This paper deals with the results of an extensive sampling of eight former channels of the Rhǒne and eight of its tributary, the Ain River, France, designed to assess the value of molluscs as ‚describers' of ecosystem dynamics. The sampling was stratified on the distribution of aquatic macrophytes. The distribution of molluscs species is shown to provide information on the rate of eutrophication, the groundwater supply and the succession of the abandonned river channels toward semi-aquatic and terrestrial conditions. Molluscs, together with other macroinvertebrates, not only describe the present stage of these ecosystems but also provide some predictive information on their successional pattern for assistance in the management of the alluvial plain.