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The western boundary of the South Platte Rll1er basin Is formed by the Continental Dillide with elevations up to about 14,200 feet and fhe Laramie Range with elevations up to about 10,000 feet. At the $Outhwostern corner of the basin, therrountl'linaraa known as South Park Is located, This area is nearly Hat and has an elevation of about 10,000 feet. It is surrounded by mountain ranges lIarying in elevation from 12,000 to 14,000 feet. The Tarryall Mountains form thenorthellstboundl'lryof the South Park area. Elevations in the plains regIon lIary from approximately 7,000 feet along the foothIlls to approximately 3,450 feet at the Colorado-Nebraska State line. Development on the flood Plain The South Platte River flood plain In Logan and Sedgwick Counties 15 utilIzed primarily for agricultural purposes. Except for the areas adjacent to Incorporated communities such as Julesburg and Sterling, existing flood plain dellelopment Is mostly farmsteads and ranch buildings. Source of Data and Records A consIderable amount of historic stream gage data does exist for the South Platte RIver which Is pertInent to lnoan and Sed~wlck , 0 Countlc~. The U. S. Geologlcai Survey has published info~tion collected at the South Platte Rlller stream gages In logan and Sedgwick CountIes. Informa,tion from these gages was used in the hydrologic anafysisconducted during This study. Information on past floods was obt'lllned from Corps of Engineers flood records. The photograph base for the flooded are~s ~re ~eri~1 photographs taken during the flood period In M.,y 1973 by the Color~do Water Conservation Board and the Corps of Engineers. Cross section d~ta utilized In thIs study was supplied by the Corps of Engineers. High water M/lrk data from the flood of "'ay 1973 was coordInated ~nd obtaIned in part by the Colorado Water Conserll~tion ~rd with 3 , I supp I em.enta I information by the Corps of EngIneers, theLl. S. Geological Survey, and the Bureau of Reclamation. LI. S. Geological Survey 7.5 minute quadrangle maps with 10 foot contour intervals were ~I$O used In the study. Flood Season and Flood Characteristics floodIng along the South PI~tte River In Logan and SedgwIck Counties normally occurs from March through September with the most frequent flooding occurring during June. Annual peak discharges, however, have been recorded throughout the year. Floods in Logan and Sedgwick Counties nor"",lly occur under non-Ice affected conditions. Ice Jams, however, 'llre possIble even though theIr historical frequency of occurrence IS rare. floodwaters on the South Platte River In Logan and SedgwIck CountIes Gan result from InTense localized raInfall or general r~lns, either of whichrn<JY be augmented by snowmelt. The Intense raInstorm floods result from Intense rainfall oller areas of relatIvely small ilre~1 extent. These storms can prod~e extremely rapid concentratIons of floodwaters and ensuant rapid rises In stream lellels. Because of lIttle werning tIme prior to streams attaining flood stages, this type of 5tO....... c.,~ produc... flood'). whiCh "r" "xtr~el'f h..u,rdous. Th;~ typ.. 01 flood prodUG9S nIgh peak diSCharge On tributarIes In the general vicinIty of the raInSTorm. The dIscharge, however, attenuates rapidly on the South Platte River downstream from the contributing tributaries due to valley storage of the floodwater. The general rainstorm floods result from ralnhll over areas of large areal e~tent. Rainfall from thIs type of storm may occur over several dl'lYs. Because of this time dur'lltion, the affected streams rise relatlllely slow, sometimes over 11 perIod of several days. The duration of the flood perIod also Is sometimes of consIderable length. Because of the large "real extent of rainfall, more tributaries contribute to the peak discharge on the South Platte River. ThIs .