ROLE OF SEEPAGE SUPPLY IN AQUATIC VEGETATION DYNAMICS IN FORMER RIVERCHANNELS - PREDICTION TESTING USING A HYDROELECTRIC CONSTRUCTION

One predicted impact of the construction of the hydroelectric plant of Bregnier-Cordon (Upper Rhone River, France) was the rise of the water table in the part of the plain situated upstream from the power plant . This impact made it possible to test the hypothesis that the slowing , indeed the halting, of ecological successions occurs in the former c hannels upstream from the power plant because of the increase of seepa ge supply through coarse alluvium. The vegetation of two braided forme r channels, one situated upstream (impacted site) and one downstream ( reference site) of the power plant was studied over a period of nine y ears, before (1981) and after (1985, 1986, 1987, and 1989) the constru ction of the hydroelectric power-plant. The two sites had similar vege tation and successional stages at the beginning of the study. The seep age supply increases in the impacted site resulted in the establishmen t of Chara globularis and Riccia fluitans, indicating groundwater infl uence, but, after halting in 1985-1986, the ecological succession proc eeded slowly again in 1987 and 1989, depicted by continuing eutrophica tion (expressed particularly by the increasing abundance of Ceratophyl lum demersum). As expected, ecological succession was not hailed in th e reference site. Terrestrialization processes were apparent and chang es were more rapid at its upstream part.