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Starting water-surface elevations for Yampa River were determinpd in a report prepared by the U.S. Army Corps of Engineers (Refer- ence 4). The starting water-surface elevations for Fortification Creek, Brotherton Gulch, and Lay Creek were determined by the slope-area method. For Pine Ridge Gulch and Cedar Mountain Gulch, starting water-surface elevations were determined from hand calcula- tions taken at culverts at State Highway 13 and the First Street Bypass. Starting water-surface elevations for Big Gulch were taken from elevations determined for Lay Creeke Starting water- surface elevations for Sand Gulch were taken from elevations deter- mined for Fortification Creek. Starting water-surface elevations for Sand Gulch Tributary were taken from elevations on Sand Gulch. Flood profiles were drawn showing computed water-surface elevations to an accuracy of 0.5 foot for floods of the selected recurrence intervals (Exhibit 1). The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the flood profiles are, thus, considered valid only if the hydraulic structures remain unobstructed, operate properly, and do not fail. The approximate 100-year flood elevations for Tributaries 1 and 2 were determined by normal depth calculations of typical sections based on field observation. All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks used in the study are shown on the maps. 4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS The National Flood Insurance Program encourages State and local govern- ments to adopt sound flood plain management programs. Therefore, each Flood Insurance Study includes a flood boundary map designed to assist communities in developing sound flood plain management measures. 4.1 Flood Boundaries In order to provide a national standard without regional discrimina- tion, the 100-year flood has been adopted by the Federal Emergency Management Agency as the base flood for purposes of flood plain management measures. The SOD-year flood is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the boundaries of the 100- and SOD-year floods have been delineated using the flood elevations determined at each cross section; between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:2400, with a contour interval of 2 feet (Reference 10) . 15