PIREN-Seine is a research group whose aim is to develop an overall vision of the operation of the system comprising the Seine hydrographic network, its watershed and the human society which depends on it. The ecological function of the river system as a whole and its modelling, from its bacteria through to its fish, are based on a detailed study of the ambient physical, chemical and biological processes. The models developed by PIREN-Seine simulate the ecological and biochemical variations within the hydro-system, from the streams until it reaches the estuary. Supported by Sorbonne Université and CNRS, it brings together teams from a number of universities, research centres, engineering schools and research bodies of the operators involved. Projects are executed with the majority of the public and private water management stakeholders in the Seine-Normandy basin, and have contributed to shaping, expanding, and disseminating knowledge about water-related processes in the local territories of the basin. The program's works have paved the way for successful forays in transdisciplinary research, an approach that will persist as the program moves forward, continually bringing together scientists from various backgrounds.