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flooding depends on the amount of snow melt nd;or <br />thunderstorm activity. Histcrically measured high discharge <br />on the Uncompahgre River occurs during the months of May <br />and;cr June and range from 200 cfs to 1740 cfs. Spring <br />runoff occurs from as early as May 1 to as late as September <br />10, depending on the snowpack i:, the area. High flcvrs <br />resulting from thunderstorms can occur any time from April 1 <br />to October 15. <br />Site measurements over the past two high flow seasons <br />(1990 and 1991) indicated a flow rate on the Uncompahgre of <br />220 cfs results in minor flooding cf the gravel bar to be <br />mined. Flooding from these flows 's confined to the three <br />high flow channels shown on Exhibit E-1. Water in these <br />channels ranged from 0.5 feet to 1.5 feet deep and were from <br />2 to 10 feet wide. Flood season flow rates for the <br />individual channels ranged from 0.5 cfs to 1.2 cfs. <br />The most recent extreme high flow of 1740 cfs occurred <br />on Jure 24, 1983 and resulted in the gravel bar to be mined <br />being expcsed. During that flood year the River channel <br />expanded east cf the normal channel by 300 feet (Plate 12, <br />Ouray County 1980 Flood Plain Study). Associated sheet <br />flows extended several hundred feet east of this boundary. <br />This flood year ;..ay represent a 50 year or a 100 year flood <br />event. <br />The Curay County Flood Plain Study of 1978 indicates <br />that a 100 year flood would raise the water levels in the <br />River in this area by 6 1;2 to 7 feet. It would also spread <br />sheet flows betweer. 1500 and 2500 feet beyond the main River <br />channel (see Plates 12 and 27 cf the Ouray County Flood <br />Plain Study;. While this mining operation will help reduce <br />the potential of flooding on the eastern and northern <br />bcundaries during average runoff high flow conditions, the <br />helpful effects would be minimal or non-existent during a 50 <br />or 100 year flood. Peck Inc. makes no promises concerning <br />increased flood protectior. during such events; but, by <br />lessening flood channel stream discharge and removing <br />gravels from the bar adjacent to the mair. River channel. <br />erosior. on the eastern and rcrthern banks will be lessened. <br />It would take a flood flow of 2 feet deep spread over a 300 <br />foot wide bar to erode the banks when mining is completed. <br />This would be a similar flow to that of the 1983 high flow <br />event. <br />2. HYDROLOGIC IMPACTS: <br />The proposed berm 10 feet wide and 2 to 5 feet high <br />left adjacent to the eastern edge of the River will keep the <br />River out of the active pit during pit operations. <br />All mining will occur at least one foot above the <br />ground water table. To enable Peck to determine if ground <br />water is present, monitoring pipes will be installed--at <br />least three for each acre to be mired. These will be <br />installed prior to mir.irg in a particular area of the pit. <br />These will be installed to a depth of 3 feet below the low <br />0 <br />