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4.1 Flood Boundaries In order to provide a national standard without regional discrimi- nation, the IOO-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 each have been del ineated using the flood elevations determined at each cross section. Between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:4800, with a contour interval of 2 feet (Reference 7l. In cases where the lOa-and SOD-year flood boundaries are close together, only the 100- yea r fl ood boundary ha s been shown. Flood boundaries for the 100- and SOD-year floods are shown on the Flood Boundary and Floodway Map. Small areas wi thi n the fl ood boun- daries may not lie above the flood elevations and, therefore. may 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 communites in this aspect of flood plain management. Under this concept, the area of the lOa-year flood is divided into a floodway and floodway fringe, The floodway is the channel of a stream, pl us any adj acent 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 FEMA limit such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. The fl oodways in thi s report are presented to 1 oca 1 agencies as minimum standards that can be adopted or that can be used as a basis for additional studies. The fl oodway presented in thi s study was computed on the basi s of a maximum one-foot rise in water-surface elevation over the 100-year water surface, for the encroached fl oOdway. The results of these canputations were tabul ated at sel ected cross sections for Willow Creek (Table 2). A floodway was not computed for the Arkansas River because it would be totally outside the corporate limits of Lamar. As shown on the Fl ood Boundary and Floodway Map, the fl oOdway bound- aries were detennined at cross sections, and between cross sections the boundari es were interpol ated. In cases where the fl oOdway and 8