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.--: ~ j . ~ , ,..." ,'" J Although several other factors besides ~recipitation commonly influence 5edi~ent yield, th;~e either are associated with precipitation or are broadly uniform throughout the Yampa River basin. The bedrock geology of the Ya~pa River basin is principally interbedded sandstone, mudstone, and shales. Likewise, hi11s10pe reI ief is general ly 5i~ilar throughout the basin so that neither of these factors can be primarily responsible for the observed variations in sediment yields. Conversely, soil type and ground cover vary in the basin, but these factors are closely related to the distribution of Drecipitation. Increasing ground cover and soi 1 development as precipfiation increases are primarily responsible for these d~cr-eases in sect1-i,nent yield. Thus, alth_ough the potential erosion by-:-air,fall';ricreClses east\.~~\d in the basin, increase~r- soil developmen-t anct"cn':o'Jndcover more ~h~r. compensate, so that sediment yield aecrea.ses ,wIth increasj-"g pre~_i.pitation:HP -..~..... Laroe increases (frol'1f""uO to 500 percen~) in coal mining in the Yampa River b~sin~afe- projected for the next is years. Host of this addit:ona\ coal produc~ion wi 11 be from surface mines. Even with the regulatory con- trols of the Surface Mining Control and,Reclamation Act of 1977 (Publ ic Law 95-8]), substantial quantities of~additional sediment are estimated to be eroded from the surface-mined areas and ray be transported into the main-stem Yampa Rive r . The potential impact of surface mining l'fOuld be the greatest in the eastern part of the basin. Existing sediment yields are less than 100 (tons/mi2)/yr or 35 (t/km?)/yr in this area. because of the extensive ground cover and soil development. Rainfall intensities ~re, however, greater in the eastern part'of the basin than in the western part. Consequently. one can anticipate significantly greater sedinent yields in the eastern part of the basin from lands which have been disturbed by surface mining. , i..-..-_J Estimated sediment yields fro~ surface-mined lands were computed by the PSIAC method. During mining and before complete reclamation, sediment yields are 1 ikely to be 4,000 (tons/mi2)/yr or 1,~OO (t/km2)/yr. Based on these estimates and the projected ranges of volumes of mined coal, the total amount of additional sediment suppl ied to the Yampa River in 1990 due to surface mining may be between 10,000 and 30,000 tons (9,000 and 27,000 t) annually. The significance of this additional sediment load within the basin depends largely upon where the majority of the sediment enters the main-stem Ya~pa River. Streams in the eastern part of the basin carry relatively small sediment loads under present conditions, and. hence, the additional sediment yield from surface-mined land could have a considerable impact. For exa~ple, if all of the surface mining I"ere loca:ed in areas of the Yampa River subbasin upstream from Hayden, the additional sediment could increase the annual load by as much as 30 percent, even though the amount of land disturbed would be less than 3 percent. As the amount of surface mining shifts to areas west of Hayden, the potential impact to the Yampa River , decreases in relative terms. Even at the greatest projected volume of coal production, the additional sediment yielcs cue to surface mining probably may not increase the present sediment load carried by the Yampa River near Maybe]] by more than an estimated 7 percent. 31