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West Elk Mine this analysis because overburden materials within the Mesaverde Formation contain numerous shale layers and lenses, which tend to undergo plastic deformation under compression. This seals fractures, which may develop in response to subsidence. In addition, siltation will assist in the filling of surface cracks, which may develop, further reducing the potential to transmit water downwazd. Another important factor in minimising the potential loss of surface water to the mine workings is the depth of cover. D. Y. Dixon states in The Impacts of Three Longwall Coal Mines on Streamflow in the Northern Appalachian Coal Fields (1989) that streamflow showed fewer effects of undermining with increased overburden thickness. Additionally, K.L. Johnson (1992) found in a study of two longwall mining sites in the northern Appalachian coal field where the depth of mining is 500-600 feet, that longwall mining beneath the two stream valleys did not lessen the quantity or quality of shallow groundwater in the valleys, nor did it affect streamflow. Subsidence impacts on streams above two longwall mines studied recently in Utah help provide a basis for further estimating impacts in the pernmit area. The basic conclusion from the studies conducted at the Utah mines is that there has been no impact to date on stream base flow where the overburden thickness is more than 500 to 600 feet. Some details aze described below for two Utah study azeas: 1. The azea is located within USFS lands on the North Fork of Miller Creek at Cyprus Plateau's Staz Point Mine. Longwall mining panels were driven beneath this perennial stream in overburden that ranges in thickness from 50 to 1,100 feet. Subsidence cracks and diversion of streamflow were observed above the mining panels where the overburden thickness was less than 300 feet to about 500 feet. However, no cracks or reduction of stream base flow were observed where the overburden exceeded 500 feet (USGS 1995). 2. This azea is located within USFS land beneath a perennial stream neaz the headwaters of Huntington Canyon. The Skyline Mine, recently started a third panel beneath the stream at an average minimum depth of approximately 600 feet. The longwall mining panel width is about 700 feet, which makes the depth roughly critical. The subsidence factor reportedly ranges from about 0.5 to 0.8. Mining just recently began in the third panel beneath the perennial stream, so all information should be considered preliminary. No impact on stream base flow has been measured thus faz, however, changes in the transverse stream channel profile and some other hydrologic parameters have been observed (USFS, 1995). The E Seam overburden thickness varies from approximately 400 feet to 1,100 feet over the three west longwall panels (i.e. the 12NW through 13aNW longwall panels) from which the E Seam, as well as the B Seam, will be mined. However, less than one percent of the mining area has an overburden depth of less than 500 feet. Overburden thicknesses between 800 and 2,250 feet exist over the four eastern B Seam panels (i.e., the 14SE through 17SE longwall panels). Minimum overburden thickness above the NE longwall panels is 600 feet. Within the Box Canyon permit revision area, the overburden thickness is between 500 and 2,200 feet. No portion of the proposed mining area has an overburden depth of less than 500 feet. Minimum overburden thickness above the NE longwall panels is 600 feet. These considerable overburden thicknesses greatly reduce the ~ potential of connection between subsidence-induced surface cracks and the mine workings. 1.05-162 Revised June 1005 PRI G