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Approximate i-percent annual chance floodplain boundaries in some portions of the study area were taken directly from the Flood Hazard Boundary Map (Reference 31). 4.2 Floodways Encroachment on flood plains, 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 resulting increase in flood hazard. For purposes of the NFIP, a floodway is used as a tool to assist local communities in this aspect of floodplain management. Under this concept, the area of the I-percent annual chance 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 i-percent annual chance flood can be carried without substantial increases in flood heights. Minimum Federal standards limit such increases to i.O foot, provided that hazardous velocities are not produced. The floodways in this study are presented to local agencies as minimum standards that can be adopted directly or that can be used as a basis for additional floodway studies. The floodways for this study were divided into two categories based upon the location of the stream. The mountain area criteria were based upon the channel of the stream plus all flood plain areas where the depth of flooding was 18 inches or greater and the floodway fringe area was the area where flooding depths were i8 inches or less and velocities were 3 feet per second or less. The plains area criteria were based upon the criteria of equal- conveyance reduction from each side of the flood plain with a O.5-foot limitation on flood height, provided that hazardous velocities are not produced (Figure i). The floodways for the Big Thompson River upstream of Lake Estes and Black Canyon Creek were based upon the criteria of equal conveyance reduction from each side of the flood plain with a I.O-foot limitation of increased flood height, provided that hazardous velocities are not produced (Figure i). The floodway for the Fall River was based upon the criteria of equal conveyance reduction from each side of the flood plain with a i.O-foot limitation on increased energy grade line. The floodway presented in the PIS report and on the FIRM for the Big Thompson River in Loveland was modified from the effective regulatory floodway. The effective floodway for the plains criteria reach of the Big Thompson River was originally developed on the basis of equal conveyance reduction from each side of the floodplain, with a rise limitation of one foot. The revised analysis defined a floodway with a rise limitation of one-half foot. Wherever possible and appropriate, the effective floodway boundary was maintained if it resulted in a base-flood rise of one-half foot or less under the new study. The floodway was widened wherever necessary to keep the rise at or below one-half foot. The floodway was made narrower in some locations because the effective floodway encroachment limit was found to be within ineffective flow areas under the revised study. The floodway for Dry Creek - BTR was computed on the basis of equal conveyance reduction from each side of the floodplain where feasible. 42