ROLE OF SEEPAGE SUPPLY IN AQUATIC VEGETATION DYNAMICS IN FORMER RIVER CHANNELS - 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 channels upstream from the power plant because of the increase of seepage supply through coarse alluvium. The vegetation of two braided former channels, one situated upstream (impacted site) and one downstream (reference site) of the power plant was studied over a period of nine years, before (1981) and after (1985, 1986, 1987, and 1989) the construction of the hydroelectric power-plant. The two sites had similar vegetation and successional stages at the beginning of the study. The seepage supply increases in the impacted site resulted in the establishment of Chara globularis and Riccia fluitans, indicating groundwater influence, but, after halting in 1985-1986, the ecological succession proceeded slowly again in 1987 and 1989, depicted by continuing eutrophication (expressed particularly by the increasing abundance of Ceratophyllum demersum). As expected, ecological succession was not hailed in the reference site. Terrestrialization processes were apparent and changes were more rapid at its upstream part.