Influence of dams on river water‐quality signatures at event and seasonal scales: The Sélune River (France) case study

International audienceams and reservoirs are known to disturb river-water composition, among other impacts, with potential implications for downstream river ecosystems and water uses. Existing studies have emphasized the variable influence of dams on water composition according to the element, its speciation (dissolved vs. particulate), reservoir properties (residence times), reservoir functions (e.g. hydropower, irrigation), and management (water releases). A now common approach to analyzing hydrological, geochemical, and biological controls on element export from unregulated rivers is to study hydrochemical signatures such as concentration-flow relationships. We investigated acase study to analyze hydrochemical signatures of a regulated river (Sélune River, western France) upstream and downstream of a chain of two hydropower dams, assuming that the dams disturbed the river’s signatures, and that those disturbances would provide information about processes occurring in the reservoirs. Both seasonal and event-scale signatures were analyzed over two contrasting hydrological years and a range of storm events. The dams nduced a chemostatic downstream response to storm events whenever elements were diluted or concentrated upstream. Dams did not disturb the seasonality of major anions but did modify silica and phosphorus concentration-flow relationships, especially during low flow. Such changes in dynamics of river-water composition may affect downstream biological communities.This study presents an initial state of the hydrochemical signature of the downstream river, before the removal of the two dam