What impact do retention basins have on the aquatic environment?

Globally, water storage provisions have increased significantly since the 1950s. These retention basins, which collect and store water, can be used in a variety of ways, both commercially and for leisure. In particular, they are used to secure water subsistence methods as well as crop yields by overcoming climate related hazards. Stored water can be used for irrigation, drinking water supply, provisions for low water, and firefighting. The number of small scale retention basins has increased; in 2000, there were around 125,000 in France.

Nevertheless, the creation of these basins has raised many questions, particularly regarding their impact on the aquatic environment. In fact, by storing and diverting water, retention basins change the natural distribution and flow paths for water and transported materials. This affects flow patterns and the transfer of sediment, nutrients and contaminants. It also modifies living conditions in aquatic environments and river continuity for organisms.


Click to enlarge image © Irstea

  • Cumulative impact: this includes any impact caused by all relevant basins across a catchment area, on both the new aquatic environments created by the retention basins and on rivers.

A unique study to clarify the debate

Since 2006, reforms on extractable volumes [1] have encouraged the reduction of water use across catchment areas. In some cases, the creation of new water storage or retention basin infrastructures is required by regional projects. Depending on their size, retention basins can't be built without a declaration or authorization. At the same time, reforms on impact studies (Grenelle 2 law of 2010) [2] have made it so that applications to build retention basins must take the cumulative effect of the projected structures into account. Some Water Management and Structure Plans (SDAGE) have already included measures to be able to evaluate the cumulative effect of existing retention basins in a catchment area.

However, faced with a lack of knowledge, tools and methods for carrying out this evaluation of the cumulative effects of retention basins, the Ministry for the Environment, Energy and the Sea has ordered a joint scientific survey from Irstea in partnership with Inra and Onema. The aim was to evaluate the cumulative impact of retention basins on the aquatic environment and suggest a methodological framework to standardize methods and improve the quality of investigation procedures.

The survey was in line with the European Water Framework Directive and the concept of "good ecological status" of bodies of water. It also supported the development of Water Management and Structure Schemes (SAGE), a planning tool used to provide balanced and sustainable management of water resources, in relation to policies for the creation of new retention basins.

The results of the study will be presented on May 19 in the auditorium of the Ministry for the Environment in Paris. Jean-Marc Bournigal, President of Irstea, will attend, along with François Houllier, CEO of INRA, and Paul Michelet, Director-General of Onema. The symposium will then travel to each of the 3 basins most concerned by the issue - Loire-Brittany, Adour-Garonne and Rhone-Mediterranean-Corsica - from September to December 2016 in order to expand the results with feedback from regional stakeholders. A methodological guide will then be produced for managers and consultants.

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For more information

[1] Law of December 30, 2006 on Water and Aquatic Environments. To reduce the structural deficit of water, the government wanted to implement quantitative management of water resources. This was to be based on a global approach to the various uses of the resource available in each catchment area in order to adapt usage.