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<br />1G68 <br /> <br />With Narrows Situation <br /> <br />Impacts to the River Channel <br /> <br />Effects on the monthly and annual volume of streamflow of the Platte <br />River at Overton were estimated using the Bureau of Reclamation model. <br />The estimated post-Narrows flows of the South Platte River at Julesburg <br />were used. Other flo~1 data incorporated in the "with" Narrows study <br />reflected the "near future condition" of development upstream of <br />Lewellen and within the modeled reach of the river. <br /> <br />Exhibit 3 shows the average annual, January-April, and May-August flows <br />at various locations within the modeled area for the period 1947-1970 <br />under the "with" Narrows condition. <br /> <br />As shown in table 1 (USBR, 1976), the Narrows Unit would deplete the <br />average annual pre-Narro~s flow of the Platte River at Overton by <br />88,100 acre-feet (121 ft Is) or about 11 percent. The greatest <br />percentage change in the Overton flows are shown to occur in May <br />(minus 19 percent) and June (minus 2B percent). Overton flows are <br />reduced an average of 8 percent during the ice-affected flow months <br />of January through April and about 20 percent during the months of <br />May through August. <br /> <br />These figures should be compared with the flow reductions that are <br />known to occur in the Platte River under the pre-Narrows conditions <br />just downstream from the CNPP&ID diversion dam, the point at which <br />the "no crane use" river reach begins. These depletions are 88 percent <br />(average annual), 92 percent (January-April), and 79 percent (May- <br />Augus t). <br /> <br />A study (USSR, 1977) of sandbar movement was made to evaluate the <br />effects that construction of Narrows Dam would have on the reach of <br />the Platte River between Overton and Grand Island, Nebraska. The <br />study purpose was to determine if there would be a detrimental effect <br />to or loss of the shallow, submerged unvegetated sandbars in this <br />70-mile river reach. <br /> <br />Use of July 1938 and August 1971 aerial photographs, gage height data <br />at Overton, and historical and computer model estimated flow data was <br />reflected in the study conclusions. <br /> <br />The decrease in mean an~ual discharge at Overton with Narrows Dam of <br />121 ft3/s from 1,150 ft Is to 1,029 ft3;s is expected to have little <br />effect on open channel widths. The open channel widths would be <br />controlled by the dominant discharge. Little change in the dominant <br /> <br />31 <br />