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-37- Evaluation of, the impacts of subsidence upon hydrologic systems requires the collection of extensive baseline data consisting of surface water and ground water monitoring (quantity and quality), precipitation gaging, evapotranspiration rates, geological mapping, topographic surveying and recording of mine inflow rates. The baseline data should be updated continuously as mining progresses to improve engineering control design parameters or to mitigate any unexpected subsidence occurrences. Such procedures are necessary at each mine within the district in order to coordinate a district wide subsidence control plan. These procedures are now being required of all operators through the Permanent Colorado Coal Mine Regulatory Program permitting process. Subsidence in the area has been studied or observed at the Somerset Mine, the Hawk's Nest Mine, the Bear Mine, and hypotheses forwarded concerning the Oliver No. 2 Mine. Dunrud, in a study regarding the Somerset Mine (USGS Prof. Paper 969), discussed numerous subsidence causes and effects, and while no specific and consistent effects were delineated, it is evident that the physical parameters mentioned above are important topics. For instance, the presence of strong rock strata (such as sandstone) at the ground surface will permit the opening of tension cracks which could extend through the effected rock unit. Such a condition could significantly affect the interception of surface water and the recharge of ground surface could result in much less pronounced surface cracking. 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 and thereby migrate downdip, eventually discharging via the portals into the Narth Fork of the Gunnison River. Possible Effects of Subsidence upon Minnesota Creek Drainage Over the life of the mine, subsidence from Mt. Gunnison's 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, & 3) by direct infiltration from streams and their alluvium.