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-58- Natural physical influences upon subsidence can be controlled to the extent that mine plans can be designed to avoid obvious hazards (e.g. shallow openings beneath perennial streams, landslide bodies, impoundments, etc.), or to limit subsidence in accordance with the current state-of-the-art of ground control (engineered control of roof and pillar failures in mines). Subsidence prediction and ground control engineering are not definitive sciences. Design engineering concepts must be verified through continuous monitoring (surface topography, ground water hydrology and surface water hydrology) and observation. Evaluation of the impacts of subsidence upon hydrologic systems requires the collection of extensive monitoring 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 discharge and consumption rates. The monitoring data should be updated continuously as mining progresses to improve engineering control design parameters or to mitagate any unexpected subsidence occurrences, Such procedures are necessary at each mine within the Somerset coal field in order to coordinate a comprehensive subsidence control plan. This monitoring is now being required of all operators through the Colorado Permanent Regulatory Program permitting process. Subsidence in the area has been studied or observed at the Somerset mine, the Nawk'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 water. The existence of weak rocks (such as shale) or soils at the 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 overburden, and 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 overburden infiltration rates, diverting water from streams and impoundments above the affected areas. Related changes in local piezometric surfaces could result i» the modification of springs and seeps. Waters, having entered the sub-surface strata, could migrate towards the underground workings, eventually permeating the strata and gradually migrating downdip beneath the Grand Mesa.