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State Reg. <br />solved solids are lowest. For water year 1979 at the U.S.G.S. gaging •, <br />station near Hayden, total dissolved solids ranged between .06 to <br />.19 tons/acre-feet between May to August. The highest concentrations <br />occur during periods of low flow (.25 - .42 tons/acre-feet) at the U.S.G.S. <br />gaging station at Hayden between October and May and August through <br />September. In addition, pH also decreases during spring runoff. <br />Table 7-17A presents data from the gaging station for the Water Year <br />1979. <br />As shown in Table 7-17, the Seneca II Mine disturbs less than .3% of the <br />entire Upper Yampa River Bosin (that portion east of U.S.G.S. gaging <br />station at Hayden) and contributes less than .I% of the flow. This <br />would indicate that a 1000 fold dilution would occur at the confluences <br />of Grassy Creek and Fish Creek with the Yampa River. In addition as <br />the mine effluent leaves the mine area, it is combined with agricultural <br />runoff which according to Woolhiser (1981) is often higher in total dis- <br />solved solids than the mine effluent. <br />~' <br />~. <br />Seneca Coals, Ltd. recognizes the importance of the main stem of the <br />Yampa River for agricultural and municipal needs. Currently the nearest <br />U.S.G.S. gaging station upstream from the confluence of Grassy and Fish <br />Creeks on the Yampa River is in Steamboat Springs. There is no quality <br />data available at this station. However, because of the large amount of <br />flow in the Yampa River, the dilution effect and the combination of <br />agricultural runoff, it would be fruitless to establish o gaging station <br />directly above the confluence of Grassy and Fish Creek to determine the <br />effect of the surface mining of Seneca II on the Yampa River. This <br />statement will be shown quantitatively in subsequent sections of this <br />chapter. <br />B. Fish Creek Basin <br />Fish Creek lies approximately 4 miles east of the Seneca II Mine area. <br />Less than 1.22 square miles of the Fish Creek Basin will be disturbed by •' <br />the mining of Seneca I1. The area of disturbance lies within two unnamed - <br />drainages in the south and southeastern portion of the permit and com- <br />prises approximately 1.5% of the Fish Creek Basin. <br />7-56 Revised 5/27/81 <br />