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<br /> <br />IllJ2639 <br /> <br />The relative significance of these estimates is shown In a comparison <br />with the existing sediment yields (fig. 5). Presently (1978), the sediment <br />yield from the Yampa River subbasin upstream from Steamboat Springs Is less <br />than 100 (tons/mi2)/yr or 35 (t/km2)/yr. Therefore, an almost fortyfold <br />increase in sediment yield Is Indicated for unreclaimed surface-mined areas. <br />This relatively large increase is due to the fact that the soil profile and <br />ground cover in this part of the basin will be changed appreciably. In the <br />western part of the basin, the soi I profi Ie and ground cover are les.s <br />developed; consequently, surface mining will not affect the erodibility as <br />much. The western area has relatively large sediment yields even though It <br />is presently largely undisturbed. Thus, surface mining will not Increase <br />sediment yields as much on a relative basis. Depending ~n the area, sediment <br />yields probably will Increase by fivefold to twentyfold due to surface <br />mining. <br /> <br />Based upon the surface-mining projections and impact estimates described <br />above, table 4 was prepared to summarize the estimated sediment yields from <br />surface-mined areas through 1990. As noted previously, not all of the <br />sediment eroded from hillslopes and small tributaries Is suppl led directly to <br />the main-stem stream. Because most of the surface mines will be located on <br />secondary tributaries, the estimated sediment yields were adjusted to reflect <br />a dra.inage area of 40 mi2 (104 km2). This area was chosen in order to <br />approximate the effect increased sediment yield from surface mines would have <br />on the main-stem Yampa River. Between 10,000 and 30,000 tons (9,DOO and <br />27,ODO t) of additional sediment are estimated to be supplied to the main- <br />stem YampaRiver annuaily due to surface mining within the basin for <br />projected conditions In 1990 (table 4). <br /> <br />The Impact of this additional sediment load In the Yampa River and Its <br />tributaries depends primarily on the location of the surface mining within <br />the basin. Sediment yields in the eastern part of the basin generally are <br />less than 100 (tons/mI2)/yr or 35 (t/km2)/yr. Surface mining in this part of <br />the basin may Increase substantially the quantity of sediment supplied to the <br />stream channels. For example, if all new surface mining In the basin were In <br />areas upstream from the gaging station on the YampaRiver below diversion <br />near Hayden (site 47, fig, 1), the estimated Increase in Sediment load <br />measured at this location would be 10 to 30 percent. This increase would be <br />the probable result of mining only about 1 to 3 percent of the contributing <br />drainage area. <br /> <br />It Is doubtful, however, that all additional mining will be located <br />upstream from the gaging station near Hayden (U.S. Department of the <br />Interior, 1976), A more realistic estimate is that only about 50 percent of <br />the new surface mining wi 11 be upstream of Hayden. If so, the likely <br />increase in the sediment load carried by the Yampa River near Hayden due to <br />surface mining through 1990 Is estimated to be 5 to 15 percent, depending <br />upon the volume of coal mined. <br /> <br />Farther west in the <br />yields due to the surface <br /> <br />basin, the relative impact of Increased sediment <br />mining will be less for two reasons. Existing <br /> <br />28 <br />