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higher percentages of sodium, magnesium, and sulfate. As flow progresses downstream along • <br />the Yampa, the water type generally remains dominated by calcium or sodium, and <br />bicarbonate. <br />Total dissolved solids (TDS) concentration varies inversely with discharge, and also <br />increases in the downstream direction. lorns et. al. (19656) presents data that portrays <br />the relation between water discharge and chemical quality of water for the Yampa River <br />near Maybell, Colorado. For a weighted average discharge of 108 cf s, TDS averages 389 <br />ppm, while for a weighted average cli scha rge of 9,750 cfs, TDS averages only 98 ppm. The <br />weighted average TDS concentration at this site is reported at 140 ppm, with an average <br />TDS discharge of 699 tons per day. Much further upstream at Steamboat Springs, Colorado, <br />the Yampa River is reported to have a weighted average TDS concentration of only 74 ppm, <br />with an average TDS discharge of only 94 tons per day. <br />lorns et. al. (19656) also includes water chemistry data for evaluating the suitability of <br />water for irrigation collected at low, medium, and high flows at three progressively <br />downstream sites along the Yampa River: 1) Steamboat Springs; 2) Craig; and 3) near <br />Maybell. For all three sites over the range of flows sampled, the water in the Yampa • <br />River is classified as either C1-51 or' C2-51, which categorizes the Yampa River as <br />exhibiting low to medium salinity hazard, and low sodium (alkali) hazard. This <br />categorization makes water in the Yampa River well suited for irrigation. Based on data <br />presented in lorns et. al. (19656), over 51,000 acres were irrigated in the Yampa River <br />Basin above Maybell, Colorado in 1957. At that time, the total annual TDS discharge was <br />estimated at 218,800 tons, and irrigation accounted for 13.6 percent (29,800 tons) of the <br />total annual TDS discharge. <br />Site Specific Surface Water Information <br />i Drainage Basin Geomorphology. Mining activities at the Seneca II-W Mine will be limited <br />primarily to two previously unnamed drainages in the headwaters of the Dry Creek <br />watershed. The southern drainage of these two, named the 006 Gulch, is part of the <br />Hubberson Gulch watershed. The northern drainage, named the 005 Gulch, drains directly <br />into Dry Creek. Names for t hese two drainages were selected based on NPDES permit outf all <br />designations, both existing (NPDES 006) and proposed (NPDES 005). Exhibit 7-6 shows both <br />drainages in relation to the proposed Seneca II-W' mining permit bou ndary. • <br />88 <br />