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<br />Using probabilities estimated from logistic <br />regression, we developed a statistical relation for <br />debris-flow frequency in which all 736 tributaries <br />had a probability greater than zero of producing a <br />debris flow each century; 60 percent of the <br />tributaries had a frequency of at least I debris flow <br />per century; and about 5 percent of the tributaries <br />had a frequency of more than 2 debris flows per <br />century. Analysis ofpar1icle-size distributions of 41 <br />intact deposits, suggests that debris flows in Grand <br />Canyon typically contain about 18 percent sand. <br />We developed a regression equation relating debris- <br />flow volumes to tributary drainage area to calculate <br />the amount of sand delivered by debris flow. By <br />combining our frequency model with relations for <br />debris-flow volume and par1icle-size distribution, <br />we developed a sediment-yield model for debris <br />flow in Grand Canyon. On average, debris flows <br />deliver between 0.14'106 and 0.30'106 Mglyr of <br />sediment to the main channel. Of that yield, <br />between 6,440 and 13,400 Mglyr of sand reacbes <br />the regulated Colorado River; while 23,000 to <br />48,400 Mg/yr is stored in unreworked par1S of <br />debris fans. Although debris flows deliver only <br />21,000 to 44,000 Mglyr of boulders (par1icles > 256 <br />mm) to the river, these few boulders have a critical <br />impact on the geomorphic framework of the river, <br />defining debris fans, rapids and related sand bars, <br />and are unlikely to be removed by regulated flows. <br />The total sediment yield by streamflow and <br />debris flow from the ungaged drainage areas is 2.8- <br />3.0'106 Mglyr. Between 4 percent and 23 percent of <br />the total is delivered by debris flow; the remainder <br />is delivered in streamflow. Of this total sediment <br />yield, 0.4" I 06 to 2.0'106 Mglyr is sand, although a <br />small amount of this sand is stored in unreworked <br />debris fans. Even with storage in debris fans, <br />between 0.1'106 and 0.5" 106 Mg/yr of sand are <br />added to the reaches between Glen Canyon Dam <br />and the Little Colorado River annually. This <br />amount is up to 33 percent of the sand delivered by <br />the Paria River, the only other source of sand-sized <br />par1icles in this critical section of Grand Canyon, <br />and double the 0.17'106 Mg/yr estimated in the <br />1995 environmental impact statement for the <br />operation of Glen Canyon Dam (U.S. Depar1ment <br />of the Interior, 1995). Sand delivered by debris <br />flows contributes up to 8 percent of the total sand <br />yields. Particles larger than sand - par1icularly the <br />boulders and cobbles delivered by debris flow - <br /> <br />are largely unaffected by regulated flows from Glen <br />Canyon Dam and continue to aggrade the Colorado <br />River in Grand Canyon. <br /> <br />REFERENCES CITED <br /> <br />Andrews, E.D., /991, Sediment transport in the <br />Colorado River Basin, in Colorado River Ecology <br />and Dam Management: Washington, D.C., National <br />Academy of Sciences, Committee to Review the <br />Glen Canyon Environmental Studies, p. 54-74. <br />Beverage, J.P., and Culbertson, J.K., 1964, <br />Hyperconcentrations of suspended sediment: <br />American Society of Civil Engineers, Journal of the <br />Hydraulics Division, v. 90, p. ] 17-126. <br />Blackwelder, E., 1928, Mudflow as a geologic agent in <br />semiarid mountains: Geological Society of America <br />Bulletin, v. 39, p. 465-494. <br />Brian, N., 1992, River to rim: Flagstaff, Earthquest <br />Press, 176 p. <br />Cooley, M.E., Aldridge, B.N., and Euler, R.C., 1977, <br />Effects of the catastrophic flood of December, 1966, <br />North Rim area, eastern Grand Canyon, Arizona: <br />U.S. Geological Survey Professional Paper 980, 43 <br />p. <br />Costa, J.E., 1984, Physical geomorphology of debris <br />flows, in Costa, J.E., and Fleisher, P.J. (editors), <br />Developments and applications of geomorphology: <br />Berlin, Springer-Verlag Publishing, p. 268-317. <br />Dendy, F.E., and Bolton, G.c., 1976, Sediment yield- <br />runoff drainage area relationships in the United <br />Stales: Journal of Soil and Water Conservation, v. <br />31, n. 6, p. 264-266. <br />Dolan, R., Howard, A., and GaHenson, A., 1974, Man's <br />impact on the Colorado River in the Grand Canyon: <br />American Scientist, v. 62, p. 392-40 I. <br />Enzel, Y., Ely, L.L., House, P.K., Baker, V.R., and <br />Webb, R.H., 1993, Paleoflood evidence for a natural <br />upper bound to flood magnitudes in the Colorado <br />River basin: Water Resources Research, v. 29, p. <br />2287,2297. <br />Farnsworth, R.K., Thompson, E.S., and Peck, E.L., <br />1982, Evaporation atlas for the contiguous 48 <br />United State: NOAA Technical Report NWS 33, 26 <br />p. <br />Flaxman, E.M., 1972, Predicting sediment yield in <br />Western United States, Journal of the Hydraulics <br />Division, Proceedings of the American Society of <br />Civil Engineers, HY 12, p. 2073-2085. <br />Folk, R.L., 1974, Petrology of sedimentary rocks: <br />Austin, Texas, Hemphill Publishing, 182 p. <br /> <br />38 Sediment Delivery by Unglged Trlbutlrles of thl Colorodo River In GrInd Clnyon <br />