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Introduction This Special Flood Halard Info~tlon Report for the South Platte River In Logan County and Sedgwick County, Colorado was prepared by the Omaha District, Corps of Engineers, under continuing authority provided by Section 206 of the 1960 Flood Control Act (Public Law 86-645l,as"men<.le<.l. Th'l report was requested by the Colorado Water Conservation Board and Is the third of three volumes presenting Infor- m~tlon relative to the South Platte River and Ifs flood plain In Weld, Morgan, WashIngton, Logan, and Sedgwick Counties, Colorado. The purpose of this report Is to present info~tlon describing the South Platte River and Its flood plain In Logan County and In Sedgwick County rela- tive to past floodIng as well as to potential future flooding, This report basically shows the relatIonship 01' the 1973 flood on the South Platte River In Logan COunty and Sedgwick COunty to the 100.year flood, Information on the 10-, 50-, and 500-year floods 15 also presented. The Information provided varIes from stage-dIscharge relationships at each cross section In the rural areas to water surface profIles In the urban areas, Throughout the study reach, flood widths have been indi- cated at each cross section locatIon, The information in this report is Intende<.l for use prl~rlly by planners concerned wIth the area. This report does not Imply any Federallnterestorl'luthoritytozoneorregu!atetheuSeoftheSouth Platt.. River flood plain I n Logan r..<lllnty nr In S..<l'Jwlck County; thl~ isaloeal responSibility. The report provIdes a suItable basis for the adoption of land use controls to guide flood plain development ~Ith consIderation for environmental attrIbutes, thereby preventIng Inten- slflcatlonof loss problems. Since It IdentIfies flood prOblems, the report will stimulate the deve I oprnent of other flood damlJge reductIon techniques such as flood control, removal of obstructions and flood prootlng, which might be used In an overall Flood PlaIn Management (FPM) program. Olstrlbutlon of the report to officials, agencIes, and Individuals concerned with planning In the area covered Is made by Logan County i!lnd by Sedgwick County. The Corps of Englne'lrswlll provide Interpretation and technlclll assistance, If requested, in application of the reportdi!ltl'l. Other guIdelines i!lvallable frOl'l1 the Corps of Engineers are a pamphlet, "Guidelines for Reducing Flood DarMges" , and a booklet, "Introduction to FlOOd Proofing". , I I , BasIn and Str~m CharacterIstIcs The South Platte River in Logan and Sedgwick Counties flows in a general northeasterly dIrectIon. Throughout this reach the South PlatteRlverflowslnarelativelywell-definedchannel. This channel Is typically braided and Is situated In a broad shallow valley ranging In width from about 7,000 feet to about 17,000 feet and Is flanked byrolllngplelns. The streambed slope of the South Platte RIver In Log<'ln"ndsedgwlckCountleslsrelatlvelyuniforrngenerallyrangIn9 between 10 feet per mIle and 4.5 feet per mile and averaging 7 feet per mile. The drainage arell of the South Platte River In Logan and Sedgwick CountIes varIes depending upon location due mainly to the tributaries draining the plains regIon located to the north and west of Logan and Sedgwick Counties. Tllble I gives drainage areas of the South Platte River at stream gage locatIons within Logan and Sedgwick Counties. Table I South Platte River Stream Gage nata In Loqan and SedgwIck Counties, Colorado GaQeLocat;on Dr~lnaq"l Area (sQuersmlles) PeriOd of Record Crook 19,238 1954 through 1958 Julesburg 23,138 1902 to 19%,1909 to 1912, 1914 to 1915, 1911t-01918,1920to 1924,1926tothepresont flxceptfor brIef periods t>etween 1940andl941,I956 and 1957, 1953 and 1959. 2