INITIATION OF DEBRIS FLOWS IN BEDROCK CANYONS IN THE COLORADO RIVER DRAINAGE, USA

Debris flows are initiated in canyons of the Colorado River system when rainfall causes failures in bedrock and colluvial deposits. Of 68 debris flows observed in Grand Canyon, 78% initiated in colluvium (62% by the fire-hose effect) and 9% in bedrock. Most debris flows in Grand Canyon are triggered during localized summer thunderstorms, but some also occur during winter frontal storms. We examined rainfall from storms associated with 27 historic debris flows in Grand Canyon. Recurrence intervals (RIs) for 1-day rainfall range from <1 to >60 yrs; most RIs were <10 yrs. Multiday storms had RIs that ranged from <1 yr to 158 yrs, but most had RIs <34-yrs. Hourly data from both seasons suggest that storms that cause debris flows terminate with a burst of intense rainfall, which complicates the use of daily rainfall to assess debris-flow hazard. The short RIs of storms contrast with the longer RI (10-50 years) of actual debris flows and underscores the importance of geologic factors in debris-flow initiation. Shales exposed at heights >100 m above the river are critical to the initiation of debris flows in the Colorado River drainage. Weathered shale fails readily to directly produce debris flows or contributes silt- and clay-size particles to the colluvial sources of other debris flows. Debris-flows along the Colorado River are associated with shales of terrestrial provenance, such as the Hermit Formation, or lacustrine/shallow marine origin, such as the Morrison Formation. Deep-water marine shales, such as the Mancos Shale, rarely produce debris flows in Holocene climates. We sampled 47 shale units and 7 debris-flow deposits on the Colorado Plateau to assess clay mineralogy and chemistry. Debris-flow producing shales and debris-flow deposits generally contain >50% illite and kaolinite, <10% smectite, and relatively high concentrations of K⁺ (2-15%) and Mg⁺⁺ (5-20%) with <1% Na⁺. Shales that rarely produce Holocene-age debris flows contain >40% smectite, generally >10% Na⁺, and only small amounts of K⁺ and Mg⁺⁺. We hypothesize that debris flows occur in the Colorado River drainage where shales contain little smectite, possibly because swelling clays inhibit deep percolation to failure planes or limit flow rheology.