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West Elk Mine <br />To determine the contribution that spring flows may make to stream flows in the West Elk lease <br />block, the spring flows measured in November 1977 were compared with average November flows <br />for Sylvester Gulch, Lower Dry Fork, Lick Creek, South Prong, and Horse Creek watersheds. In <br />making this comparison, it was assumed that seeps did not contribute to stream flow, that none of <br />the spring water was lost by infiltration or evapotranspiration, and that the springs flowed at the <br />same rate all month. The November 1977 sampling period was chosen because spring <br />contributions would be most important at this time to water use in the basin and because a good <br />data base was available for this period and represents the lowest flow of the year. <br />The results of this study indicate that springs contribute to 11 percent of the flow in Lower Dry <br />Fork, 4 percent of the flow in Lick Creek, 12.6 percent of the flow in South Prong, and 0.09 percent <br />of the flow in Horse Creek. Sylvester Gulch data indicated that springs contribute 102 percent of <br />the flow, but this is a fictitious number, since the Bear Mine (which was operating on Atlantic <br />Richfield leases as a contract miner at the time of the study) removed water from Sylvester Gulch <br />upstream of the gaging station. <br />The assumptions made above for calculating the spring contribution to stream flow probably result <br />in high estimates. However, ignoring Sylvester Gulch data, it is possible that springs could <br />contribute up to 6.2 percent of stream flow for the entire lease block in the dry part of the year. <br />North Fork <br />Baseline characterization of streamflow conditions for the North Fork at the West Elk Mine was <br />• developed using data from the USGS Gage near Somerset (09132500). This gage is located <br />immediately upstream of the mine as shown on Map 34. Discharge volumes on the North Fork <br />have averaged 330,500 acre-feet per year based on the 1934 to 2002 monitoring period. The <br />average annual yield of 330,500 acre-feet is equivalent to approximately 628 acre-feet-ger- square <br />mile per year. The mean daily flow during the period of record is 453 cfs. Flows at the Somerset <br />gage are influenced by approximately 3,000 acres of irrigation upstream of the site, as well as <br />storage in sizable reservoirs including Overland Reservoir (capacity 6,280 acre-feet) and Paonia <br />Reservoir (capacity 18,300 acre-feet). <br />Extreme conditions are of particular concern when assessing hydrologic impacts. Low flow <br />conditions are relevant for the evaluation of minimum streamflows and their relationship to aquatic <br />life, design of water supply systems, assessment of impacts of wastewater discharges, and the <br />determination of water rights yields. Two low flow conditions have been identified in this baseline <br />assessment of North ork streamflow: <br />1. The historic daily low flow of 17 cfs occurred in November 1950. <br />2. The annual 7-day to flow of 25 efs oea urD-ed in February 1978 <br />An analysis was performed to determine the frequency that North Fork flows dropped below the <br />Colorado Water Conservation Board's (CWCB) minimum streamflow right of 30 cfs for December <br />through February, and 60 cfs for March through November. Since Paonia Reservoir was <br />0 constructed in 1961, the North Fork near Somerset gage records were analyzed for the 40-year <br />period of 1963 to 2002. As shown in Figure 10, flows are below CWCB's minimum flows 22 days <br />2.04-96 Revised A'0vember 2004 PRI G; Rev. APri1 2006 PRIO; Sep. 2007 PR12; Feb. 2008 PR12