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_ ~,_.„~ Therefore, concentrations of TDS decrease during peak flow periods. In the summer, when ground water discharge makes up a larger percentage of the flow in the rivers, TDS values increase. The Colorado Water Conservation Board (1969) reports that intermittent (and ephemeral) drainages at lower elevations contribute most of the dissolved and suspended solids that leave the basin. The dominant cations in the Yampa and Williams fork Rivers include calcium, sodium, and magnesium, while dominant anions are bicarbonate and sulfate with some chloride present. The total dissolved solids (TDS) load in the Yampa River is increased by discharges from the Williams fork River. In the Williams Fork River the average dissolved solids concentration is 311 mg/1. Dissolved solids concentration of the Yampa River average 264 mg/1 upstream from the confluence with the Williams Fork River and 284 mg/1 downstream. Water within the Yampa River Basin is consumed through the irrigation of croplands, municipal water supplies, stock watering, cooling water for power plants, evapotranspiration by riparian vegetation and phreatophytes, and transbasin diversions. Irrigation of cropland constitutes the largest of these uses.. Surface water consumed within the Yampa River drainage basin 1n 1976 totaled approximately 445,000 acre-feet. Of that, near]y 399,000 acre-feet were used for irrigating croplands and hay meadows or for watering livestock. Other uses included 5,478 acre-feet for industrial purposes, 2,555 acre-feet for municipal water suppl#es, and 8,283 acre-feet for other unspecified uses (Steele et al., 1979). Industrial consumption has since increased by a total of 18,720 acre-feet/year due to use by the Craig generating station. Water for agricultural irrigation is generally obtained by simple stream diversion structures and networks of ditches for flooding grasslands and meadows during the summer months. The short growing season precluded growing of warm weather crops such as corn. In the Williams Fork River, an even higher proportion of the water used is for irrigation of grasslands and hay fields. D. Probable Hvdroiovic Conseouences of the Eagle No. 5 and No. 9 Nines The Eagle No. 5 and Eagle No. 9 Mines are located in a hydrologically sensitive area; there are four rock aquifers and two stream/alluvial systems that can potentially be affected by mining. Underground mining in these mines creates the potential for depleting the quantity of water in the four rock aquifers, the two alluvial aquifers, and the Yampa and Williams Fork Rivers. Underground mining in these mines also creates the potential for causing water quality degradations in the rock aquifers, alluvial aquifers and the rivers. This'section will go into detail on the impacts of underground mining in the Eagle Mines No. 5 and 9 to the hydrologic regime.