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<br />ALTERNATIVES <br /> <br />Eight different conditions have been modeled. In addition to an <br />analysis of historic data and present conditions, the effects of <br />bringing Grayrocks Reservoir online, depleting the flows of the Laramie <br />and South Platte Rivers, and the impact of developing additional ground <br />water on surface flow conditions were each modeled singul arly and in <br />combinations. Finally, an effort was made to simulate flow conditions <br />estimated as necessary to maintain channel width and curtail seedling <br />establ ishment in the central Nebraska portion of the PI atte River. <br />These alternatives are not all inclusive; rather, they are intended to <br />be illustrative of various conditions imposed upon a hydrologic system <br />of the Upper Platte River Basin. Each are summarized and their impacts <br />generally described in the remainder of this chapter. <br /> <br />Historic Condition <br /> <br />The period of record used in the modeling portion of this study <br />extended from 1941 through 1977. Data were gathered from records <br />of the Geological Survey, appropriate state offices, and the Bureau <br />of Recl amation. Monthly values in thousands of acre-feet were <br />tabulated for each of the points in the system and were averaged <br />over the period of record. Table 10 presents these monthly values <br />for eight major streifll gage points. <br /> <br />Nearly all of the eight points exhibit a similar pattern of increasing <br />flows during th,e spring runoff period, marked decreases in the summer <br />months (due primari ly to diversions for irrigation) and a somewhat <br />constant flow in the winter months. <br /> <br />Exceptions can be noted, however. Point No.1, North Platte River below <br />Guernsey, is indicative of a period of record below a storage reservoir. <br />In the winter period--October through April--flows of the North Platte <br />are bei ng stored in accord ance with the decree and operat i ng criteri a. <br />Thus, flows past this point are minimal during this period. The <br />remainder of each year--May through September--flows increase sub- <br />stantially, indicating releases from storage for downstream irrigation <br />requi rements. <br /> <br />Another point of interest in the historical record is pOint No.6, <br />PI atte River at Overton, Nebraska. The summer peri od--July through <br />September--refl ects the impact of i rrigat i on canal divers ions between <br />Overton and the upstream point No.5, Platte River near North Platte. <br />Seven major canal diversions occur in this reach including the Tri- <br />County Canal. The Tri-County diversion, however, has a year-round <br />impact, as water is diverted through this system to generate power. <br />The return flows from this power operation are reflected in the <br />substantial winter and spring flows at Overton. <br /> <br />47 <br />