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<br />.' <br /> <br />" <br /> <br />OOO[IU;, <br /> <br />, <br /> <br />hYdroperiod are now weak or non-existent. The first flow pulse <br />in the Duncan hydrograph typically started about halfway through <br />February and went through the middle of March. The second flow <br />PUlse in the Duncan hydrograph typically started about two-thirds <br />the way through May and went through the middle of June. It <br />should be noted that the 1929-38 peaks are very noticeable even <br />though a drought occurred during much of this period. <br /> <br />The following output from a duration-frequency analysis <br />illustrates how the flow regime once functioned. The early <br />spring time-frame is from February 1 to April 15 and the late <br />spring time-frame is from April 16 to June 30. <br /> <br /> Duncan Grand Island Percent <br /> 1929-38 1941-89 Exceedence <br />Early Spring <br /> 3 days 12,000 cfs 5,588 cfs 20% <br /> " " 9,000 cfs 3,200 cfs 50% <br /> " " 2,250 cfs 1,400 cfs 100% <br />Early Spring <br /> 7 days 7,240 cfs 4,740 cfs 20% <br /> " " 6,010 cfs 2,900 cfs 50% <br /> " " 2,200 cfs 1,250 cfs 100% <br />Late Spring <br /> 3 days 15,000 cfs 8,456 cfs 20% <br /> " " 4,930 cfs 4,160 cfs 50% <br /> " " 1,150 cfs 620 cfs 100% <br />Late Spring <br /> 7 days 12,200 cfs 5,312 cfs 20% <br /> " " 3,440 cfs 2,800 cfs 50% <br /> " " 934 cfs 606 cfs 100% <br /> <br />Under this alternative, managed water would be released to <br />emulate a semblance of the pre-development hydroperiod. The flow <br />level would be based on forecast basin runoff. Managed water <br />releases would be combined with natural flow conditions to <br />attempt to emulate the two hydrograph flow pulses. The early <br />spring peak would begin in February and follow the above <br />mentioned flow duration-frequencies. The late spring peak would <br />begin in May and follow the above mentioned flow duration- <br />frequencies. If there was not enough managed water to fulfill <br />both spring peaks, then the time period where flows were met less <br />frequently would be given first priority. If any managed water <br />remains after the two different peaks were attained, then that <br />water would be released during the summer when Platte River flows <br />become very low (e.g., 400 cfs or less). <br /> <br />4 <br />