Trajectories and future of the anthropized Seine River, France

The Seine River has been profoundly modified by the multiple uses to which it has been devoted (water supply, navigation, energy, pollutant receiver, fishing, swimming). From mid-19th century, the development of river transport induced irreversible modifications of its course. At the same time, the Seine suffered a noticeable degradation of its water quality due to the discharge of wastewater from the City of Paris, to which this river is intimately linked (Lestel et al., 2023). Since the 1950s, housing has become denser in the alluvial plain of the Seine, making the metropolis extremely exposed to the risk of flooding.In a general manner, the co-evolution of river basin systems and sociosystems remains poorly addressed, particularly over long periods (>100 y). Medium-sized river basins make it possible to perform such detailed analyses, which combines historical river fluxes, material flows, river ecology, environmental history and political ecology. Such an interdisciplinary approach at the scale of the Seine basin, undertaken by the PIREN-Seine research program, started in 1989, has shown how scientific and technological knowledge, environmental awareness, environmental regulations and policies, and political decisions have played a role on water quality during that period (Meybeck et al., 2022).In this presentation, we will describe some examples of trajectories of the Seine River under the influence of the City of Paris. The controls exerted on the river course and aquatic ecosystem by Paris now far exceed the natural hydromorphological and hydraulic variations in many ways. Many of the existing transformations – channelization, bank artificialization, floodplain sand mining, and damming - might be considered permanent, given the current management policy in the Seine basin, which still gives priority to navigation. But the Seine is a complex place where this vision can be counterbalanced by that of a river managed according to sustainable principles, whose beauty was magnified by its impressionist painters and recognized by the classification of its banks by Unesco in 1991. In conclusion, we will discuss the fact that meeting the new challenges of the 21st century (whether from climate change, or new societal expectations, such as recreational bathing and bank restoration) will require the development of river care, more integrated management, overall plan and better coordination among the different institutions in charge of the river and its functions. Lestel L., Meybeck M., Carré C., Belliard J. (2023). The Seine, the River dedicated to Paris, In River cultures: life as a dance to the rhythm of the waters, Wantzen K.M. ed., Unesco, 669-693. Meybeck, M., Bouleau, G., Carré C., Garnier, J., Lestel, L. (2022). Rivers help us to quantify the socio-ecological functioning of their basin at the Anthropocene: the Seine example (1850-2020), Comptes-rendus, Geoscience, doi.org/10.5802/crgeos.14