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The ~OO- and 500-year f~ood plain boundaries are shown on the Flood Insurance Rate Map (Exhibit 2)~ On this map, the ~OO-year f~ood p~ain boundary corresponds to ~he boundary of ~he areas of specia~ f~ood hazards (Zones AE and AO), and the 500-year f~ood p~ain bound- ary corresponds to the boundary of areas of moderate f~ood hazards. In cases where the ~OO- and SOD-year flood plain boundaries are close together, on~y the 100-year f~ood plain boundary has been shown. Sma~l 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 app~oximate methods, on~y the ~OO-year flood p~ain boundary is shown on the Flood Insurance Rate Map (Exhi- bit 2). Approximate 100-year flood p~a~n boundaries in some portions of the study area were taken directly from the F~ood Hazard Boundary Map (FHBM) for Pitkin County (Reference 24). 4.2 F~oodways Encroachment on flood p~ains, 'such as structures and fill, reduces f~ood-carrying capacity, increases f~ood heights and ve~ocities, and increases flood hazards in areas beyond the encroachment itse~f. One aspect of flood plain management involves balancing the economic gain from flood plain deve~op~ent against the resu~ting increase in flood hazard. For purpose$ of the NFIP, a f~oodway is used as a tool to assist local communities in this aspect of f~ood plain management. Under this concept, the area of the 100-year flood plain is divided into a flood~ay and a floodway fringe. The flood- way is the channel of a stre~, plus any adjacent f~ood plain areas, that must be kept free of encroachment so that the ~OO-year flood can be carried without substantia~ inoreases in f~ood heights. Minimum Federal standardslim1t such increases to 1.0 foot, provid- ed that hazardous velocities ~re not produced. The floodways in this study are presented to ~oca~ agencies as minimum standards that can be adopted directly or that can be used as a basis for additiona~ f~oodway studies. The floodways presented in this study were computed for certain stream segments on the basis df equal conveyance reduction from each side of the flood plain. Floodway widths were computed at cross sections. Between cross sections, the floodway boundaries were interpolated. The resu~~s of the floodway computations are tabulated for se~ected cross aections (Table 3). The computed floodways are shown on the F~qod Insurance Rate Map (Exhibit 2). In cases where the f~oodway aqd ~OO-year flood plain boundaries are either close together or qOllinear, on~y the floodway boundary is shown. 16