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-25- Some fractures and faults transect the Mesaverde Formation and extend vertically to the surface. These faults and fractures produce narrow bands of secondary porosity within the rock strata. Due to the low permeability of the rock strata within the Mesaverde Formation, these faults and fractures provide the primary path through which water flows both vertically between rock strata and horizontally within rock strata. When faults and fractures are encountered within the mines in the North Fork region, they generally produce mine inflows. The flow characteristics of each mine inflow associated with faults and fractures depend upon the lateral extent and the proximity of the fault or fracture to a stream valley. All inflows from these sources are characterized by an initial surge of water which then either decreases, or ceases completely with time. Since most stream channels in the North Fork drainage basin are developed in zones of weak fractured rock associated with fracturing and faulting (Dunrud, 1976), mine inflows from fractures and faults below these streams mimic the temporal flow characteristics of the overlying streams. In the Hawk's Nest Mine, an inflow was encountered under the ephemeral drainage of Hawk's Nest Creek. The rates of inflow beneath this drainage fluctuate seasonally with a slight lag time in response to the flows in the overlying stream. In the Somerset Mine, inflows beneath Hubbard Creek are continuous and may reflect the perennial nature of this stream. Inflows from faults and fractures located outside stream valleys (such as in the Orchard Valley Mine) generally dry up with time or flow intermittently at discrete points along the fault or fracture. Those which continue to flow have flow rates which diminish to a trickle. Such inflows may represent the dewatering of lenticular sandstone units with limited recharge areas, or may represent flows through fracture zones extending to the surface which have narrow recharge zones on steep slopes. 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-mine areas are all tributary to the North Fork (Figure 1). 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 annual streamflow at Somerset of approximately 313,500 acre-feet per year (1962-1979). The flow is regulated by the Paonia Reservoir 5 miles upstream of the town of Somerset, which became operational in 1962. Water yields during that period have ranged from a high of 451,300 acre-feet per year in water year 1962 to a low of 82,270 acre-feet in water year 1977. Flow records for the North Fork of the Gunnison 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.