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<br />just two years of data and one of those years was an extremely high flow year. <br />Butler also inferred that a minimum streamflow hydrograph cannot be <br />characterized by volume or peak discharge alone, both the frequency and duration <br />of the peak flows are also important. He concluded that the effects of changing <br />sediment loads are not limited to Yampa Canyon, and that ".. .changes in the total <br />sediment yield will affect the Green River. II <br /> <br />Rivers in disequilibrium between channel geometry and sediment transport <br />are the norm rather than the exception according to Andrews and Nelson (1989). <br />Such disequilibrium can be persistent over a period of decades. Andrews and <br />Nelson (1989) reported that the mean annual sand-sized sediment load decreased <br />from 2.3 million tons/yr pre-reservoir to 0.84 million tons/yr post-reservoir at the <br />Jensen gage (a 64% decline). Despite this decrease, they concluded that an <br />approximate equilibrium had now been attained between the sediment supplied <br />and sediment transported out of the Green River reach from the confluence with <br />the Yampa River to the confluence with the Duchesne River. Their reason for the <br />decrease in the mean annual sediment load at the Jensen gage for the period 1962 <br />to 1982 when compared to the pre-reservoir period was the decrease in the <br />magnitude of flows equal to or exceeded less than 30% of the time. They inferred <br />that there was no change in the overall sediment supply to the Green River. They <br />further claimed that the bankfull channel has adjusted slowly to the decrease in <br />peak flows. <br /> <br />Schumm and Gellis (1989) reported that sediment loads in the Colorado <br />River system have been declining since the late 19th century and early 20th <br />century when arroyo development (incision) resulted in a high sediment load in the <br />Colorado River. The annual sediment load in Grand Canyon has displayed a <br />marked decrease since 1942. Some of the decrease may be attributed to soil <br />conservation efforts and flood control, but Schumm and Gellis (1989) indicate that <br /> <br />41 <br />