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The floodway for the Big Thompson River near Johnstown was computed on the basis of equal conveyance reduction from each side of the floodplain. Because the Town of Johnstown regulates to the one-foot floodway requirement and the County regulates to the more stringent half-foot floodway requirement different floodway requirements were used. For the portion of the reach that flows in or near Johnstown the one-foot floodway requirement was used. This section is from cross-section F to cross-section U. For the rest of the reach the half-foot floodway requirement was used. Computation of portions of the floodway for the Cache La Poudre River in the study reach required nonstandard solutions due to the numerous divided-flow paths. In location of divided flow, it was attempted to channel all of the flow into the mainstream path, limiting the flow to the I-percent annual chance flood plain. If the water-surface elevation increase was less than the allowable surcharge, the main flow path was encroached by proportional conveyance reduction. If the water-surface elevation increase caused by the additional discharge was greater than the allowable surcharge, then water was allowed to flow both along the main flow path and the involved divided-flow paths. The divided-flow paths were then encroached using proportional conveyance reduction until the allowable surcharge was reached. The floodway was then defined as the outer limits of the two paths. In areas where divided-flow paths were not present, the floodway was computed using proportional conveyance reduction from each side of the flood plain with adjustments as necessary to obtain a consistent and reasonable f1oodway. Floodways for the revised areas of Cache La Poudre River were based on equal reduction in conveyance from both sides of the flood plain. Although both O.5-foot- and I-foot-rise floodways were computed, the I-foot-rise floodway was mapped. In the lower reach of the study, divided floodways were developed along the Cache La Poudre River and Cache La Poudre LPATH due to split-flow conditions. Just east of Wastewater Treatment Plant Number 2, the main flow of the Cache La Poudre River leaves its channel during large flood events. The floodway was delineated along the breakout flow path. No floodway was mapped for the Lowflow channel of the Cache La Poudre River. Just below Taft Hill Road, a flow diversion was modeled by subtracting the left overbank flow from the total discharge and confining the remaining flow to the channel. It was assumed that the confining banks fail for the purpose of computing a floodway. For the revised area of the Cache La Poudre River from Wood Street to Watson Lake, a 0.5 foot floodway was computed. 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 revised 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. A 0.5 foot floodway was computed for Boxelder Creek and Cooper Slough based on the equal conveyance method. 43