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- OD1579 . the capacity of the river to transport that load, and the river has been shifted into a condition of sediment deficit. Operation of the dam has greatly reduced the floods that exported large quantities of fine sediment from Grand Canyon and has eliminated the naturally occurring lower flows that allowed seasonal sediment storage. Sediment delivery to Marble Canyon decreased by about 99.5-99.6% after comple- tion of Glen Canyon Dam. Transport past the Grand Canyon gage decreased between 81 and 85%, because some fine sediment continues to enter the Colorado River from the Paria and Little Colorado Rivers and from smaller tributaries (Topping et aI., 2000a). The role of sediment availability in determining the con- centration of suspended sediment at any discharge is now greater than during the pre- dam period. . 3.0 THE VALLEY OF THE COLORADO RIVER . The study area consists of four segments: Glen Canyon, upper Marble Canyon, lower Marble Canyon, and upper Grand Canyon. Glen Canyon is the segment between River Mile -15 and River Mile -1, where the river flows through a canyon of Jurassic Navajo Sandstone and upper Triassic Kayenta forma- tion, and between River Mile -1 and + 1 where the river flows in a relatively open valley of lower Triassic sedimentary rocks. Upper Marble Canyon begins at River Mile 1, and river-level bedrock is Mississippian to Permian sediments. The downstream end of this seg- ment occurs where the valley widens and approximately occurs at River Mile 40. Lower Marble Canyon extends to the mouth of the Little Colorado River at River Mile 61; river- level bedrock is primarily Cambrian sedimen- tary rocks. Upper Grand Canyon occurs between River Mile 61 and 87, and river-level rocks include Cambrian sandstones and Prot- erozoic sedimentary, volcanic, and metamor- phic rocks. We use the informal names Tapeats Gorge, Big Bend, and Upper Granite Gorge to refer to the parts of this segment between River Miles 61 and 65,65 and 77, and 77 and 87, respectively. Finer resolution segmentation of the study area has been pro- posed (Schmidt and Graf, 1990; Melis, 1997), but is inappropriate for the types of data analyzed here. Channel width varies in relation to bed- rock lithology. The average width of the channel at base flow of227 m3/s in Marble and upper Grand Canyon is 86 m. Average channel width ofa flood of2750 m3/s is 130.9 m, 54% greater than channel width at base flow. The channel is widest where river-level bedrock is shale or inter-bedded shale, sandstone, and limestone. Average channel width at base flow is typically greater than 110m and is greater than 160 m at flood flow between River Mile 47 and 57 and between River Mile 66 and 73 (Fig. 5). The narrowest parts of the Colorado River are in upper Marble Canyon and Upper Granite Gorge (Table 1). Reach average channel width at base flow is less than 70 m and is less than 100 m at flood flow between River Miles 11 and 31 and between River Miles 78 and 87. There are 3 types of unconsolidated deposits that are widespread in the valley of the Colorado River: alluvium, colluvium, and eolian deposits (Hereford, 1996; Hereford et aI., 1993, 1998, 2000a, 2000b). The depth of these unconsolidated deposits that fill the bedrock trench of the Grand Canyon varies from 0 to 45 m, based on side-scan sonar records, geophysical studies, and borings at proposed dam sites (Table 2). Alluvium includes gravel bars and fine-sediment deposits that occur as terraces and flood plains and gravel bars (Fig. 6). These deposits occur mostly within the active channel and form narrow deposits beyond the channel. Collu- vium includes landslide deposits, talus, and debris-flow deposits. Eolian deposits mantle parts of debris fans, high terraces, and talus, especially in lower Marble Canyon and parts of upper Grand Canyon. 2.0 Water and Fine-Sediment Fluxes 7