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The semnon

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The Semnon Catchment

What is a catchment?

A catchment is all the land irrigated by the same river network and delimited by the watershed (or ridge line) : a stream or river with all its tributaries and all the watercourse which feed it.

These lands collect the precipitation which contributes to the volume of flow. Within the same catchment, all the waters follow, the geographic relief, a natural slope and are concentrated at the same point of exit called the outlet.
The catchment of the Semnon
The Semnon catchment is situated to the south-eat of Rennes and belongs to the catchment of the Vilaine. It extends over about 495 km², 2 regions (Brittany and  Pays de la Loire), 4 departments (Ille et Vilaine, Mayenne, Loire-Atlantique and Maine et Loire) and 37 communes (in part or in whole).
The Semnon is a tributary of the Vilaine. It takes its source in the commune of  Congrier in Mayenne and it discharges to the Vilaine at the junction of the communes of de Bourg des Comptes and Pléchâtel. It flows from east to west for approximately  73 km and includes 15 main tributaries, these represent in total (Semnon and very small watercourses included) a length of close to 440 km of watercourse.
Hydrographic map of the Semnon & the limits of the catchment
The Intercommunal Union of the Semnon  Catchment:
The Intercommunal Union of the Semnon Catchment was created by inter-prefectorial orders of Ille-et-Vilaine and Loire atlantique dated  26 March and 6 April 1979.
It has as its objective the restoration of the watercourses of the catchment thus to achieve or promote all actions necessary for the conservation, improvement and a better utilisation of hydraulic heritage.
At its creation in 1979, the Intercommunal Union of the Semnon Catchment comprised15 member communes, all on the banks of the Semnon :
-          Bain de Bretagne, La Bosse de Bretagne, Bourg des Comptes, Eancé, Ercé en Lamée, Lalleu, Martigné-Ferchaud, Pancé, Pléchâtel, Poligné, Teillay, Thourie, Tresboeuf in Ille et Vilaine ;
-          Fercé et Soulvache in Loire-Atlantique.
In 2008, with the perspective of leading a coherent and homogeneous global policy of water management on almost the whole catchment, the boundary of the  Intercommunal Union of the Semnon Catchment was extended to10 new communes of Ille et Vilaine : Janzé, Le Sel de Bretagne, Le Theil de Bretagne, Sainte Colombe, La Couyère, Retiers, Rannée, Chelun, Saulnières and Coesmes.
At present, the Intercommunal Union of the Semnon Catchment has a project for its extension to 12 other communes :
-          Forges la Forêt in Ille et Vilaine ;
-          Rougé, Noyal sur Brutz, Villepot, Ruffigné, Soudan in Loire-Atlantique ;
-          Senonnes, Saint Erblon, Congrier, La Rouaudière, Saint Aignan sur Röe in Mayenne ;
-          Pouancé in Maine et Loire.
Since its creation in1979, the maintenance of watercourse, the management of riparian forest, the renewal of works, … have been accomplished. In 2003, a preliminary study for a 1st Contract for Restoration and Maintenance of the Semnon itself and its 4 principal tributaries. The following 5 years (from 2004 to 2008) saw the putting in place of a programme of works established by the 1st contract.
Since 2008, the Intercommunal Union of the Semnon Catchment has been in a preliminary study phase for the preparation of a future contract for the catchment. To this end, different studies are under way :
-                     A preliminary study for a 2nd contract concerning all the tributaries not included in the initial contract (140 km of watercourses studied) : this study will permit the definition of the priority sectors of watercourses for intervention(planting works, diversification, bank reprofiling, elimination of livestock access to watercourses, removal of dumped refuse, …)
-                     An inventory of wet zones and watercourses: this inventory is being finalised. This will enable the identification, at communal level, of all the wet zones and watercourse with the goal of better preserving them.
-                     A study of the biological, physical and chemical quality of the water: this study will the identification at catchment level of the causes of the degradation of water quality and to propose in consultation with the different actors concerned (communes, communities of communes, farmers, sewage treatment unions, …) a programme of actions for the reconquest of water quality.
-                     A diagnostic study of wooded areas which will permit the identification of the zones where there is a priority need for the creation and restoration woodland. The woodland, like the wet zones, has a not inconsiderable role in the preservation of water quality.
In order to follow all these studies and to take in hand the future actions included in the catchment contract, the Intercommunal Union of the Semnon Catchment has recruited, since April 2009, an animator for the catchment.
Today, conforming with the European Directive on the water environment of 2000 and the French laws on water of 1964, 1992 et 2006 which impose, amongst others, a management of water by catchment and the achievement of a « good ecological state » of the water here in 2015, it is therefore urgent to embark upon the actions for the reconquest of water quality. That is why, the Intercommunal Union of the Semnon Catchment, with the signature of the future contract for the catchment expected to be in early, has the legitimacy to be the bearer of the project of actions for the reconquest of water quality and the aquatic environment in our territory.
The contract for the catchment, what is it?
The contract for the catchment is a contractual engagement between the master of works (here, the Intercommunal Union of the Semnon Catchment) and the different financiers (Water Agency of Loire-Bretagne, regions Brittany and Pays de Loire, departments) to reach the good ecological state of water in 2015. It permits the definition of a programme of measure and actions thanks to several operational tools such as : the contracts restoration-maintenance, the programme Breizh Bocage (restoration of the woodland), the agri-environmental measures,…
It is a document which plans the assignments and the work of the union on the territory of the Semnon for the 5 years to come. It is a programme multi-thematic which concerns also good purification to master the pollutions diffused from origins both urban and agricultural(pesticides, nitrates, phosphorus) and the restoration of the aquatic environment.
Contact :
For all questions, you can contact Sophie DUCHANGE, animator of the Intercommunal Union of the Semnon Catchment, at

Université du Temps Libre


Université du Temps Libre


Université du Temps Libre