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Section 780.21(b) Continued. <br />• <br />tation ponds in the mine plan and tipple areas equals 66.9 acre- <br />feet for the Canadian River Watershed, and 0.74 acre-feet for the <br />Illinois River watershed. The mean annual discharge volume of the <br />Canadian River ~uat upstream of the mine plan area is 111,360 <br />acre-feet. This 1s based on measurements taken at a stream gaging <br />station operated by the Water Resources Division of the U.S. Geolo- <br />gical Survey from 1978 to 1980 at their Station No. 06619400 lo- <br />cated in Section 6, Township 8, North, Range 77 West. This station <br />is shown on Map 5, North Park Hydrology. The average discharge for <br />the Illinois River at Walden ie 26,520 acre-feet, based on measure- <br />ments taken at a stream gaging station operated from 1923 to 1947 <br />by the Water Resources Division of the U.S. Geological Survey at <br />their Station No. 06618500 located in Section 29, Township 9 North, <br />Range 79 West. Comparison of these data demonstrates that the <br />gross impact of sedimentation pond water retention decreases the <br />total discharge of the Canadian River by 0.060 percent and decreas- <br />es the total discharge of the Illinois River by 0.003 percent. In <br />addition to the fact that these percentages ere so low as to be <br />insignificant, potential impact is even leas because the impact of <br />sedimentation pond retention will occur in the spring when runoff <br />is much higher than the mean annual discharge rate, and when there <br />is no shortage of water availability for any purpose. <br />Detention of surface water in mine pits will result in a temporary <br />reduction in downstream water availability, but if necessary, can <br />be quickly alleviated by pumping water out of the pits. This <br />pumping can be continuous, if necessary, immediately after water <br />begins to accumulate in the pit so that the coal and overburden can <br />be removed. Rates of discharge are discussed later under Section <br />780.21 (c) under the sub heading, "Dewatering". <br />I . <br />In the extremely remote event that retention of runoff by mining <br />operations creates an adverse impact within the mine plan or tipple <br />780-101 <br />