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<br /> <br />002641 <br /> <br />sediment yields are already relatively large due to a lack of soil <br />development and sparse ground cover. Thus, because surface mining will not <br />greatly alter these factors, the relative increase In sediment yield will be <br />small. Furthermore, areas of surface mining in the western part of the basin <br />actually being disturbed are expected to be relatively small. Thus, the <br />additional sediment load carried by the Yampa River at the Maybell gaging <br />station due to surface mining will be only an estimated 2 to 7 percent of the <br />present mean-annual sediment load passing that site, depending upon the <br />volume .of coal production. Thus, even at the greatest projected volume of <br />coal production in the basin by 1990, the additional quantity of sediment <br />contributed to the streams will be small relative to the total quantity of <br />sediment being transported out of the Yampa River subbasin or the basin in <br />its entirety. <br /> <br />SUMMARY <br /> <br />The mean-annual sediment loads at 17 gaging stations in the Yampa River <br />basin of Colorado and Hyoming were computed by the flow-duration, sedlment-' <br />transport-curve method (Miller, 1951). Sediment-transport curves for each <br />gaging station were constructed by combining separate curves for suspended- <br />and bedload-sediment discharges. The suspended-sediment curves were <br />determined by fitting a mean relation between measured suspended-sediment <br />discharge and water discharge. The bedload-transport curves were derived <br />from bedload-transport rates computed for various discharges by the Meyer- <br />Peter and Mueller (1948) relation. The annual sediment loads were computed <br />by combining the total sediment-transport curve with the observed cumulative <br />frequency of water discharges and summing the products. <br /> <br />Average sediment yields for the contributing drainage areas upstream <br />from the 17 gaging stations show that sediment is not contributed equally <br />throughout the basin. The most significant sediment-source area lies in the <br />downstream part of the Little Snake River subbasin. This qrea supplies about <br />60 percent of the entire sediment load passing in the Yampa River at <br />Deerlodge Park, although it Is less than 35 percent of the total basin <br />drainage area, and contributes less than 3 percent of the total runoff. In <br />contrast, the eastern part of the basin contributes only 14 percent of the <br />sediment load and 76 percent of the annual runoff. <br /> <br />The distribution of sediment yields In the Yampa River basin closely <br />reflects the variations in annual precipitation. The largest sediment yields <br />are found in those parts of the basin which receive from 10 to 14 Inches <br />(250 to 360 mm) of precipitation annually. This observation Is in agreement <br />with the conclusion of Langbein qnd Schumm (1958) that the greatest sed.iment <br />yields in the United States occur from drainage areas having about 12 inches <br />(305 mm) of annual precipitation. As the mean-annual precipitation increases <br />above 12 inches, the sediment yield decreases. Thus, as precipitation <br />increases from west to east across the Yampa River basin, sediment yields <br />decrease. <br /> <br />30 <br /> <br />,to <br />