centered over northwestern New Mexico (Magirl and
others, 2007). These increasingly wet storms culminated on July 31, 2006, when the last pulse of rain fell
on already saturated watersheds. Floods of record occurred in several washes throughout the region,
and hundreds of hillslope failures and debris flows were generated in at least four southeastern Arizona
mountain ranges (Pearthree and Youberg, 2006). While debris flows were previously recognized as
significant hazards in Arizona’s mountains and canyons (Wohl and Pearthree, 1991; Melis and others,
1997; Pearthree, 2004), the number and extent of debris flows from the 2006 storm event was
unexpected.
Location
The 2006 Santa Catalina Mountain debris flows occurred in response to extreme, prolonged
precipitation, with debris flows exiting or nearly exiting the mountains in four southeastern canyons:
Soldier, Gibbon, Sabino, and Bird. Canyons farther west along the south side of the Santa Catalina
Mountains did not have debris flows, but abundant evidence exists of past debris flows near canyon
mouths. AZGS geologists mapped debris‐flow deposits at the mouths of all major canyons on the south
side of the Santa Catalina Mountains: Agua Caliente, La Milagrosa, Molino, Soldier, Bear, Sabino, Bird,
Esperero, Ventana, Pontatoc, Finger Rock, and Pima Canyons (Figure 1). Four smaller, informally named
canyons were also mapped: Linda Vista Canyon on the northwest side of Pusch Ridge, Pusch Canyon just
west of Pima Canyon, Cobblestone Canyon just west of Finger Rock Canyon, and Gibbon Canyon
between Bear and Soldier Canyons. Gibbon Canyon is the only smaller, informally named canyon that
had 2006 debris‐flow activity.
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