Synthesis of the main ecological impacts resulting from hydroelectric developments and their consequences on river operation

Based on a large bibliography, this document proposes a synthesis of present knowledge on the main ecological impacts resulting from hydroelectric installations and their consequences on river operation. These main impacts are the modification of flow conditions (upstream and downstream the works), and the perturbation or even the blockage of flows (sediments, organic matter, nutrients) and of displacements of aquatic organisms. Consequences for running water ecosystem operation are discussed upstream the weir or dam and within the reservoir (modifications of hydraulic and morphological conditions and of habitats, modifications of water thermal regime and quality, evolution of biological communities, modification of solid transport mechanisms and entrapment of materials in reservoirs), downstream the weir or dam (modification of solid transport mechanisms and morphological adjustments, modifications of water thermal regime and quality, modifications of the hydrological regime), and in terms of disturbances for species migrations and displacements). The report finally outlines that impacts and consequences may vary according to developments