Cumulative impact of reservoirs on the aquatic environment. Joint scientific appraisal

Water storage has increased considerably worldwide since the 1950s. Water is collected and stored in reservoirs to supply towns and cities with drinking water, and for agricultural and industrial purposes, fish farming, leisure activities and low-flow period support. In the late 20th Century, there was a considerable increase in small reservoirs in France, reaching a total of 125,000 by the early 2000s. The creation of new water storage structures continues in parallel with the exploration of water use reduction solutions. These developments raise a whole host of environmental issues, such as the impact of reservoirs on the aquatic environment, particularly in areas where there are already a number of reservoirs and water resources are in high demand. By law, building a new reservoir requires a planning application or government authorization, which require an environmental impact study. Such studies must now assess the cumulative effects of the project together with other known similar projects. The “cumulative” aspect of the impact of water storage structures on a single catchment area is often poorly understood, probably due to a lack of relevant knowledge and methods. Consultants and government services therefore face a lack of operational tools for processing new reservoir applications, which gives rise to other problems around water management planning and the supervision of the development of new reservoirs. In this context, the French Ministry of the Environment, Energy and Marine Affairs (MEEM), supported by ONEMA, requested a joint scientific assessment (ESCo) from Irstea, in partnership with INRA, on the cumulative impact of reservoirs on the aquatic environment. It was produced by around fifteen experts from a range of disciplines and research organizations, and is based on analysis of a thousand or so international scientific articles and reports.