Riparian wetlands of the middle reach of the Seine river (France): historical development, investigation and present hydrologic functioning. A case study

Along the middle reach of the Seine river, upstream from Paris, riparian wetlands that have been functioning for centuries, first as a result of a succession of sedimentological/hydrological processes and later of continuous management practices where the value of the wetlands has been acknowledged as well as their importance for the ecology. Here, we present the results of a multidisciplinary research programme focused on this system with emphasis on its present hydrological functioning and on the tools used to understand and quantify it. The major hydrological and geochemical processes that are active at present are reviewed, but past processes are also described as well as the potential long-term evolution of the system. This study uses several innovative tools to investigate the wetlands and analyse them in space: thermographic aerial survey and electromagnetic prospecting resulting in a map of clay-layer thickness. Concerning the wetland hydrology, water budgets, established at different time scales (several years, one year, a season) illustrate the exchanges between the watershed, the river and the wetland and water storage in the wetlands during flood events. The results show how the upstream reservoirs, built during the last 20 years, presently control the preservation and functioning of downstream riparian wetlands. Monitoring and modelling of the hydrological functioning of one wetland site have made it possible to identify and quantify exchanges between the wetland (especially in the unsaturated upper layer) and its environment. In situ measurements of denitrification rates in the wetland were used to make initial estimates of its nitrate-elimination capacity.