CHANGES WITHIN THE UPPER RHONE-RIVER MACROBENTHIC COMMUNITIES AFTER THE COMPLETION OF 3 HYDROELECTRIC SCHEMES - ANTHROPOGENIC EFFECTS OR NATURAL CHANGE

In this paper, we study an Upper Rhone River impoundment (Bregnier-Cor don section) composed of a dam and a diversion canal. We present the e cological impact of this impoundment on the benthic invertebrate commu nities over 17 years (from 1975 to 1991) that shows a progressive long -term change at the five prospected sites. Before impoundment benthic communities were dominated by rheophilic species (hydropsychids) where as after impoundment a majority of lentic and thermophilic species wer e collected. This long-term change in benthic invertebrate communities was partly related to the warming of water. The warming of water was attributed to the synergy between local climatic change during the stu dy and the completion of hydroelectric schemes upstream. Due to a high degree of human impact, the diversion canal demonstrated a lower taxa richness than the other sites of the former Rhone. In the by-passed s ection, the level of the compensation flow was sufficient to maintain a species richness close to the normal situation, though a global chan ge in community composition also occurred. Seasonal patterns were pres erved before and after impoundment; however, after impoundment, specie s such as Hydra sp. blew up during spring.