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-26- laterally continuous Rollins sandstone. The laterally discontinuous lenticular sandstones within the Upper Mesa Verde Formation support only localized ground water flows, and are considered to be insignificant in terms of the overall hydrologic balance. Recharge to the Rollins sandstone occurs along outcrops, along subcrops beneath stream alluvium and in the channels of the North Fork and its tributaries. However, due to the steepness of the topography in the outcrop areas (i.e, sandstones are cliff formers) and the narrowness of the stream valleys, the Rollins sandstone receives little recharge, except in the area under the North Fork of the Gunnison River and its associated alluvial deposits. Some recharge to the Rollins sandstone may come indirectly from the percolation of snowmelt and precipitation downward through faults and fractures. Drill holes which penetrated the Rollins sandstone near the Orchard Valley Mine were initially dry, but after allowing these holes to recover, some accumulated up to 300 gallons of water in the casing. However, this amount of water is insufficient to provide a desirable well yield. This indicates that recharge along outcrops and subcrops above the level of the North Fork is insufficient to sustain ground water wells developed in the Rollins. The only domestic use of the water from the Rollins sandstone is from wells along the North Fork near the Hawk's Nest Mines. The water quality elsewhere is considered too saline for domestic use (Prince and Arnow, 1974). DESCRIPTION OF THE SURFACE WATER REGIME The North Fork of the Gunnison River is the major drainage for the general area. Smaller streams in or adjacent to the life-of arsine areas are all tributary to the North Foric. Figure 4 contains the drainage map of the study area with the locations of streams and reservoirs which are described in the following discussion. The North Fork has an average annual streamflow at Somerset of approximately 313,500 acre-feet per year (1962-1979). The flow is regulated by the Paonia Reservoir on Muddy Creek five (5) miles upstream of the town of Somerset, which became operational in 1962. River yields during that period have ranged from a high of 601,800 acre-feet per year in water year 1984 to a low of 82,270 acre-feet in water year 1977. Flow records for the North Fork of the Gunnison River near Somerset are given in Table 2. Water quality parameters for the North Fork are given in Table 3. As indicated, waters in the North Fork are a calcium bicarbonate type. There are moderate levels of sulfate. Salinity averages less than 100 mg/1. Just below Paonia Reservoir, Muddy Creek and Anthracite Creek join to form the North Fork of the Gunnison River. Anthracite Creek contributes about two-thirds of the flow to the North Fork. Anthracite Creek is a perennial stream with an average annual yield of 157,313 acre-feet for the period of record between July, 1977 to October, 1980 (U.S.D.I., U.S.G.S., 1980). This corresponds to an average flow rate of 217 cfs. The highest discharge recorded was 4,200 cfs on May 27, 1979. Low flows of 12 cfs have been recorded several times during the period of record.