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<br />Sediment Delivery by Ungaged Tributaries of the
<br />Colorado River in Grand Canyon, Arizona
<br />
<br />By Robert H, Webb, Peter G, Griffiths, Theodore S, Melis, and Daniel R, Hartley
<br />
<br />Abstract
<br />
<br />Sediment input to the Colorado River in Grand Canyon, Arizona, is a valuable resource
<br />required to sustain both terrestrial and aquatic ecosystems, A total of 768 ungaged tributaries
<br />deliver sediment to the river between Glen Canyon Dam and the Grand Wash Cliffs (river miles -
<br />15 to 276), The 32 tributaries between the dam and Lee's Ferry produce only streamflow floods,
<br />whereas 736 tributaries in Grand Canyon produce streamflow floods and debris flows, We used
<br />three techniques to estimate annual streamflow sediment yield from ungaged tributaries to the
<br />Colorado River, For the Glen Canyon and Marble Canyon reaches (river miles -15 to 61.5),
<br />respectively, these techniques indicate that 0,065.106 and 0.610.106 Mg/yr (0,68.106 Mg/yr of
<br />total sediment) enters the river, This amount is 20 percent of the sediment yield of the Paria River,
<br />the only gaged tributary in this reach and a major sediment contributor to the Colorado River. The
<br />amount of sand delivered ranges from 0,10.106 to 0,51'106 Mg/yr, depending on the sand content
<br />of streamflow sediment. Sand delivered in Glen Canyon is notably coarser (Dso = 0.24 mm) than
<br />sand in other reaches (Dso = 0,15 nun), A relation is given for possible variation of this sediment
<br />delivery with climate,
<br />Debris flows transport poorly-sorted sediment onto debris fans in the Colorado River, In the
<br />pre-dam era, debris fans were completely reworked during Colorado River floods, liberating all
<br />fine-grained sediment to the river; in the post-dam river on average only 25 percent of debris-fan
<br />volume is reworked, leading to storage of sand in the matrix of debris fans, We develop a
<br />sediment-yield model for debris flows that uses a logistic-regression model of debris-flow
<br />frequency in Grand Canyon, a regression model of debris-flow volumes, particle-size distributions
<br />of intact debris-flow deposits, and debris-fan reworking, On average, debris flows deliver between
<br />0.14'106 and 0.30'106 Mg/yr of sediment to debris fans throughout Grand Canyon. Together,
<br />streamflow and debris flow deliver nearly 2.8'106 Mg/yr of sediment to the Colorado River from
<br />ungaged tributaries, In the post-dam era of minimal debris-fan reworking, the combined sand
<br />delivery rate in Glen and Marble Canyons averages 0,32.106 Mg/yr, which is 20 percent of the
<br />sand delivery of the Paria River and double the 0,17.106 Mg/yr of sand estimated for this reach in
<br />the 1995 environmental impact statement for operation of Glen Canyon Dam.
<br />
<br />INTRODUCTION
<br />
<br />grained sediment thaI used to enter Grand Canyon,
<br />replenish beaches, and provide substrate for the
<br />riverine ecosystem. With the closure of the dam in
<br />1963, sources of fine-grained sediment in Grand
<br />Canyon have been limited to major tributaries, such
<br />as the Paria and Little Colorado Rivers and Kanab
<br />and Havasu Creeks (Andrews, 1991), and num-
<br />erous small tributaries. The major tributaries have
<br />
<br />Because of the presence and operation of Glen
<br />Canyon Dam on the Colorado River, sediment
<br />supply and transport in Grand Canyon is an
<br />important management issue (V.S, Department of
<br />the Interior, 1995; Schmidt and others, 199B), Glen
<br />Canyon Dam blocks the prodigious input of fme-
<br />
<br />INTRODUCTION 1
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