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lOt) reported that nine forms of invertebrates found in the Green River at Little Hole, just downstream from the Flaming Gorge Dam, before the closure of the dam were not found after the closure. On the other hand, ten groups of invertebrates that had not been found previously were found ~ .J a.. ~ 2.5 < en ~ Lt.. ~ 2.0 o after the dam was completed. Downstream, at Carr Ranch and Echo Park, the river environment showed signs of transition resembling the pre- impoundment condition. Still further downstream, at Island Park, the river appeared the same in 1964-65 as it did before the dam was completed OTHER AQUATIC INVERTEBRATES ~ (1963). This study and others have shown that disruption of an ecosystem d ~ en 15 ". en ~ en z 1.0 < C) a: o o z , \ \ \ \ \ \ \ \ \ , , , . ,. .. .-, ..... .... -...,..... .... .... .". '.... " ,-------,,-- can cause changes in the species composition and abundance of macroinver- tebrates. Pronounced water level fluctuations can also have a great impact on macroinvertebrates. For example, an estimated 25 percent of the stream- bed that had been submerged for the previous five months was exposed by a drop of more than 1 foot in the water level of the Green River at Island Park on 7 September 1964 (Pearson and Franklin, 1968). As the water level o C) o .J o 1200 SEPT 7 SEPT 8 Drift Rates of Chironomidae and Other Aquatic Invertebrates During a 1-1/2 ft, Drop and Rise in Water Level, Island Park, Green River, 7-9 September 1964, Source: W, D, Pearson and D, R, Franklin, "Some Factors Affecting Drift Rates of Baetis and Simuliidae in a Large River," Ecology vol. 49, no, 1, 1968. dropped, the drift rates of Baetis sp., other mayflies, caddisflies, and stonef1ies reached the highest ever recorded at this station, especially after sunset (figure 3). Pearson and Franklin reported that they ob- served nymphs of Baetis sp. moving (crawling and swimming) toward deeper water as the river level dropped. As the water level returned to normal two days later, the drift of macroinvertebrates decreased. The wide fluctuations in streamflows released from the Green Moun tain Reservoir on the Blue River, a tributary to the Colorado River, were ~lso shown to have a great impact on invertebrate populations (Powell, 1958). Powell reported that the average weight per unit of area was six 0.5 CHIRONOMIDAE \ ...... . . . . 2400 1200 2400 Figure 3, , WATER " lEVEl---, I I I I I I I t.. . . .-1 ... ~ 1.5 w Lt.. I en C) Z o < 1.0 W a: . . . w C) < C) ...J 0.5w > w ...J a: w ... ~ o 1200 2400 SEPT 9 " I( t, ... .. .. .. , , .. ... - ~