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width is about 11 miles. Topography of the area is relatively steep with elevations in the headwaters region averaging about 7,500 ft, m.s.l. falling to about 5,200 feet, m. s .1. in the lower reaches of the basin in Denver. Cherry Creek flows in a northerly direction and empties into the South Platte River in the heart of the business district in downtown Denver. Slope of the stream varies from about 100 feet per mile in the headwaters area to about 25 feet per mile in the lower reaches of the stream. The lower 5.5 miles of Cherry Creek flows through a residential and industrial district of Denver. A map of the Cherry Creek basin is shown in Figure 1. b. Climate. Temperatures in cherry Creek basin normally vary during the year from about 10 degrees below zero to 100 degrees above zero. The high temperatures normally occur in the lower end of the basin near Denver with the lower temperatures occurring in the central and upper end of the basin. Evaporation observations made at Fort Collins, Colorado show an average monthly pan rate varying from 1.3 inches in December to 6.23 inches in July. c. precipitation. The average annual precipitation over Cherry Creek basin is 16.3 inches. The precipitation occurs primarily as rain during the months of April through August. Isolated high intensity, short duration thunderstorms occur frequently throughout the basin during the spring and summer months. Snowfall occurs throughout the basin during the fall and winter months with the larger amounts being received in the higher altitudes. None of the major floods of record have occurred directly as a. result of snowmelt runoff. d. Runoff Characteristics. Major floods in the Cherry Creek basin are usually caused by short periods of intense rainfall occurring over a small portion of the basin. Ninety percent of the floods on Cherry Creek have occurred during the period of May to September. The steep slopes in the basin cause rapid concentration of runoff and peak discharges are experienced at the Cherry Creek Dam within 3 to 4 hours after the beginning of runoff producing rainfall. 4. Applicable Requlations and References Derivation of the Inflow Design Flood, Unit Hydrograph Peaking Analysis, and various routing assumptions are based on ER-1110-8-2 (FR) dated 1 March 1991 and HMR 55A dated June 1988. Derivation of the Standard Project Flood is based on EM 1110-2-1411 dated March 1965. Background information and data used in this study were obtained from USACE, Omaha District's, Reconnaissance Report, Hydrologic Improvement Assessment, Cherry Creek Lake, Colorado, dated September, 1990. 2