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Hydrologic analyses for the approximate study areas were carried out to establish the peak discharge-frequency relationship for floods of the 100-year recurrence interval. The U.S. Soil Conser- vation service report on urban hydrology for small watersheds was used in the analysis for the nine individual basins that were studied by approximate methods (Reference 9). A study was also made of the areas damaged by the July 24, 1977, flood. Information from a report on debris-flow hazard analysis and mitigation (Reference 10): a study of the mud flows (Reference 11): past newspaper articles: and photographs of the July 24, 1977, flood (Reference 12) were analyzed. The historical and geological data and the results of the hydrology study by the u.S. Soil Conservation Service method were used to determine the 100-year flood plain boundaries for the basins studied by approximate methods. Because of the general shape of the alluvial fans, the most extensive damage during 100-year frequency floods occurs in areas directly in the discharge paths of the basins. As the 100-year frequency flood extends over the widening alluvial fans, it turns to sheetflow, thereby reducing considerably the damage from the 100-year frequency floods. For this reason, the 100-year flood plain usually does not extend to the Roaring Fork River. Peak discharge-drainage area relationships for the Colorado and Roaring Fork Rivers and Threemile and Mitchell Creeks are shown in Table 1. 3.2 HydraUlic Analyses Analyses of the hydraulic characteristics of flOOding from the sources studied were carried out to provide estimates of the eleva- tions of floods of the selected recurrence intervals. The water-surface profiles for the 10-, 50-, 100-, and 500-year floods were computed using the U.S. Army Corps of Engineers HEC-2 step-backwater computer model (Reference 13). Cross section data for the Colorado River and portions of the Roaring Fork River were obtained from available topographic maps and field measurements (References 14 and 15). Cross section data for Threemile Creek and other portions of the Roaring Fork River were digitized from aerial photography (Reference 16). Cross section data for Mitchell Creek were field surveyed. All bridges were surveyed to obtain structural geometry and elevation data. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway was computed (Section 4.2), selected cross section locations are also shown on the Flood Boundary and Floodway Map (Exhibit 2). 14