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The 100- and SOD-year flpodplain boundaries are shown on the Flood Boundary and Floodway Map (Exhibit 2). In cases where the 100- and SOD-year floodplain boundaries are close together, only the 100-year floodplain boundary has been shown. Small areas within the flood plain 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. For the streams studied by approximate methods, only the 100-year floodplain boundary is shown. .,. The 100-year floodplain boundaries for Viele Channel, Davids Draw, Anderson Ditch, and Little Dry Creek, studied by approximate methods, were taken from the effective Flood Insurance Rate Map for the City of Boulder (Reference 14). The 100-year floodplain boundaries for Gregory Creek, Bluebell Canyon Creek, Kings Gulch, Sunshine Gulch, upper reaches of Bear Canyon Creek, and Skunk Canyon Creek, were taken from the Boulder and adjacent county drainageways - FHAD prepared by Greenhorne and O'Mara (Reference 18). 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 resul t ing increase in flood hazard. For purposes of the NFIP, a Hoodway is used as a tool to assist local communities in this aspect of floodplain management. Under this 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 100-year flood can be carried without substantial inc reases in flood heights. Minimum Federal standards lirni t such increases to 1.0 foot, provided that hazardous velocities are not produced. The floodways in this study are presented t.o local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway studies. :; The floodways presented in this study were 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 is computed (Table 3). As shown on the Flood Boundary and Floodway Map (Exhibit 2), the floodway boundaries were computed at cross sections. Between cross sections, the boundaries were interpolated. In cases where 16