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<br />. <br /> <br />. <br /> <br />Sediment Loads <br /> <br />Colorado River tributaries are among the most silt-laden <br />streams known, and include the Dirty Devil, San Juan, Paria, and <br />Little Colorado Rivers; however, less is known upstream reaches <br />and tributaries (Fig. 12). Muddy Creek drains the region south of <br />Rabbit Ears Peak (elevation 3,231 m) and flows 56 km to join the <br />Colorado River at Kremm~ing, Colorado (elevation 2,207 m). With a <br />drainage area of 751 km , and a mean discharge of 4.1 cms <br />(146 cfs; range = 0.03 cms to 47.3 cms, with peak discharge in <br />May) between 1982 and 1985, the estimated mean annual total <br />sediment discharge of Muddy Creek was 75,281 mt/yr (Ruddy 1987). <br />Sediment loads of this magnitude will reduce the capacity of the <br />proposed Wolford Mountain Reservoir by 10% per century. <br /> <br />The San Juan River rarely runs clear, its turbidity due to <br />input from its many tributaries and particularly those that cut <br />through the Mancos and Chinle shales. The metn annual discharge <br />of the San Juan River is approximately 2.6 km/yr (2.1 maf; Hunt <br />1974). Sediments carried by the river past the town of Bluff, <br />Utah between 1914 and 1965 typically consisted of fine sand and <br />silt, in roughly equal proportions (Mundorff 1968). Suspended <br />sediment loads, which are correlated with discharge, were <br />extremely variable, ranging from 4.6 million mt/yr during the <br />1958-59 water year to 101.6 million mt/yr during the 1940-41 water <br />year. The maximum daily record occurred in September 1929, when <br />10.4 million mt of suspended sediment passed Bluff in one day, a <br />sediment transport rate of 120 mt/sec. <br /> <br />The Colorado River near De Beque, Colorado, cuts through the <br />easily eroded, Eocene Wasatch Formation. That area has been <br />proposed as the dam site for the Una Reservoir. Butler (1986) <br />reported that sediment river at that site was 0.97 mt of sediments <br />per year between 1966 and 1984. Reservoir capacity at this site <br />would decrease by 30% per century. <br /> <br />Prior to impoundment in 1963, the Colorado River transported <br />a mean of 61.7 million mt of sediments through the Grand Canyon <br />annually, a value that tripled in high run-off years (Thomas <br />et al. 1960). Because competence increases with discharge, peak <br />sediment transport was correlated with peak runoff during the <br />spring months. Summertime peaks in sedimentation were timed with <br />tributary contributions from monsoonal rain fschmidt and Graf <br />1987). Siltation of Lake Powell to the leve of the penstock <br />intakes will 700 yr (U.S. Bureau of Reclamation); however, the <br />storage capacity will be significantly reduced long before that <br />time. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />30 <br /> <br />. <br />