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Roughness coefficients (Manning's On") for the Colorado and Roaring Fork Rivers and Threemile and Mitchell Creeks were estimated by field inspection. For the Colorado and Roaring Fork Rivers, roughness values for the channel areas ranged from 0.033 to 0.045 and from 0.050 to 0.160 for the overbank areas. Threemile Creek had coefficients ranging from 0.045 to 0.05 and 0.08 to 0.09 for the main channel and overbank areas, respectively. A coefficient of 0.04 was used for all Mitchell Creek computations. Starting water-surface elevations for the Roaring Fork and Colorado Rivers were taken from water-surface profiles developed for the Colorado Water Conservation Board Flood Plain Information Report (Reference 2). Starting water-surface elevations for Threemile and Mitchell Creeks were derived from critical-depth computations. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus con- sidered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. All elevations are referenced to the National Geodetic Vertical Datum of 1929. Elevation reference marks used in this study are shown on the maps. 4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS The National Flood Insurance Program (NFIP) encourages State and local governments to adopt sound flood plain management programs. Therefore, each Flood Insurance Study produces maps designed to assist communities in developing flood plain management measures. 4.1 Flood Boundaries To provide a national standard without regional discrimination, the 1 percent annual chance (100-year) flood has been adopted by FEMA as the base flood for flood plain management purposes. The 0.2 percent annual chance (SOO-year) flood is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the 100- and SOO-year flood plain boundaries have been delineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were inter- polated using topographic maps at a scale of 1:2,400, with contour intervals of 5 and 2 feet (References 14, 15, and 17). The extreme upstream boundaries of the Colorado River were inter- polated using USGS 7.S-minute series topographic maps at a scale of 1:24,000, with a contour interval of 40 feet (Reference 18). 16