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flood which equals or exceeds the 100-year flood (1 percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10); and, for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community at the time 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 peak discharge- frequency relationships for each flooding source studied by detailed methods affecting the community. Recorded flood information for the tributary streams studied by detailed methods is nonexistent. Good records do exist for the South Platte River. However, due to the construction of Chatfield Dam," the recorded information on the South Platte River is not appl icable. As a resul t, synthet icall y deri ved hydrographs were computed to determine the potential flood magnitudes for those tributary streams with relatively small drainage basins. Thes.e hydrographs reflect the effects of precipitation characteristics of the basin, ground cover, geology, slope, drainage area, and other physical characteristics of the drainage basins. To assure correctness of the synthetic data, recorded flood data in adjacent areas were also analyzed. Flood frequency determinations were based on the standard log- Pearson Type III statistical analysis of flood discharge records of several streams adjacent to the study area, as well as in eastern Colorado. The discharge records for the streams considered covered periods of 10 to 74 years. The records and flood frequency determinations were generated at various gaging stations by the USGS and published as regional IOO-year data (Reference 4). In addition, the analyses were correlated with the Colorado State University study on flood magnitudes in the area (References 10 and 11). In order to assure consistency in the hydrology, the recorded data analyzed, as discussed above, were plotted on a graph of lOO-year discharge versus drainage area. This curve was used as a comparison for synthetically generated hydrograph flows for each stream in the study. Rainfall data for the synthetic analysis were taken from the Denver Regional Council of Governments' Urban Storm Drainage Criteria Manual (USDCM) (Reference 26). Synthetic hydrograph procedures used in the study included the Colorado Urban Hydrograph Procedure, outlined in the Denver Regional Council of Governments' USDCM (Reference 26), and the COE HEC-l, computer hydrograph package (Reference 27). The South Platte River peak discharges for the IOO-and SOQ-year frequency floods below the dam were computed to reflect the COE 10