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• The Dry Fork above Minnesota Reservoir (Basin 35) is the receiving water body for Basins 4 <br />through 6 and 26 through 33; Basin 34 is tributary to the Dry Fork below Minnesota Reservoir. <br />The channel thalweg for the Dry Fork spans multiple E Seam panels. Maximum subsidence <br />along the stream profile is approximately 9.3 feet. Channel slope in the lower reach is upwards <br />of 5.6 percent and average slope for the entire reach is approximately 5.3 percent. Slope <br />changes due to mining subsidence will range from a minimum of .1.5 percent to a maximum of <br />2.0 percent. Figure 17 depicts the pre-and post-mining stream profiles and demonstrates the <br />overall channel slope is not significantly impacted by subsidence. <br />The Dry Fork mining area includes the upper reaches of Deep Creek (Basin 37) and the <br />extension of Basin 35 (Dry Fork). The mining under Basins 35 and 37 is deeper (more <br />overburden) and the coal seam is thinner. Thus, the impacts on the stream should be less <br />severe. <br />The average pre-mining slope for Basin 37 is 7.2 percent while the slope in the lower basin is <br />5.4 percent. The maximum change in slope could be estimated as the maximum tilt estimated <br />in Exhibit 60E. This was estimated to be 1.9 percent for panels E3 and E4. Thus, as shown on <br />Figure 19A the expected change in slope should not impair any stream flow. <br />For Basin 35 the average slope for the creek channel in the Dry Fork area is 6.3 percent. The <br />impacts would be similar to discussed in Basin 37. Although not a stream, the Deep Creek <br />Ditch was also considered in this evaluation. It has an average slope of approximately 1.6 <br />percent. As shown on Figure 17B, the only area where subsidence could impact this ditch is <br />near its end. Since the ditch appears only affected by a portion of longwall panel E5 the slope <br />may increase and not decrease. If, however, any ponding should occur, Mountain Coal <br />Company has committed to perform the necessary construction to rectify the problem. <br />• <br />CHANGES IN STREAM CHANNEL HYDRAULIC CHARACTERISTICS <br />The channel characteristics for each of the basins (as shown in Table 3) were then analyzed <br />using geomorphological and sedimentation engineering relationships and formulas to determine <br />the extent and type of change to each channel segment and then to estimate the amount of <br />sediment yield change. <br />Hydrologic review has determined that the following basin characteristics will not change <br />significantly as a result of subsidence: <br />1. Mean annual runoff <br />2. Peak discharge <br />3. Dominant discharge <br />4. Forest cover <br />Due to the fact that the mean annual runoff, peak discharges, and the dominant discharge are <br />greatest for the lower end of each stream segment studied, which is where the channel changes <br />would be maximized, this analysis has concentrated primarily on the lower portion of each <br />stream segment. The computations were performed for the lowest channel reach within the <br />influence of the mining. <br />A principle of fluvial morphology, as confirmed by Manning's equation, is that the stream <br />channel characteristics described in Table 3 for channel width and depth will change as shown <br />below. <br />Tetra Tech - 0307161P 5