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4.1 Floodplain. Boundaries To provide a national standard without regional discrimination, the 1 percent annual chance OOO-year) flood has been adopted by FEMA as the base flood for floodplain 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 by detailed methods, the lOa-year floodplain boundaries have been delineated using the flood elevations determined at each cross section. Floodplain boundaries were plotted at surveyed and synthesized cross sections and interpolated between cross sections based on the contours shown on the 7.S-minute topographic quadrangle maps of Steamboat Springs, Rocky Peak, Mud Creek, Cow Creek, Blacktail Mountain, and Clark, Colorado, at a scale of 1:24,000, with a contour interval of 40 feet (Reference 7). Flood boundaries for cross sections R to Y were transferred from maps supplied by the CaE, because these maps are at smaller scales, and thus have more detail and definition (Reference 4). The downstream boundary of upstream portion of the Yampa River (located at Cross Section AL) matches the upstream boundary of the Flood Hazard Information Study prepared by the CaE in cooperation with FEMA (References 4 and 3). The lOa-year floodplain boundaries are shown on the Flood Insurance Rate Map (Exhibit 2). On this map, the lOa-year floodplain boundary corresponds to the boundary of the areas of special flood hazards (Zones A and AE). Small areas within the floodplain boundaries may lie above the flood elevations but cannot be shown due to limitations of the map scale and/or lack of detailed topographic data. Approximate lOa-year floodplain boundaries in some portions of the study area were taken directly from Flood Prone Quadrangle Maps (Reference 8). 4.2 Floodways Encroachment on floodplains, such as structures and fill, reduces flood-carrying capacity, increases flood heights and velocities, and increases flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under thi s concept, the area of the 100- year floodplain is divided into a floodway and a floodway fringe. The floodway is the channel of a stream, plus any adjacent floodplain areas, that must be kept free of encroachment so that the lOa-year flood can be carried without substantial increases in flood heights. Minimum Federal standards limi t such increases to 1.0 foot, provided that hazardous velocities are not produced. The floodways in this study are presented to local agencies as minimum 8