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4.1 Flood Boundaries In order to provide a national standard without regional discrimina- tion, the IOO-year flood has been adopted by the Federal Insurance Administration as the base flood for purposes of flood plain manage- ~ent measures. The SOQ-year flood is employed to indicate additional areas of flood risk in the community. For each stream studied in detail, the boundaries of the IOO-year and the SOQ-year floods have been delineated using the flood eleva- tions determined at each cross section; between cross sections, the boundaries were interpolated using elevations from field survey measureme~ts. In cases where the lOO-year and the SOQ-year flood boundaries are close together, only the IOO-year boundary has been shown. The flood boundaries are snown on the Flood Boundary Map (Exhibit 2). Since the Yampa River does not present a flood threat to the incorpo- rated area of Hayden, the flood boundaries are not included in this report. Flood boundaries for areas studied by approximate methods were determined through interviews with residents, historical records, the rational formula method, and maps (References 9 and 10). Small areas within the flood boundaries may lie above the flood elevation and, therefore, not be subject to flooding; owing to limitutions 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 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 lOO-year flood is divided into a floodway and a floodway [r,inge. 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 IOO-year flood be carried hiithout substantial increases in floud hGig~ts. As minimum standards, the Federal Insurance Administratjon limits such increases in flood heights to 1.0 foot, proviced that hazardous velocities arc not produced. The floodway designated for Dry Creek was computed on the basis of equal conveyance reduction from each side of the flood plain witnout permitting encroachment on the channel. The results of these compu- tations are tabulated at selected cross sections for each stream segment for which a floodway is computed (Table 2). As shown on the Floodway f'1ap (Exhibit 3), the floodway boundaries were determined at cross sections; between cross sections, the boundaries w~re interpolated. 8