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South Platte River. Heavy rainfall occurred over portions of the northern sections of the South Platte River basin on the 14th and 15th of June. As the storm system moved southward, torrential rain- fall centered princIpally over the Plum Crook watorshed on 16 June and on the Bljou Creek watershed on 17 June. Storm rainfall of the periOd extended over ~e 3,000 square miles of the South Platte River basin, Including the Plum Creek. Cherry Creek, and Sand and Toll Gate Creek watersheds In the Denver region, and the Bljou Creek. Kiowa Creck, Oommanche Creek, Badger Creek, and Beaver Creek water- sheds to the east. Flooding occurred on the South Platte Rivar from Plum Creek downstream to North Platte, Nebraska as a result of this rainfall. discharges were experienced along portions of the main stem of the South Platte River tor most of tho month of May and on into June. High water mark data from this flood In Morgan and Washington COun- ties Is contained In table 5. The flooded Areas. plates 3 through 19S, sho" the area flooded by thIs flood. Future Floods Floods of the same or larger magnitude as those that have occurred could occur In the future. To determine the flood hazard posed by the South Platte RIver In Morgan and Washington COunties, the IO-year, 50-year, IOO-Year, and 500-year floods were analyzed. The magnitudes of these future floods are presented In this report in table 6 and In the form of "ater surface profiles, stage- discharge curves, flood width delineations, and the relationship of these future floods to the flood of May-June 1973. 1969 _ Heavy rains during this perIod started on the after- noon of ~ May and continued with only 'ntermlttont breaks until 8 May. The storm covered an arca along and near the eastern slopes of the mountains and extendeQ into portions of tho hiqh pialns. The heaviest amounts were c~ntered 25 miles south"ost of OQnver and extonded In 8 band along the foothills north"ard to near Estes Park. The weather station at Morrison reported a total storm rainfall of 11.27 Inches and a maximum dally amount of 5.77 Inches, General fl~lng resulted along the South Platte River. 1973 - Snowmelt runoff from the lower mountain area of the South Platte River basin began about the middle of Apri I. Rainfall, amountIng to as much as 6 Inches, "hlch was the ~jor causatiVe tector of the flooding In the South Platte River basin, began on 5 May. Sharp Increases In flow as a result of the rainfall runoff "ere recorded ~t ~ll gdglng statIons along tho South Platte River from Littleton to tho Colorado-Nebraska State lIne. The ralnfell runoff waS eug~nted by mountaIn snowmelt runoff whIch was olso Increasing during this period. The result was general flooding throughout the South Platte River basin; flooding w~~ characterized by hIgh, ~harp hydrograph peaks from the rainfall runoff followed by a sio" reces- sion because of the continuing mountain snowmelt runoff. 8ankfull frequency The 500-year flood is not the large~t flood toat can occur but tM probability of lerger floods Is remote. As Ciln be soon frot'l the gaging records for the South Platte River. discharges smaller than either the lOO-year or 500-year floods are much more eommon. Largo floods, ho"ever, can happen; thIs was clearly demonstrated by the Denver erea floods of 1965, the Black HIlls, South Dakota area floods of JUnA 1972, and the July 1976 floods in the elg Thompson Canyon near Loveland. Colorado. Hazards of LerQe floods The extent of d~~go caused by any flood dApAnd~ on the topog- raphyof the area flooded. depth and duration of flooding. velocity of flow, rate of rise, developments on the 11006 plain, amount of debrislntl'9flocdwator,<>nd Ice condItIons, if Ice is a problem In that partIcular ar"". A lOO~year flood or a "OO-year fiood "" . 12 "