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stream of Montrose. These frequency curves were computed by statis- tical analyses of stream gage data for 58 years of record at the Delta gage and 57 years of record at the Colona gage. The indepen- dent frequency curveS for rainfall and snowmelt events were combined statistically to get "all-event" frequency curves at the gage loca- tions. Because the two all-event frequency curves were nearly identical at the two gage locations, the peak discharges for the 10-, 50-, 100-, and 500-year floods were computed to be constant between the gages. This indicates that attenuation and stream diversions counterbalance tributary inflow between the stations. Peak discharge-drainage area relationships for the study streams in the City of Montrose are shown in Table 2. The peak discharges for Cedar Creek and Montrose Arroyo were deter- mined by the CWCB in April 1979 (Reference 2). Rainfall data for the CWCB study were statistically analyzed to define recurrence interval probabilities and the resultant rainfall- frequency relationship for both 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 precipitation pattern shown in the National Oceanic and Atmospheric Administration Atlas (Reference 8). 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 two major basins (Cedar Creek and Montrose Arroyo). The result was composite hydrographs at each design point for each design storm. The impact of snowmelt was examined (and is included) for the over- all flood plain analysis. Snowmelt affects peak flow rate, as it establishes 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 statisti- cally combined with the corresponding curve based on rainfall. Snowmelt data rainfall data were combined after the rainfall-generated runoff hydrographs for the various subbasins were combined. Ditch crossings in the study basins, with the exception of the South Canal, were all handled in the same manner for the purposes of the hydrograph routing procedure. The ditches were assumed to 10