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DEBIIS FLOIIS FlOK TlIBUTARIES OF THE COLORADO RIVER, GRAND CANYON NATIONAL PAlUC, ARIZONA: EXECUTIVE SUHKARY By Robert H. lo1ebb ABSTRACT Debris flows are a major process of sediment transport to the Colorado River from ungaged tributaries in Grand Canyon National Park, Arizona. Debris flows are slurries of clay- to boulder-sized particles of large magnitude aud short duration that occur infrequently. They are the source for potential large volumes of sand for beaches on the Colorado River. Debris flows create and malntain hydraulic controls (rapids) on the Colorado River at tributary mouths. INTRODUCTION A potentially large source of sand for Colorado River beaches in Grand Canyon National Park, Arizona, is dArived from sediment transported from small drainages (fig. 1). Little is known about the annual sediment yield from these drainages, and existing methodology for predicting sedillentyields from small b'\sins is not designed for high- relief basins with a large potential for slope failures. The key to estimating sediment transport is an understanding of the sediment-transport process. A previous flood report (Cooley and others, 1977) and recent mapping of alluvial deposits in tributary canyons during this project indicate that debris flows are the dominant process of sedillenttransport In aaall drainages in Grand Canyon National Park. Debris flows are common in ari~ and semiarid regiona,butthe!r importance ~, supplying sediment to the Colorado R.iver baa not been previously recognized. The pUrpose of this report is to doCUllent the oceUl'rence of debrb flows in Color~~o River tributaries. . Th!'~~_~tJ:'1h\lta~_~_.c:any0l'ls---were studlf!dfil-u detaU for ,- --'---~-_. --debn.;;f1ow--rr-equency and the ....gn1 tude of recent events. KETBODS "'';''-... --~ Debris flows are flowing water-based slurries of poorly sorted clay- to boulder-sized particles (Costa, 1984). Debris flows typically have volu.etric vater content of 15 to 40 percent coqpared With 40 to 80 percent for hyperconcentrated flow and 80 to 100 percent for streaaflows (Beverage and Culbertson, 1964). Debris-f.low deposita Were identified in tributary canyoft8 on the basis of poor sorting of particle sizes, lack of sed1aent~ry structures, and matrix support of cobbl.s and boulders. -1-