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the 100-year flood (1 percent ,chance of annual exceedence) in any 50- year period, is approximately 40 pe1:1cent (4 in 10), and, for any 90-year period, the risk increases to apprQximately 60 percent (6 in 10). The analyses reported herein,reflect flooding potentials based on conditions existing in the community at the t~me of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak dis- charge-frequency relationships for each flooding source studied by detailed methods affecting the community. For Plum Creek, East Plum Creek, West plum Creek, Hangmans Gulch, Sellers Gulch, Unnamed Tribu~ary to Sellers Gulch, Carpenter Cr",ek. Happy Cal\yon Creek, NElwlin Gulch, Baldwin Gulch, Sulphur Gulcll, Tallman Gulch, Bayou Gulch, and West Creek, the synthetic ,hydrograPh method was used td obtain the peak discharge. The . analyses were ,based on a storm duration of 24 hours and a Type II 'A'di~tribution, as described in the SCS National Engineering 'Han<Uiook (Reference 8). The amount of rainfall was obtained from a".precipitation-frequency atl;as (Reference 9), and areal adjust- nie'n:t;was applied to convert t;he point precipitation values to 'a:-iI'e:t'a\{e precipitation over the watershed area. Hydrologic soil . cover: complexes and associat~d Runoff Curve Numbers were obtained , "f'rOlll';J:ielli investigations, a ;soil survey of Castle Rock (Refer- ence:l0), an unpublished SCS !study of the area, and land use and naturaiplant cover maps of Qouglas County (References 3 and 11). Values of 10-, 50-, 100-, and 500-year peak discharges were ob- tained using the computer program developed by the SCS (Refer- ,ence 12). The computed peak ,discharges compare favorably with the peak discharges estimated using the USGS Technical Manual No.1, (Reference 13). Discharge magnitudes for floqds in Cherry Creek analyzed in this FIS were based on an analysi~ of stream gaging data at the USGS stream gages located approximately 2.5 miles south of Franktown and 6 miles northwest of Parker. This information was obtained from a COE Flood Plain Information report (Reference 14). Discharge-frequency relationships for the upstream and downstream limits of the study reach were developed using data from the Franktown and Parker stream gages, respectively. The p:t'esence of 32 flood-retarding structures in the Cherry Creek basin I constructed by the SCS was taken into consideration. I ' A continuous record of flows 'at stream gaging station No. 0670500, located on the South Platte River at South Platte, is available from 1900 to the present, and stream gaging station No. 06701500, 'l;ocated below'Cheesman Lake,has a continuous record of flows from '., 1925 to'the present. 18