GROUNDWATER SURFACE-WATER ECOTONES AT THE UPSTREAM PART OF CONFLUENCES IN FORMER RIVER CHANNELS

Two slow flowing former channels of the Rhone River, which are supplied by nutrient-poor groundwater and subject to backflows of nutrient-rich water from the Rhone, were studied in order to demonstrate the occurrence of an ecotone located upstream to their confluences with the main channel. In one of the channels, the long-term stability of the boundary zone compared to the other sections was investigated by examining floristic changes over the period of a decade, demonstrating its high temporal stability compared to the adjacent systems. This stability is seen to be a result of both external and internal processes: backflows of river water rich in suspended sediment favour alluvial deposition of silt in the ecotone area, which in turn allows the growth of numerous hydrophyte species, many of which have specific environmental requirements. These macrophytes promote sediment retention and deposition (a positive feed-back mechanism), thus acting as a filter for the surface water flow. Finally, flash floods regularly scour fine sediment deposits and rejuvenate these areas on a patch by patch basis. The occurrence of boundary zones is demonstrated in the areas located upstream from the confluence in both channels under study. The seasonal chemical variability of the channels and the Rhone itself, as compared to the backflow area, was analysed using chemical variables that are influenced by water fluxes. The fluctuations of the chemical parameters of the boundaries are seen to be intermediate between those of the adjacent systems in one case and higher in the other. With reference to recent definitions in the scientific literature, the overall characteristics of these boundary areas lead us to conclude that they can be considered as ecotones between two water fluxes.