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other significant structures that might divert or restrict flood- flows were surveyed. Additionally, because the digitized cross sections show the water-surface elevation in each stream on the date of photography, the cross sections were adjusted by field measurements to reflect the streambed elevations and an area beneath the mapped water-surface level for a discharge of 300 cfs in the stream at the time of photography. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). Flood plain roughness factors (Manning's "n") used in the hydraulic computations were selected based on field inspection of the actual flood plain conditions. Manning's linn values for the Mancos River ranged from 0.035 to 0.045 in the channel and 0.060 to 0.10 in the overbanks. For Chicken Creek, Manning's "nil values ranged from 0.030 to 0.050 in the channel and 0.040 to 0.070 in the overbanks. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus consid- ered 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 (NGVD). Elevation reference marks used in this study are shown on the maps; descriptions 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 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 a contour interval of 2 feet (Reference 1). 8