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4.1 Flood Boundaries In order to provide a national standard without regional discrimina- tion, the lOa-year flood has been adopted by the Federal Insurance Administration as the base flood for purposes of flood plain manage- ment measures. The SOO-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 SOa-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 scale of 1:24,000 enlarged to a scale of 1:7200, with a contour interval of 10 feet (References 9 and 11). Where there was an apparent error in the topographic maps, the flood boundaries were interpolated using the surveyed cross sections. In cases where the 100- and SOO-year flood boundaries are close together, only the IOO-year flood boundary has been shown~ Flood boundaries for the 100- and SOO-year floods are shown on the Flood Boundary and Floodway Map (Exhibit 2). Small areas within the flood boundaries may lie above the flood elevations and, therefore, not be 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 communities in this aspect of flood plain management. Under this concept, the area of the lOa-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 in order that the lOa-year flood be carried without substantial increases in flood heights. As minimum standards, the Federal Insurance Administration limits such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. The floodway for this study was computed on the basis of equal conveyance from each side of the flood plain, using the Union Pacific Railroad berm as a boundary between the split flows described in Sections 2.1 and 3.2. The results of these computations are tabulated at selected cross sections for each stream segment for which a floodway is computed (Table 2). 8