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II-a I I I II ,I I I I II I I I I I I I I, I I 4. Nat~re and Frequency of Flooding (11 )(24) Flo)ding along the main stem of the Colorado River is nonnally the l~esult of rapid melting of the mountain snowpack in late May through early July. These snowmelt floods are occasionally augmented by rain. General rainstorms alone and convecti ve-type thunderstorms nonnally have little effect on the Colorado River due to the large size of the drainage basin. The Colorado River has a long history of snowmelt flooding but for the period prior to initiation of the gaging stations little detailed infor- mation is available. The estimated flood of record, based on high water marks only, occurred in June-July 1884. This flood is estimated at 125,000 cfs at (10) Fruita below the confluence of the Colorado River with the Gunnison River and 50,000 cfs on the Gunnison River at Grand Junction. Based on these estimates, the 1884 flood was probably in the neighborhood of 70,000 cfs at the Una dam sUe. Peak flows recorded at the Cameo gage since 1934 are as shown in Table II-3. The annual peak flood frequency curve, as computed by the USGS based upon the Cameo gage record and standard Water Resource Council (WRC) (29) guid.~lines, is shown in Fig. II-4. The 100-year and 500-year re- turn period flood peak discharges at the dam site are approximately 39,000 cfs and 44,000 CfE:, respectively. The annual, seasonal, and monthly 5- and 10-year return period flood peak discharges at the dam site are shown in Table II-4. The peak flows at the dam site are approximately the same as those measured at the Cameo gage, based on correlation studies of the period of overlapping record (1967-1980). 5. Reservoir Evaporation Average annual Una Reservoir evaporation was estimated based on infonnation published by the Department of Commerce in Climatic Atlas of the United States, (22) 1968. The annual lake evaporation is approximately 34