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4,1 Flood Boundaries In order to provide a national standard without regional discrimination, the 100-year flood has been adopted by FEMA 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 for 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 orthophoto contour maps at a scale of 1:2400, with a contour interval of 2 feet (References 6 and 7), In cases where the 100-and SOD-year flood boundaries coincide, only the 100-year flood boundary has been shown. Flood boundaries for the 100- and SOD-year floods are shown on the Flood Boundary and Floodway Map. Small areas within the flood boundaries may lie above the flood elevations and, therefore, not subject to flooding; owing to limitations of the map scale, such areas are not shown. 4,2 Floodways Encroachment on flood plains, such as artificial fill, reduces the flood-carrying capacity and increases flood heights, thus increasing flood hazards in areas beyond the encroachment itself. One aspect of flood plain management involves balancing the economic gain from flood plain development against the resulting increase in flood hazard, For purposes of the National Flood Insurance Program, the concept of a floodway is used as a tool to assist local communites in this aspect of flood plain management, Under this concept, the area of the 100-year flood is divided into a floodway and a floodway fringe, The floodway is the channel of a stream, plus any adjacent flood plain areas, that must be kept free of encroachment so that the 100-year flood may be carried without substantial increase in flood heights. Minimum standards of the FIA limit such increases in flood heights to 1,0 foot, provided that hazardous velocities are not produced, The floodways in this report are presented to local agencies as minimum standards that can be adopted or that can be used as a basis for additional studies, The floodways presented in this study were computed on the basis of equal conveyance reduction from each side of the flood plain, The results of these computations were tabulated at selected cross sections for each stream segment for which a floodway was computed (Table 2), As shown on the Flood Boundary and Floodway Map, the floodway widths were detennined at cross sections; between cross ,sections, the boundaries were interpolated. In cases where the boundaries of the floodway and the 100-year flood are either close together or collinear, only the floodway boundary has been shown, 8