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Discharges for streams with drainage areas greater than 15 square miles were calculated using a log-Pearson Type ill frequency analysis (Reference 41). As is typical in Colorado, flood events in the study area comprise two distinct and generally independent populations (i.e., cain and snowmelt floods). For this reason, it was necessary to analyze the rain and snowmelt events separately. Each type of flood event was assumed to follow log-Pearson Type ill distribution. At each of 15 USGS gaging stations with at least 15 years of record, two annual peak flood series were determined, respectively, for the cain events and snowmelt events from streamflow charts. Three statistical parameters, the mean, standard deviation, and skew, which define the log-Pearson Type ill distribution, were then computed for each type of flood event. The statistical parameters are intuitively dependent on watershed characteristics and hydrometeorologic conditions in the region. Two basin parameters, drainage area and mean watershed elevation, which most significantly affect the magnitudes of floods, are regarded as independent variables in the discharge predictive regression analyses. The regression equations, including only the significant independent variables for each type of event, were used to compute the flood-frequency curve at any ungaged site, given the drainage area and mean watershed elevation. These two frequency curves, one for rain events and the other for snowmelt events, were then statistically combined to give a composite flood-frequency curve that defines the flood-frequency curve for the un gaged site in question. Discharges for streams with less than 15 square miles of drainage area were calculated using the Colorado Urban Hydrographic Procedure (CUHP) (Reference 42). The 10-, 50-, and 100-year discharges were calculated directly, whereas the 500-year was estimated by extrapolation. Peak discharges at selected locations in the study area were obtained by routing CUHP flood hydrographs for each subbasin. Peak discharges for the South Platte River were developed as part of the FIS for Douglas County (Reference 20). In that study, a log-Pearson Type III analysis (Reference 41) was used to calculate the discharges. Streamflow data from USGS stream-gaging stations located at South Platte (Gage No. 06707500) and below Cheesman Lake (Gage No. 06701500) were used in the frequency analysis. The hydrologic analyses for Lena Gulch upstream of West 6th Avenue, Jackson Gulch, Kenneys Run, and Clear Creek, upstream of the Burlington Northern Railroad were developed as part of the FIS for the City of Golden (Reference 5). In that study, the peak discharges for Clear Creek above the Burlington Northern Railroad were obtained from the USACE, Omaha District (Reference 43). The USACE established peak discharge frequency relationships for floods of 10-, 50-, 100-, and 500-year recurrence intervals. A log-Pearson Type ill analysis (Reference 41) was conducted on the discharge records for the Clear Creek USGS stream gages at Golden (1911-76) and Derby (1934-76); however, the statistical parameters computed by these methods were not sufficiently reliable to predict the frequency of extreme events. In lieu of a discharge-frequency analysis, a rainfall-runoff approach was used. The Massachusetts Institute of Technology Catchment Model (Reference 44) was constructed foc the 400-square-mile area above the Golden gage, and a storm water management model (Reference 45) was constructed for the 175-square-mile area between the Golden and Derby gages. The rainfall depths used in the analysis were based on data obtained from the 1973 National Oceanic and Atmospheric Administration (NOAA) report, 25