Differential Effects of Floods on the Aquatic Vegetation of Braided Channels of the Rhône River

Abstract

Six stations on cut-off braided channels of the Rhône River differing in substrate grain size, distance from the main channel, overflowing frequency, and riparian forest protection, were studied over five years to compare effects of high discharge and floods and subsequent recovery by aquatic vegetation. Less frequently disturbed stations have finer substrate than the more disturbed ones. In the latter, competition processes should not have time to take place, because of flood frequency, so the occurring pioneer species should allow faster recovery from flood disturbance. Vegetation cover (by species) and environmental variables were surveyed from 1988 to 1992, 2-4 times per year. In two fine-substrate stations subjected to floods, the vegetation exhibited no directional change in species composition and abundance following flood events, because these stations are located far from the Rhône River and are sheltered by riparian forest. In the third fine-substrate station, a major flood had a considerable, unexpected, indirect and long-lasting effect: it caused a lowering of the water table, and terrestrial plants progressively replaced the aquatic ones that dried up. In the three stations that are close to the main river and have coarse substrates, vegetation cover was reduced after the major floods. Rate of post-flood recovery was most rapid at stations where flood scouring frequency was highest.