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<br />15 <br /> <br /> <br />1973 - Snowmelt runoff from the lower mountain area of <br />the South Platte River basin began about the middle of April. <br />Rainfall, amounting to as much as 6 inches, which was the major <br />causative factor of the flooding in the South Platte River basin, <br />began on 5 May. Sharp increases in flow as a result of the rain- <br />fall runoff were recorded at al I gaging stations along the South <br />Platte River from Littleton to the Colorado-Nebraska State line. <br />The rainfal I runoff was augmented by mountain snowmelt runoff <br />which was also increasing during this period. The result was <br />general flooding throughout the South Platte River basin; flood- <br />ing was characterized by high, sharp hydrograph peaks from the <br />rainfall runoff fol lowed by a slow recession because of the con- <br />tinuing mountain snowmelt runoff. Bankfull discharges were exper- <br />ienced along portions of the main stem of the South Platte River <br />for most of the month of May and on into June. High water mark <br />data from this flood in Weld County is contained in table 8. The <br />Flooded Areas, plates 3 through 22, show the area flooded by this <br />flood. <br /> <br />Future Floods <br />Floods of the same or larger magnitude as those that have <br />occurred could occur in the future. To determine the flood hazard <br />posed by the South Platte River in Weld County, the 10-year, 50- <br />year, 100-year, and 500-year floods were analyzed. The magnitudes <br />of these future floods are presented in this report in table 9 and <br />in the form of water surface profiles, stage-discharge curves, <br />flood width delineations, and the relationship of these future <br />floods to the flood of May-June 1973. <br /> <br />Frequency <br />The 500-year flood is not the largest flood that can occur <br />but the probability of larger floods is remote. As can be seen <br />from the gaging records for the South Platte River, discharges <br />