Partner NRP : Haute Vallée de Chevreuse

INRAE departments and units : AQUA-Hycar

The biodiversity of the Haute Vallée de Chevreuse Park is largely due to its wet valley bottoms. However, the 330 kilometres of watercourses that drain the area are impeded by more than 200 dams, both artificial and natural, breaking their ecological continuity.

The park and INRAE are working together to re-establish this continuity and, in particular, to encourage the movement of fario trout, a migratory species of heritage and fishing interest. The study focused on restoring ecological continuity at two mills, one on the Mérantaise (Ors mill) and the other on the Aulne (Béchereau mill), chosen from among the 31 structures prioritised by the RNP.

The hydro-ecologists have committed to a ten-year monitoring programme, before and after the restoration work.

Between 2011 and 2022, before the works, they monitored the hydromorphological evolution of the two watercourses, as well as the presence of invertebrates and fish upstream and downstream of the structures. INRAE also recorded the passage of trout equipped with electronic chips using fixed antennae placed at the mills. It was found that the presence of mills reduces trout movements and their access to potential spawning areas.

As a result, work was carried out on the beds of the Mérantaise and Aulne rivers in 2022 and 2023. This involved creating a new riverbed at the bottom of the valley at the Béchereau mill site and a bypass around the Ors mill weir. Monitoring is continuing to assess the effectiveness of these works.