Scientific program to understand the mechanisms of restoration of the Selune River (France) following the removal of two large dams
Résumé
By disrupting ecological connectivity and causing fragmentation in river systems, dams impose significant changes to hydrological regimes, water temperature and biological and bio-chemical fluxes. This is the case on the Selune River, one of the four coastal rivers that ends in the Bay of Mont Saint-Michel in Normandy, France. Two large dams (36 and 16m high) and many other secondary obstacles hinder free water flow and migrations of diadromous fish species. Their dismantling was the subject of many political, ecological and societal debates and finally started in 2019. The largest dam has now been completely removed. A breach at the bottom of the second dam will be done by summer 2021. After a century of lockdown, the upper 60 km of the Selune River, representing about 1000 km of flowing habitats if one considers the numerous tributaries of the river, will be reconnected to the ocean. To understand the mechanisms of restoration of the Selune River, a scientific program was initiated in 2012 and will continue until 2027. This unique and multi-disciplinary program aims at characterising the physical, chemical, biological and societal processes involved in the ecological restoration of the river and its valley. The scales of study are multiple, cascading from landscapes to chemical elements. The objective of this program is to produce a full experience feedback on the restoration of river connectivity. So far, scientists have worked on understanding how the river works with dams. Current research also focuses on anticipating the changes that may occur following dam removal. From 2022, research will focus on understanding how the aquatic and riparian ecosystems will evolve without dams. By comparing these different phases, it will be possible to characterize the effects of the removal of dams and to identify the societal and ecological costs and benefits associated with the return of a free-flowing fluvial ecosystem.
