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-23- C. Surface Water The study area includes the Yampa River upstream of Hayden, Colorado and all tributaries. The Yampa River is the major perennial stream in the area. Its headwaters lie in the mountainous areas to the north, east and south of the study area. Flow in the Yampa River is dominated by snowmelt runoff with more than half the annual flow occurring in April, May and June. The U.S. Geological Survey maintains a gaging station on the Yampa River near Hayden, Colorado. Data on the quantity and quality of streamflow at this station are published annually in the U.S. Geological Survey's Water Resources Data for Colorado. The Yampa River basin at the U.S.G.S. gaging station 4.7 miles east of Hayden, has a drainage area of 1,430 square miles. The altitude at this gaging station is 6,380 feet. Natural flow of the Upper Yampa River is affected by diversions for irrigation of about 30,000 acres. There are also transbasin diversions and storage reservoirs located along the river. The average discharge for the Yampa River, at the Hayden station, was 1,043 cfs over a 17 year recording period from 1965 to 1472. The water quality of the Yampa River at Hayden is excellent for irrigation purposes with low SAR values (less than 1.0), specific conductance averaging 243 umhos/cm, and total dissolved solids averaging 157 mg/1. The levels of suspended solids are also low for the 1981 water year averaging 26 mgJ1. Chemistry and discharge data on some of the tributaries to the Upper Yampa River are available from U.S. Geological Survey gaging stations. More site specific data can be found in the mine permit applications on file with the Division. The major tributaries to the Upper Yampa River are, from upstream to downstream, Oak Creek, Elk Creek, Trout Creek, and Grassy Creek. These streams are all perennial. Only Trout Creek and Grassy Creek are affected by mining. Trout Creek has three major tributaries; Fish Creek, Foidel Creek, and Middle Creek. Many of the smaller drainages of the area, such as Foidel Creek, have intermittent flow. Peak discharges occur during the period from March through June, from a combination of snowmelt, incident precipitation, and ground water contribution. Surface water quality in the area is affected at the baseline level by farming, road drainage, and previous mining. Concentrations of major and trace constituents vary greatly with the rates of flow and evaporation. Total dissolved solids levels in Fish Creek and Foidel Creek have range from 216 mgJl to 1,350 mg/1 and 380 mg/1 to 3,460 mgJl, respectively. Total dissolved solids concentrations are highest during low flow months and lowest during spring runoff. Total suspended solids show the reverse seasonal trends.