ROCK FALLS AND DEBRIS FLOWS IN SEMIARID CANYONS OF THE SOUTHWESTERN UNITED STATES

Much research on mass wasting processes has been completed in susceptible mountainous regions of the United States. However, less attention has been paid to landslide activity in semiarid canyons of the desert Southwest, despite its common occurrence. Semiarid deserts of this region are ideal for the initiation of rock falls and debris flows due to a unique set of environmental variables, including climatic conditions, sparse vegetation, and a propensity of exposed sedimentary rock. Using the Grand Canyon as a study area, we examined characteristics of the regional climate, vegetation, and geology in order to develop a model that roughly predicts where and under what conditions mass wasting is most likely to occur in similar areas of the Southwest. The canyon itself provides an ideal setting for rock falls and debris flows because of the near limitless expanse of thousands of feet of steep exposures of alternating layers of weak and resistant sedimentary rock units, as well as a vast network of high-gradient, bedrock-floored tributary canyons. Rapid erosion of weaker, slope-forming sedimentary rocks undercuts and destabilizes more resistant, cliff-forming layers causing overlying cliffs to collapse, initiating landslides. Debris is rapidly mobilized on steep slopes and quickly directed into the tributaries which create ready-made chutes for the channeling and transport of mass wasted material, often associated with flashfloods. The probability of mass wasting is heightened where bedrock has been shattered by fault movement and by the bending of brittle rock layers by folding. The likelihood of landslides is only increased within the canyon by the lack of vegetation in this semiarid to arid desert region because of reduced cohesion. The drastic diurnal temperature variations and greater moisture occurring over an extended fall through spring season is essential for enhancing the freeze-thaw process that wedges fractured bedrock apart on steep slopes and cliffs, initiating rock falls; while late summer, monsoonal precipitation often falls in short, but heavy downpours, and is the main initiator of debris flows within the canyon.