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-38- The contribution of salts to leachate from existing and proposed waste piles along the North Fork of the Gunnison River will double the TDS for the 10-year -day low flow, but increase the TDS only seven percent for the average flow (Table 11 ). This is probably an overestimation of the impact since seepage from piles is the result of snowmelt and would probably occur when stream flows are much higher than average. Subsidence Impacts on Surface Water Hydrology Subsidence 1n the North Fork of the Gunnison Ya11ey could modify the area's surface water hydrology. These modifications can be divided into three categories--storage capacity or streamflow reduction, a change in the drainage's surface water hydraulics, and geomorphic impacts. These will be discussed in greater detail below. _ _ _ fie surface expression of subsidence fractures may impact a dam's capability to hold water. Consequently the Orchard Ya11ey Mine monitors the volume of water in stock ponds located in affected areas and in areas projected to be affected. Two large reservoirs occur within or on the life-of arsine lease boundary of WECC Mt. Gunnison No. 1 Mine--Minnesota (Monument) Reservoir and Beaver Reservoir. Both are located below the outcrop of the F seam, and the portions of lower coal seams beneath these reservoirs are legally severed from the lease. Therefore, these two water bodies are not likely to be affected by mining. • Streamflow reduction could develop from three separate scenarios--increased infiltration of runoff through subsidence fractures, direct stream recharge to the mine in zones of stream undermining, and loss of spring flow. Infiltration of runoff could occur on the surface or along fractures beneath colluvial or alluvial deposits. Despite the presence of subsidence features at several of the mine sites, this impact has not been identified by arty of the monitoring programs. Subsidence could reduce streamflow by directly diverting surface flow and alluvial ground water into the bedrock ground water system. Extensive mining under streambeds developed along strong fracture zones or in areas of thin overburden could result in substantial Interception of surface water by the ground water system. Information presented in the Nawk's Nest, Blue Ribbon and Somerset permit applications indicates that mine inflows in areas underlying streams are related to the flow 1n streams, as evidenced by higher inflows during snowmelt and lower inflows in late fall and winter. The duration of stream flow affects the severity of impacts and is related to the stream's maturity. Mine inflows are not normally significant when ephemeral streams are undermined and most of the streams to be undermined in the region are ephemeral. Flow 1n these ephemeral streams are concentrated in periods of snowmelt and high intensity precipitation events. fie stream gradients are steep and their channels contain little alluvium. Perennial streams, however, may experience significant depletions of flow if mine workings are extended adjacent to and/or below the level of the stream valley . (e.g., the bedrock alluvial contact). The lower stream gradient and the thicker alluvial deposits in perennial stream valleys favor flow retention and recharge to the ground water systems. The Mt. Gunnison mine wilt undermine several perennial drainages during the life of the mine--the Dry Fork of