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f I , I , I i fl 1 I: I~ 1 I I ~' : , I: J I~ I. Ii . ,t. IL a. Comparison of Colorado River Discharges at Several Locations along Study Reach To determine if the discharges estimated by the Corps downstream of ,Plateau Creek (listed in Section No. II A) were appropriate to use at Palisade and points downstream, gauging station records at several points were examined. Figures 2 and 3 compare flows at the state Line gauge during the 1983 and 1984 snowmelt flood with flows computed by adding records of mean daily flows at the Cameo, Plateau Creek, and Gunnison-Grand Junction gauges. (The Plateau Creek gauge did not record any flows during 1984.) It is apparent that the sum of the flows of the three upstream gauges is nearly equal to the recorded values at the State Line. Therefore, one may conclude that since there are no major t r ibuta ries on the Colorado between Plateau Creek and the Gunnison or between the Gunnison and the State Line, any additional runoff from intervening drainage areas is balanced by channel and overbank storage and diversions into irrigation ditches in the area. Thus, there is no overall change in peak discharges and it is reasonable to use the peak flows estimated by the Corps below Plateau Creek for the stream reach between Plateau Creek at Palisade to a point just upstream of the Gunnison River. b. Analysis of Gunnison River Flood Flows Downstream of the mouth of the Gunnison, peak discharges on the Colorado River increase significantly. To determine how much effect the Gunnison River has on floods on the Colorado, two questions were addressed: 1) what are the magnitudes 'of the floods on the Gunnison; and 2) when do peak discharges on the Gunnison occur, relative to peaks on the Colorado. Records from the long-term gauge on the Gunnison were analyzed, using the log Pearson Type III distribution to estimate peak discharges near Grand Junction. The entire period of record was not used because Blue Mesa Dam, completed in 1965, significantly affects downstream flows. The flood flow frequency analysis performed for the period 1966 to 1985 gives the following results. Return Interval Results of Frequencv Analvsis Previous FIS 10-year 50-year 100-year 500-year 17,700 cfs 28,700 cfs 34,400 cfs 50,400 cfs 15,000 16,600 19,000 25,000 -5-