_GENERAL DOCUMENTS - C1981017 (262)

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-25- All three mining operations in the general area are underground operations. The life-of-mine areas of these mines are plotted on Figure 2. In addition, the coal seams to be mined, the area of surface disturbance, the area to be undermined during the first 5-year permit period and the area to be undermined during the life-of-mine are tabulated in Table 1 . This assessment is divided into the three major subsections: Description of the Hydrologic Environment, Prooable Hydrologic Consequences of Mining and Cumulative Hydrologic Impact Assessment. Uescriotion of the Hydrologic Environment The first part of this assessment describes the hydrology of the Roaring Fork River Drainage Basin. This includes a regional description of 1 ) topography, 2) precipitation and runoff, 3) geology, 4) ground water, and 5) surface water. Topography The topography of the Crystal-Roaring Fork Drainage Basin consists of broad flood plains and terraces in the lowland river valleys flanked by steep walled canyons and broad park areas along the tributary streams. The topography is strongly controlled by the erosional resistance of the sedimentary rock strata along the Grand Hogback. Resistant sandstone strata form steep sloped water gaps along stream channels, and form prominent ridges and cliffs. Erosive shale strata form the broad river valleys and intra-mountain parks. The tributaries that flow through the mine sites have steep relief and steep stream gradients. Fourmile Creek varies in elevation from 10,361 feet at Bald Mountain to 7,831 feet at the Sunlight Mine to 5,950 feet at the Roaring Fork River. Thompson Creek varies from 10,927 feet at Twin Peaks to 7,500 feet at the (North Thompson Creek Mines to 6,350 feet at the Crystal River. Coal Creek varies from 11 ,852 fleet at Huntsman Ridge to 8,000 feet at the surface facilities of the Coal Basin Mines to 7, 165 feet at the confluence with the Crystal River. Precipitation and Runoff Mean annual precipitation varies with elevation and slope attitude, that is precipitation is orographically controlled. The mean annual precipitation at Glenwood Springs (lowest end of the Roaring Fork watershed) is 16 inches per year; at cne Sunlight "tine, it is aoout 13 inches; at Basalt, Colorado, it is 15 inches; at the Coal 3asin ;-lines, it is 26.4 inches; and, at Ine upper elevations along Huntsman `2idge, the mean annual precipitation is in excess of 40 inches. Over half of the precipitation occurs as snowfall . The remaining precipitation is mostly concentrated in high energy thunderstorm events in late spring and early summer. Some of the runoff flows during the