7049 - Laserfiche WebLink

Home Browse Search

,cot' The .elative sig.!lificance of these estimates is shOlvn in a comparison \vith the existing sediment yields (fig. 5). Presently (1978), the sediment yield fro~ the Yampa River subbasin upstream from Steamboat Springs is less than 100 (tons/mi2)/yr or 35 (t/km2)/yr. Therefore, an almost fortyfold increase in sediment yield is indicated for ur.reclaimed surface-mined areas. This relatively large increase is due to the fact that the soi I profile and ground cover in this part of the basin wi 11 be changed appreciably. In the western part of the basin, the soil profile and ground cover are less developed; consequently, surface mining vlill not affect the erodibi-cl)ty as much. The \';estern area has relatively large sed.inent,y>i'elds even theu.gh it is presently ,Iargely__ undisturbed. _Thus,-'surf2ce mining will not fncrease sediment yields as much on a relative basis. Depending on the area, sediment yields probably wi 1'1 increase by fivef::ld to twenty,fold due to surface mining. .~~~~~ Based upon ~he surface-mining projections and impact estimates described abo'le, tabl,: 4 -w'as prepared to summarize the estimated sediment yields from surface-mined areas through 1990. As noted previously, not all of the sediment eroded from hi 1 lslopes and small tributaries is suppl ied directly to the main-stem stream. Because most of the surface mines wi I I be located on secondary tributaries, the estimated sediment yields were adjusted to reflect a drainage area of 40 mi2 (104 km2). This area was chosen in order to approximate the effect incr~ased sediment yield from surface mines would have on the main-stem Yampa River. Between 10,000 and 30,000 tons (9,000 and 27,000 t) of additional sediment are estimated to be suppl ied to the main- stem Yampa River annually due to surface mining within the basin for projected conditions in 1990 (table 4). The impact of this additional sediment load in the Yampa River and its tributaries depends primarily on the location of the surface mining within the basin. Sediment yields in the eastern part of the basin generally are less than 100 (tons/mi2)/yr or 35 (t/km2)/yr. Surface mining in this part of the basin may increase substantially the quantity of sediment suppJ ied to the stream channels. For example, if all new surface mining in the basin were in areas upstream from the gaging station on the Yampa River below diversion near Hayden (site 47, fig. 1), the estimated increase in sediment load measured at this location would be 10 to 30 percent. This increase would be the probable result of mining only about 1 to 3 percent of the contributing drainage area. It is doubtful, however, that all additional mining will be located upstrea~ from the gaging station near Hayden (U.S. Department of the Interior, 1976). A more realistic estimate is that only ab'out 50 percent of the new surface mining wi II be upstream of Hayden. If so, the J ikely increase in the sediment load carried by the Yampa River near Hayden due to surface mining through 1990 is estimated to be 5 to 15 percent, depending upon the volume of coal mined. Farther '.vest in the yields due to the surface basin, the relative impact of increased sediment mining wi 11 be less for two reasons. Existing 28 "';':'-,7rl~"m,~,