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s I i i PEARSON, KRAMER, FRANKLIN: MACROINVERTEBRATES 151 TABLE 1 MEAN MONTHLY DISCHARGE OF THE GREEN RIVER BEFORE AND AFTER CLOSURE OF FLAMING GORGE DAM, GREENDALE, MAY-OCTOBER, EXPRESSED IN M3/SEC. Year Month May June July Aug. Sept. Oct. Preimpoundment (1950-1960 mean) 129.8 206.9 98.4 48.5 26.3 26.1 Postimpoundment 1963 3.7 3.5 2.9 2.9 3.2 3.6 1964 41.8 40.9 69.1 56.4 62.3 73.1 1965 30.5 40.7 13.4 14.1 20.8 36.2 1966 38.3 45.0 47.9 54.4 58.8 62.3 1967 54.3 91.4 77.9 85.3 85.6 88.2 TABLE 2 MAXIMUM WATER TEMPERATURES RECORDED AT LITTLE HOLE, GREEN RIVER, MAY-OCTOBER, 1963-1967, EXPRESSED IN °C Year Month May June July Aug. Sept. Oct. 19631 .... 16.0 15.5 15.5 16.5 19641 9.5 7.0 8.0 14.5 10.0 11.0 1965 .... 11.0 15.0 16.0 13.5 12.0 19661 6.0 10.0 11.5 11.0 12.5 11.5 1967 6.0 6.5 8.5 9.5 10.0 'Data from Utah Fish & Game records. Water Temperature The general effect of the dam has been to increase river tem- peratures during the winter and decrease them during the summer, compared to preimpoundment years. '`lean monthly preimpound- ment water temperatures ranged from 0.5° C to 21.0° C at Greendale, and mean monthly postimpounInent temperatures have ranged from 2.0° to 12.5° C. At Little Hole, maximum summer water temperatures are now inversely related to stream flow, and the lowest temperatures during the June-October period were recorded in 1967, the year of highest summer flows (Tables 1 and 2). Near Jensen, Utah (150.3 KBD), seasonal changes in water temperature were affected little by the dam in 1963, but summer temperatures during 1964-67 were lA° to 3.5° C lower than the preimpoundment average. Winter temperatures at Jensen have been similar to those during preimpoundment years (0-1° C in December, January, and February). r ..