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-37- rock unit. Such a condition could increase the interception of surface water. However, since the sandstones in the area above the coal seams tend to be lenticular and isolated, the underlying clays should tend to inhibit deep percolation of water. The dimensions of surface cracks appear to relate to the type of rocks in which they occur, the thickness of coal removed, the depth of the location of crack occurrence. Cracks in competent rocks tend to remain open while cracks in incompetent rocks and soils are likely to fill. In light of these observations a range of subsidence impact scenarios are plausible. Subsidence related cracking could increase surface and over-infiltration rates, diverting water from streams and impoundments above the affected areas. Related changes in local piezometric surfaces could result in the modification of springs and seeps. Waters, having entered the subsurface strata could reemerge downstream in the same drainages or migrate towards the underground workings. In some cases this water may reemerge as a discharge from the portal or as springs along the subcrop or outcrop. Over the life of the mine, subsidence from underground mining operations could affect surface water hydrology in the North Fork of the Gunnison and the Minnesota Creek watersheds. Subsidence effects would reduce the surface flow from an affected watershed in several ways: 1) by increasing infiltration which would, in turn decrease runoff; 2) by lowering the piezometric surface and causing springs to dry up; and, 3) by direct infiltration from streams and their alluvium, or terraces In areas not covered by soils, subsidence fractures would directly divert runoff into the ground water system. In such an area, numerous open fractures could significantly reduce surface runoff. In areas covered with soils (which would not maintain an open fracture), any reduction in runoff would be less severe. Subsidence fractures would increase deep percolation of soil water, but the infiltration characteristics of the soil would still control the amount of runoff. Infiltration might still increase slightly, because deep percolation might more rapidly deplete soil moisture and decrease anticedent moisture. Impacts on Surface Waters The Division, based on review of the permit applications for the mining operations considered in this assessment, has identified several possible affects of mining on surface water quality and quantity Surface water quality might be affected by the discharge of mine water from underground workings or by seepage from waste rock or coal refuse piles to the stream - alluvial aquifer system. The underground mining operations, by altering ground water flow, reversing hydraulic gradients, and through subsidence effects, may deplete springs and seeps or stream flows. In addition, the withdrawal of surface water for mine usage may deplete surface flows.