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West Elk Mine . All of these streams (with the exception of Raven Creek, and Deep Creek) within the South of Divide pernut azea are ephemeral, based on U.S. Geological Survey topographic maps and extrapolation of gaged streamflow data. Flows occur only in response to snowmelt and significant rainfalls. As discussed in Exhibits 55 and SSA, WWE and Mr. Pemberton used a variety of statistically-based methods to determine a representative average annual yield value that would apply to these drainages. Exhibits 55 and SSA concludes that for the purpose of computing pre and post-mining average annual sediment yields, an appropriate mean annual runoff for the subject basins of 475 acre-feet per square mile per yeaz should be adopted, even though site-specific data for the basins indicate a mean annual runoff of considerably less than this amount. From the standpoint of water rights, analyses of water yield by WWE for the Division No. 4 Colorado Water Court for the 1986 West Elk Mine water augmentation plan indicated that the typical annual water yield for tributaries to the Dry Fork of Minnesota Creek were approximately 200 acre-feet per square mile per yeaz. This is consistent with the average annual yield of the Dry Fork basin, which is also about 200 acre-feet per square mile per year based on the available data. In short, the appropriate average annual streamflow for the channels ranges from 200 and 475 acre-feet per square mile per year, with 200 acre-feet per square mile per year being used for water rights purposes and 475 acre-feet per square mile being used for sediment yield purposes. In addition to computing average annual yields, W WE and Mr. Pemberton calculated floodflows for multiple return frequency events. These are presented in Exhibits 55 and SSA. This is important because it is necessary to evaluate how the stream channels will respond to large flood flows after subsidence has occurred, especially with respect to sediment transport. The projected subsidence for the stream channels was determined using output from the CISPM, Version 2.0, calibrated using site-specific subsidence data. The stream channels were analyzed to determine the magnitude of change resulting from the change in channel slope. The changes to stream channel chazacteristics were analyzed using standazd procedures of the sedimentation and geomorphic engineering professions based on the effects of thalweg slope changes (either increase or decrease) due to mining-induced subsidence. Maximum estimated change in channel width is calculated to be seven feet, and the maximum change in channel depth is 0.4 feet. Changes in new regime width and depths occur in a slow process that may take from three to five yeazs or more. The likely change in sediment yield is not expected to be more than 5 percent. Reaches where the slope increases will undergo degradation (erosion), and reaches where the channel slope decreases will experience aggradation (deposition). Overall, the mining-induced subsidence impacts on existing stream channel pazameters and basin sediment yield aze not significant. See Exhibits 55 and SSA. As discussed in the subsidence section of this pemut document [(Section 2.05.6 (6)(e)(i)(C&D)], the height of the caved zone above B Seam mining is conservatively estimated at 2.St or 30 feet. The height of the fractured zone is conservatively estimated at 20t or 240 feet when t = 12 feet. Summing these two figures yields a total combined thickness for the caved/fractured zone of 270 feet above the B Seam. This can be contrasted with the minimum overburden thicknesses in the mine plan area of 375 feet. 1.05-158 Revised June 1005 PRIO