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DISCUSSION AND CONCLUSIONS The debris flow. had similarities indicative of the cause and nature of debris flow~ in Grand Canyon National Park. All three debris flows were initiated at slope failures in the Hermit Shale and Supai Group, especially the Esplanade Sandstone in the Supai Group. All debris flows transported a poorly sorted lIixture of clay- to boulder-sized particles with water contents that ranged from 23 to 33 percent by volume. The largest boulders transported ranged froll 9 tons in the Lava-Chuar Creek drainage to 37 and 47 tons in the Konument Creek and Crystal Creek drainages, respectively. Two of the three debris flows significantly constricted the Colorado River at the tributary 1I0uths. The frequency of debris flows reaehing the Colorado River is tentative; however, available data suggest that one debris flow reaches the Colorado River every 20 to SO years in the.e drainages. A cOllpilationof historical information on flow events froll Grand Canyon tributaries, however, indicates that debris flows occur more frequently throughout the park. The bedrock geology of Grand Canyon Nat~onal Park provides an ideal location for the initiation ofdebrls flows. The high relief combined with differential strength properties of the rocks results in a high potential for slope failures. The 1I0St cOllllon sources of mobilized sed1llents for debris flows are the Pomian. Hermi t Shale and underlying Esplanade Sandstone of the Supai Group. Other sources include in descending order the Pe,mian Kaibab Lilies tone , Toroweap Formation, and Coconino Sandstone (sequence overlies the Hermit Shale); Cambrian Muav u...tone and underlying Bright Angel Shale; and Quaternary basalts in the western Grand Canyon. Dispersive and swelling clays in some of these f01"1lations aid Inthe Inltlatlon of debris flows. The llagn1tudeand frequeney of debrls flows control the hydr.ulie. of the Color.do RiverlnGrandCanyon Natlonal Park. Debris fiov. fro. the ...lltrlbutarie..ggr.da fans that typlcally ,force the river.g.inst the oppo.ite v.ll of the canyon (flg. 2). Theabillty of ...11 dr.lnage., such uKonuaent Creek, to fom hydraulic controls (r.plds) on one of the l.rge.t river. in the United States is hydrologie.lly .ignifieant.The debris fans also cause flow separation zone. eonducive todepo.ition andstor.ge of.and on beaches. Reworkingof debris fans by 41.charge. ofebe Colorado River creates.econdaryriffles or r.pids (fig. 2). Debris flow. .re the .ource of largevolwaes of sand ent.ring the river.t cliseretepointa, although the debris flows occur infrequen~ly. Knowledge. o.fthe llagnltude---and-f-requencyof--:debrls flows-Is nec....ry for any under.tanding or long-tem estimates of sedillent transport in the Colorado R.iver in Grand Canyon National Park. lInURCIS CITED Beverage, J. P., and Culbert.on, J. K., 1964, Hyperconcentrations of .upended sedillent: American Soeiety of Civil Engineer., Journal of the Hydraulics Divi.ion, v. 90, p. 117.126.