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Starting water-surface elevations for the Crystal River and Castle Creek were taken at their confluence with the Roaring Fork River. Starting water-surface elevations for Maroon, Snowmass, Coal, and Hunter Creeks were determined by the critical depth method. The starting elevation for the Roaring Fork River was obtained by the slope-area method. The starting elevation for the Fryingpan River was taken from a 1978 Flood Plain Information report (Reference 15). 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. Hydraulic analyses of approximate-study areas were based on normal- depth computations and information obtained from available topo- graphic maps and field reconnaissance. An area of shallow flooding exists along the southern bank of Coal Creek. This area is specified as a shallow flooding source in the Floodplain Information Report for Crystal River and Coal Creek (Reference 16). All elevations are referenced to the National Geodetic vertical Datum of 1929 (NGVD). Elevation reference marks used in this study are shown on the mapsl the descriptions of the marks are presented in Elevation Reference Marks (Exhibit 3). 4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS The NFIP encourages State and local governments to adopt sound flood plain management programs. Therefore, each Flood Insurance Study provides 100-year flood elevations and delineations of the 100- and 500-year flood plain boundaries and 100-year floodway to assist communities in developing flood plain management measures. 4.1 Flood Plain Boundaries To provide a national standard without regional discrimination, the 1 percent annual chance (lOO-year) flood has been adopted by FEMA as the base flood for flood plain management.purposes. The 0.2 percent annual chance (500-year) flood is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the 100- and 500-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 scales of 1:600 and 1:1,200, with a contour interval of 2 feet (Reference 12)11:2,400, with contour intervals of 2 feet (References 13 and 16) and 5 feet (References 6 and 21)11:4,$00, with contour intervals of 5 and 40 feet (References 14 and 22, respectivelY)1 and 1:24,000, with a contour interval of 2 feet (Reference 23). 15