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management 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 SOO-year floods have been delineated using the flood elevations determined at each cross section; between cross sec;tions, the boundaries were in- terpolated by photogrammetric methods using topographic maps from aerial photos at a scale of 1:2,4_00 with a contour interval of 2 feet (Reference 14). Areas of shallow sheet flow outsiC\e the aerial photomapping limits were delineated using a topographic map at a scale of 1:24,000 with a contour interval of 10 feet (Reference 8). Floodplain boundaries are indicated on the Flood Insura\ice Rate Map. On this map, the lDO-year boundary corresponds to the boundary of areas of special flood hazards (Zone AE); and the500-year boundary corresponds to the boundary of areas of moderate flood hazards (Zone X). In cases where the 100- and SOO-year floodplain boundaries are close together, only the 100,-year bounda:ry has been shown. Small areas within the floOdplain boundaries may lie above the flood elevations, and therefore, not b~ subject to flooding; owing to limitations of the map scale, such ,!reas are not shown. 4.2 Floodways Encroachment on floodplains, such ,as artificial fill, reduces the flood-carrying capacity and increas~s flood heights, thus increasing flood hazards in areas beyond the :encroachment itself. One aspect of floodplain management involves palancing the economic gain from floodplain development against the resulting increase in flood hazard. For purposes of the NFIP, the concept of a floodway is used as a tool to assist local communi des in this aspect of floodplain 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 floodplain areas, that must be kept free of encroachment in Qrder that the 100-year flood be carries without substantial increas,es in flood heights. As minimum standards, the Federal Insuranqe Administration limits such increases in flood heights to 1.9 foot, provided that hazardous velocities are not produced. The revised floodw/ly for Sand Creek was computed on the basis of equal conveyance reduction from each side of the floodplain. The results of these computations are tabulated at selected cross sections for each stream segment for which a floodway was computed (Table 2). The hydraulic reanalys'i s of the 2.3-mile reach of the South Platte River did not include: the calculation of a floodway, primarily due to the split-flow sit4ation (Reference 13). , As shown on the Flood Insurance Rate Map (Exhibit 2), the floodway boundaries on Sand Creek were deter~ined at selected cross sections; between cross sections, the boundar-ies were interpolated. In cases where the floodway and 100-year: floodplain boundary are close together, only the floodway has been shown. The area between 14