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<br />PEARSON, KRAMER, FRANKLIN: MACROINVERTEBRATES 165 <br />The reduction in standing crop of Simuliidae larvae at Little <br />Hole in 1967 may also have been caused by low water tempera- <br />tures. <br />A comparison of pre- and postimpoundment data indicates that <br />species composition of the invertebrate community of the Green <br />River has not been noticeably altered below Echo Park, and per- <br />haps not below the mouth of Lodore Canyon. Six of the nine <br />forms not found during the present study but reported previously <br />from the Green River had been collected above the Flaming <br />Gorge Dam site. Of these six, Perlesta, Genus et species novum <br />(Ephemeroptera), Pseudiron, Pentagenia, and Helodidae were prob- <br />ably rare before September, 1962, since each was represented by <br />fewer than five specimens in preimpoundment collections. The <br />sixth, Baetis insignificans, may have been collected in the present <br />study but assigned to one of the ten unidentified categories. <br />Of the remaining three groups, Argia and Gyrinidae were <br />frequently seen but not captured with the sampling gear used <br />in this study. The last form, Claassenia sabulosa, was reportedly <br />common in the study area before closure of the dam, and it should <br />have been collected with the techniques used in this study. Since <br />no specimens were found, Claassenia sabulosa, therefore, is the <br />only macroinvertebrate form to have been definitely adversely <br />affected by the fish-control project and/or the operation of Flam- <br />ing Gorge Dam. C. sabulosa is common, however, in several other <br />areas in the Intermountain region (Jewett, 1959). <br />CONCLUSIONS <br />The installation and operation of Flaming Gorge Dam has <br />affected the river environment for at least 150 km downstream. <br />Stream flows have been stabilized on a seasonal basis, but daily <br />flows fluctuate widely. Water temperatures are now lower in <br />summer and higher in winter than before impoundment. Inverte- <br />brate communities between the dam and Carr Ranch have been <br />altered, probably as a response to lower summer water tempera- <br />tures. Baetis sp. I responded spectacularly to low water tempera- <br />tures in 1967 and practically disappeared from the first 9.5 km <br />below the dam. <br />Species composition in the Green River below Lodore Canyon <br />in 1964-65 appeared to be much the same as that reported in <br />preimpoundment studies with the singular exception of Claassenia <br />sabulosa, which has apparently disappeared from the fauna. As <br />r