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-15- For these reasons, the bedrock ground water impacts of each mine are localized to the immediate vicinity of the mine. It is assumed that ground water will not migrate along the strike. However, cumulative impacts to the hydrologic balance due to localized degradation or diminution of bedrock ground water could affect the nature of surface water and alluvial aquifers of the region. The Eastside and Coal Ridge No. 1 Mines are both located within the Colorado River Basin. The Coal Ridge No. 1 Mine is directly adjacent to the Colorado River, with the surface facilities located on alluvialjcolluvial fan terraces approximately 50 feet above the Colorado River. The Eastside Mine is located on Harvey Gap, a tributary to the Colorado River. The probable hydrologic consequences of mining on Harvey Gap (Eastside Mine) are detailed in the following section. This CHIS addresses the potential of the hydrologic impacts predicted for the two mines to cumulatively affect the Colorado surface water system and the related ground water of the alluvium. TABLE 2 DISTURBED ACREAGE DUE TO COAL MINING IN THE CHIS AREA Disturbed Acreage Mine Surface Underground Eastside 12 299 Coal Ridge No. 1 35 2640 The main stem of the Colorado River below Glenwood Springs, Colorado, has a drainage area of 6013 sq. miles. The natural flow of the stream is affected by trans-mountain diversions, storage reservoirs, power development, and diversions for irrigation of approximately 110,000 acres. The average discharge of the stream at Glenwood Springs, for seventeen years of record, is 3,357 cfs. The maximum discharge on record occurred on June 25, 1983, and was 27,900 cfs. The 1984 water year maximum discharge is reported to have exceeded the previous maximum but the date of exceedance has not yet been published. The minimum discharge on record occurred on February 11, 1981-- 870 cfs. Selected chemical parameters and flow for the 1983 water year are presented in Table 3 for the Glenwood Springs U.S.G.S gaging station. Also given is the 1983 water year flows for the Colorado River near De Beque, Colorado. The De Beque gaging station is located approximately 54 river miles downstream of the Glenwood Springs station. The drainage area of the Colorado River at the De Beque station is 7370 sq. miles. The Coal Ridge No. 1 surface facilities are proposed for location approximately 11 miles downstream of the gaging station at Glenwood Springs. The confluence of Harvey Gap with the Colorado River is approximately 20 miles downstream of the Glenwood Springs gaging station.