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For the San Juan River, North Fork McCabe Creek, West Fork McCabe Creek, and McCabe Creek, the boundaries of the 100- and SOO-year floods were adopted from the results of the flood hazard study by the u.S. Soil Conservation Service (Reference 8). Flood boundaries for the 100- and SOO-year floods are shown on the Flood Boundary and Floodway Map (Exhibit 2). In cases where the 100- and SOO-year flood boundaries are close together, only the 100-year flood boundary has been shown. ,. Approximate boundaries were delineated according to hydraulic calcula- tions and engineering judgment, using topographic maps at a scale of 1:24,000, with contour intervals of 20 (Reference 14) and 40 feet (Reference 15), and at a scale of 1:62,500, with contour intervals of 40 and 80 feet (References 16 and 17, respectively). , Approximate flood boundaries in some portions of the study area were taken from the Federal Insurance Administration's Flood Hazard Boundary Map (Reference 18). Small areas within the flood boundaries may lie above the flood elevations and, therefore, 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 invclves 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 communities in this aspect of flood plain management. Under this concept, the area of the lOa-year flood is divided into a floodway and a floodway fringe. 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 lOa-year flood be carried without substantial increases in flood heights. As minimum standards, the Federal Insurance Adminis- tration limits such increases in flood heights to 1.0 foot, provided that hazardous velocities are not produced. The floodways for the San Juan River, North Fork McCabe Creek, West Fork McCabe Creek, McCabe Creek, and the Rio Blanco were computed on the basis of equal conveyance reduction from each side of the flood plain. The results of these computations are tabulated at selected cross sections for eac~ stream segment for which a floodway is computed (Table 3). 13