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4 5 '.0 portat,ions bav~ been used in the upper basin ubove t.he Narrows area. The lower baSin has benefited (rom t.he use of return flows. j\.fosl oC tbe return flows have been used by Tlleans of ground water witb- drawnh, le~\'ing surface flows in the river about the ~ame 8.S pre- Colorado-BIg Thompson comlit.ions, The J66,370 acres of land that ,,,:0 uld. obt.ain supplemental iITigation waLer are in Morgan, Logan, Wasbmgton, and Serlgwick Counties, along t.be main stem of the lower South Platte River in 110rtheasterl1 Col<imdo, All of the land is now or has in tbe past been irrigated. The South Platte River is the only silrnificant source of surface wate~ for irrigation in the proposed N arrov.""S unit. Ground water also constItutes n major SOllrce of water in tbe basin. Thirty-three irriga.tion systems now serve lands ,\'ithin tbe Narrows area. Four offstream reservoir:3 serve about 10 to 12 or the dit.ch aystems. None of t.he reservoirs have sufficient ca.pacity to assure adequate su:pplies for the systems served. The r~maining ditch s:.yst.ems rely on dlI'ect stream- flows fo~ the.ir major SOllf(~e of suPp.!y: Water shortages occur every Y~B.r, prlmarJly beea.use st.orage facilities are lacking on BOIDe of the dltch systems, The principal feature of tbe proposed development is the N arro"s Dam and Reservoir to be constrllcted on the South Pla.tte River near Fort MOI:g8:n, Colo. Aho proposed is ~be.acgu~ition and development of the eXlStmg Jackson Lake ReservolI', U1cludmg Borne rehabilitation of tbe dam, for recreation and fish and wildlife enhancement, Federal construction of irrigation disLribution facilities would Dot be involved since the water developed by the unit would be conveyed hy existing systems. Hydroelectric power development was found to be economi- caUy: i,nfeasible., Lack .of any forp.se"abJe requirement for additional mUDlclpal and mdustnal water precludes the need for !;ucb facilities at tbis time. The wat.er supply tor t.he Na.rro\\"S unit would be derived from regul~tion or S~LTr]us 6treamflow and from direct How water right~ assoClated WIth JrrJgaLed lands to he acquired for the proposed Narrows Dam and Reservoir. Irrigation storage capacity in Jackson Lake Res~r\Toir would be transferred to tbe proposed -N arrov.'S Reservoir. Project water stored in the propoged Narrows Reservoir would be released either into the South Platte River through n. river outlet works for delivery to existing dO\\11stream diversion works or through a canal outlet works directly into the Fort Morgan Canal. Supplemental water requirements for 23 of the ditches which serve 98.4 percent of the irrigahle land in the Lower South PJat.te Wat,e,. Conservallcy District would be 178,000 acre-reet at. tbe river headgo.te ~as~d on the 1947-61 period of study. Reservoir operation studies mdlcate that ~verage annual supplemental yields of 119,400 acre-reet would be obtamed unuer the proposed plan of development. In addi- tlOD, t.be divertible return flow from project deliveries would average 21,300 aere-feet, These sources would yield a total or 140,700 acre-feet of supplemental water at the river headgates. The difference of 37,300 acre-feet annually represents diversion shortages, which occur in 3 of the 15 years of study, The BlITeau of Sport Fisheries and Wildlife has formulated a pIa" for conservation and development of the fish and \dJdlife resources. It would include ~inimum flow of 50 cubic feet per second in nnd public aceese to 5,5 miles of the South Platte River immediately below the proposed Narrows Reservoir, rehabilitation and stabilized operation of t,he existing Jackson Lake Reservoir, development of a wildlife manageJ?lent area, and a zOning plan for tbe proposed Narrows ReservolI' . The National Park Service has recommended a recreation plan that would include four puhlic Use areas, Two of tbe areas would Oe located adjacent to tbe proposed reservoir shoreline and would function at all pool levels, a tbird area would be located below the proposed dam and reservoir, and hhe fourt.h would be at Jaekson Lake Reservoir. The Bureau (If Outdoor Recreation finds that the recreation development recommended is consistent with the objectives of the Colorado outdoor recrea.tjon plan. The Corps of Engineers evaluated the flood control req,uirements for t.he unit. It recornroended the provision of reserVOIr storage capacity of 550,000 acre-feet lor downstream flood protection. or that capacity, 75,000 acre-feet would be used jointly for flood control and water conservation purposes. The dam would bave a river outlet cB.[lacit..r ot 8,600 cubic feet per see-ond and a spillway capacity of 27,600 cubic feet per second, Operation of the reservoir for flood control would be in accordance with regulations prescribed \'y the Secretary of the Army, In its preliminary report, Lhe Public Health Secvice indicated that a high degree of waste treatment and a minimum st.rea.mflow of 25 cubIC feet per second are needed below the proposed Narrows Reservoir to ohtain the deaired water quulity, Normal re.~ervoir seepage, tributary in110~"5, and return flows would provide more than the desired flows, The Federal Water PoUution Control Admin- istration (FWPCA) recommends that the operating procedw:es for the unit provide for maintenance of that streamilo,,- buL finds Lhat. the water would be usable for ot.her purposes rather than wa.ter quality control as discussed later in this report in connection with the alternative plan, As recommended hy the Public Health Service, every effort v.ill be made in cooperation with the affected interests to assure that development a..u.d a.cth"ities on the unit. wiJ1minimize anv pot.ential public health hazards, The Bureau of Mines advised that petroleum and natural gas are the only significant mineral resources present in tbe dam and reservoiI' area. A small field has shown a steady decline in production sinc.e 1956, In October 1965 only 34 of 108 welb that had been drilled within the reservoir area at tbe flood control pool elevation were still prod\lcin~. Considering a construction perIOd of 9 years, the Bureau of Mrnes anticipate:s tbat all oil Rod gas wells in tbe site will have been abandoned, N; presented in the attached regional director's report, the estimated Lota! construction cost of the Narrows Unit would be $61,820,000, based on October 1965 price levels, Interest during construction would add $4,918,600 making the total investment cost $66,738,600, Opera.tion, maintena.nce, a.nd replacement. costs would total $216..500 annually. The annual costs f>f the unit for economic analysis, wbich includes the annual equivalent or tbe net project investment and annual opera.tion, maintenance, and replacement costs, have been computed to he $2,275,200, The evalllated annual benefits to be derived by the several fUnctions are as follows: . '-J" ].- (.;~