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4 11\ Supplying sediment to the Colorado River has not been previously r('cogni~ed. The purpose of this report is to document the extent of debris flows in Grand CJnvon National Park and the occurrence and magnitude of debris flows :n' three Colorado River tributaries. The effects of these events on the mainstem-channel morphology are necessary in understanding sediment trans?ort and hydraulic controls in the Colorado River. This study was funded in cooperation with the U.S. Bureau of Reclamation as part of a larger sttldy enti tIed "Glen Canyon Environmental Studies." AcknowledQ:ments He authors thank J. T. Brown, H. D. Daggett III, S. L. Reneau, and V. A. Y0Cum for their help with the fieldwork and companionship during the riVer trip. R. M. Turner of the U.S. GeOlogical Survey rephotographed historic Scenes in Monument, Crystal, and' Dragon Creeks. R. C. Euler of the National Park Service shared his Grand Canyon archaeological experiences and provided photographs of the Crystal Creek and Lava-Chuar Creek drainages. HalG . Stevens. formerly of the U.S. GeologicaL Survey, generously provided his matched photograph of Hillers' view of the mouth ,of Monument Greek. V. A. S. McCord and<T. P. Harlan of the Laboratory ,of Tree-RingR~search, University of Arizona, examined tree-ring specimens that exhibited damage from debris flows. J. D. ' Rogers, Rogers/Pacific Consultants, Lafayette, California, provided published and unpublished manuscrip;ts on debris flows and geology of the Grand Canyon. Bryan Brown, Kim Crumbo,Larry Stevens, and Mike Yard, Colorado River boatmen, shared their experiences with Grand Canyon navigation and changes in rapids. Mike Walker and Mike Yard provided eyewitness accounts.of Grand Ganyon debrIs flows. S. W. Kieffer andT. C. Pierson of the U.S. Geological Survey and S. L. Ret:reau of the University of California at Berkeley critically reviewed:';.the manuscript. Jack C. Schmidt and Julia B. Graf, Geological Survey, Tucson, discussed aspects of the research and provided a different perspective on the hydrologic effects of debris flows on the Colorado RiVer. We especially thank Curt Green, Bob Marley, and Tom Wise of the Geological Survey for being excellent field assistants and skeptics; we hope to..share more field seasons with them. The senior author especially thanks Green for his hydraulics lessons in t:he 416-2. EXTENT OF DEBRIS FLOYS IN THE GRAND CANYON Debris flows Are flowing water-based slurries of poorly sorted clay- to boulder-sized particles (Costa, 1984). Terms sometimes intended to be synonymous include mudflows, debris slides, debris torrents, mud slides, or lahars (volcanic debris flows). Debris flows occur in many different environments ranging from deserts (Johnson and Rodine, 1984) to montane forests (Gallino and Pierson, 1985) and offshore continental shelf slopes (Piper and others, 1985). Debris flows tend to form their own cha~ls between levees that are parallel to the flow direction when unconfined by an existing channel. These levees are composed of boulders which commonly appear to be floating in a matrix of gravel, sand, and clay