HYDROSEDIMENTARY CLASSIFICATION OF NATURAL AND ENGINEERED BACKWATERS OF A LARGE RIVER, THE LOWER RHONE - POSSIBLE APPLICATIONS FOR THE MAINTENANCE OF HIGH FISH BIODIVERSITY

At the end of the last century, the shorelines of the Lower River Rhon e were embanked and submersible dikes were built in the main channel, delimiting dike fields. We compared the morphological, hydrological an d sedimentary variables of nine dike fields, four sites in dead arms a nd one in the main channel, in order to describe the fluviosedimentary functioning and to produce a classification of these different natura l and engineered backwaters. Within these 'artificial floodplains', th e different types of backwaters and their long-term changes bear a str ong similarity to those observed within a natural floodplain. The long -term frequency of submersion by the water of the main river channel i s the first factor that controls the erosion and sedimentation within the different backwaters. Three types of site can be distinguished: th e lotic sites are the main channel and one dike field, five dike field s comprise the semi-lotic sites and the cluster lentic sites include t he three remaining dike fields and the four dead arms sites. These thr ee groups also differ in their riparian habitats and in their juvenile fish assemblages. As juvenile fish were most abundant in the semi-lot ic dike fields, the use of such structures can be envisaged in order t o restore high fish diversity in rivers where the former floodplain ha s disappeared or is no longer accessible to fish because of human modi fications. (C) 1997 John Wiley & Sons, Ltd.