<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
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