Debris flows occur in 600 tributaries of the Colorado River in Grand Canyon, Arizona when intense precipitation causes slope failures in bedrock or colluvium. These slurries transport poorly sorted sediment, including very large boulders that form rapids at the mouths of tributaries and control the longitudinal profile of the Colorado River. Although the amount of rainfall on the days of historic debris flows typically is not unusual, the storm rainfall on consecutive days before the debris flows typically had recurrence intervals greater than 10 yrs. Four types of failure mechanisms initiate debris flows: bedrock failure (12 percent), failure of colluvial wedges by rainfall (21 percent), failure of colluvial wedges by runoff (the “firehose effect;” 36 percent), and combinations of these failure mechanisms (30 percent). Failure points are directly or indirectly associated with terrestrial shales, particularly the Permian Hermit Shale, shale units within the Permian Esplanade Sandstone of the Supai Group, and the Cambrian Bright Angel Shale. Shales either directly fail, produce colluvial wedges downslope that contain clay, or form benches that store poorly sorted colluvium in wedge-shaped deposits. Terrestrial shales provide the fine particles and clay minerals—particularly kaolinite and illite—essential to long-distance debris-flow transport, whereas marine shales mostly contain smectites, which inhibit debris-flow initiation.
Using repeat photography, we determined whether or not a debris flow occurred in the last century in 164 of 600 tributaries in Grand Canyon. We used logistic regression to model the binomial frequency data using 21 morphometric and lithologic variables. The location of shale units, particularly the Hermit Shale, within the tributary is the most consistent variable related to debris-flow frequency in Grand Canyon. Other statistically significant variables vary with large scale changes in canyon morphology. Standard morphometric measures such as drainage-basin area, channel gradient, and aspect of the river corridor are the most significant variables in the narrow and deep eastern section of Grand Canyon. Measures of the location of source lithologies are more important in western Grand Canyon, which has broader and low-gradient drainages. Measures of geologic structure, and other standard hydrologic variates, were not significant.
Our results show that the probability of debris-flow occurrence is highest in eastern Grand Canyon. Throughout Grand Canyon, the probability of debris-flow occurrence is highest in reaches of the Colorado River that trend south-southwest. This direction is significant because most summer storms originate from a southerly direction, and the maximum slope of the regional structure is to the southwest. The binomial frequency of debris flows is not random in Grand Canyon, and tributaries of similar debris-flow frequency are clustered in distinct reaches.
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