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for the four frequency floods, and discharges for the san Miguel River were determined from these regional curves. The peak discharges for Hieroglyphics Canyon and Happy Canyon Creek were determined by the study contractor in April 1979 (Reference 8). A regional floodflow frequency analysis was performed on gaging station data from the study basin, from which drainage area discharge curves were derived. A mixed population of rainfall and snowmelt data was used. The results of this analysis were combined statis- tically to give a maximum flow for both events. Regional curves of drainage area versus discharge were computed for the four fre- quency floods. The peak discharges for Cedar Creek, Dry Cedar Creek, and Montrose Arroyo were determined by the Colorado Water Conservation Board in April 1979 (Reference 4). Rainfall data for the Colorado Water Conservation Board study were statistically analyzed to define recurrence interval probabilities and the resultant rainfall-frequency relationship for both of the upper basin areas, as well as the City of Montrose. This rainfall relation was then graduated between the City of Montrose and the upper basin areas according to the predominant regional precipita- tion pattern shown in the National Oceanic and Atmospheric Adminis- tration Atlas (Reference 9). For the overall flood plain analysis, the runoff hydrographs were developed by using the U.S. Soil Conservation Service procedure for each of the individual subbasins. All channel routings were developed using the Muskingum procedure. This process was accom- plished for the various subbasins comprising each of the three major basins (Cedar Creek, Montrose Arroyo, and Dry Cedar Creek). The result was composite hydro graphs at each design point, for each design storm. The impact of snowmelt was examined and included for the overall flood plain analysis. Snowmelt impacts peak flow rate as it estab- lishes the base streamflow to which the peak flow is added. Numerous drainage basins, judged to be reasonably similar to the study basins, for which snowmelt data are available were examined. From these data, the 100-year frequency snowmelt relation of flow to drainage basin area was defined for the Montrose study area. These data, in turn, were used to define the snowmelt-frequency flow relation. The snowmelt-frequency flow curve was then statistically combined with the corresponding curve based on rainfall. Snowmelt data and rainfall data were combined after the rainfall-generated runoff hydro graphs for the various subbasins were combined. Ditch crossings in the study basins, South Canal, were all handled in the of the hydrograph routing procedure. with the exception of the same manner for the purposes The ditches were assumed to 18