Impact of Civil Engineering Works on the Successions of Communities in a Fluvial System: A Methodological and Predictive Approach Applied to a Section of the Upper Rhône River, France

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Abstract
A study of fluvial dynamics allowed us to characterize the main geographical patterns along the Upper Rhone River, France (between Geneva and Lyon) from which we chose a braided and anastomosed section (aggradation of the long profile, lateral flow into sinuous channels) at the southern end of the Jura Mountains for more in-depth study. Fluvial dynamics determine the habitat diversity by the erosion-deposition processes that create the biotopes. The functional units (communities of aquatic or terrestrial living organism bound to particular water dynamics) are organized in chronosequences (time successions on a determined land form) and described on the plain in spatiosequences. Historical analysis clarifies the determinism of the spatial organisation of communities and their temporal succession in four situations: 1) in natural conditions, the original succession are reconstituted by a retrospective study; 2) the construction of the submersible embankment at the end of the 1880s allowed the complete development of formally-truncated sequences and initiated new sequences but impeded the renewal of some other units; 3) a break in the embankment (1917) initiated a new type of succession; and , 4) the construction of a hydroelectric project is a case study in order to predict the succession of the ecosystems beyond 1984. The development speed of the ecological successions, and therefore the life span of the communities, depends on the equilibrium between allogenous and autogenous processes. Consequently, for ecological management, it is recommended to conserve those units with a slower succession rate and to promote the regenerative capacity of the fluvial land forms and associated ecosystems.