Résumé : The Seine River and its basin (70 000 km2, 500 m3/s mean annual discharge) are studied since 1989 by an interdisciplinary research programme (https://www.piren-seine.fr/). The river receives the effluents of about 17 M inhab (70% from Paris conurbation) and from industries. Intensive agriculture using agrochemicals is another cause of water quality alteration. Morphological alterations (such as channelization and reservoirs construction) were considerable.