DIFFERENTIAL-EFFECTS OF FLOODS ON THE AQUATIC VEGETATION OF BRAIDED CHANNELS OF THE RHONE RIVER

Six stations on cut-off braided channels of the Rhone River differing in substrate grain size, distance from the main channel, overflowing f requency, and riparian forest protection, were studied over five years to compare effects of high discharge and floods and subsequent recove ry by aquatic vegetation. Less frequently disturbed stations have fine r substrate than the more disturbed ones. In the latter, competition p rocesses should not have time to take place, because of flood frequenc y, so the occurring pioneer species should allow faster recovery from flood disturbance. Vegetation cover (by species) and environmental var iables were surveyed from 1988 to 1992, 2-4 times per year. In two fin e-substrate stations subjected to floods, the vegetation exhibited no directional change in species composition and abundance following floo d events, because these stations are located far from the Rhone River and are sheltered by riparian forest. In the third fine-substrate stat ion, a major flood had a considerable, unexpected, indirect and long-l asting effect: it caused a lowering of the water table, and terrestria l plants progressively replaced the aquatic ones that dried up. In the three stations that are close to the main river and have coarse subst rates, vegetation cover was reduced after the major floods. Rate of po st-flood recovery was most rapid at stations where flood scouring freq uency was highest.