Pont de Rhodes Weir, France

Introduction

The weir at Pont de Rhodes mill is situated on the Ressègue, a tributary of the Escalmels stream, which in turn feeds into the Cère River. This location falls within the Natura 2000 special conservation area FR8302033: ‘Affluents de la Cère en Chataigneraie’, notable for its substantial population of pearl mussels and recent evidence of their reproduction. The area is affected by agricultural activities, particularly livestock farming, impacting the water and environmental quality. Several dams and weirs, remnants of former mills like the one at Pont de Rhodes, present significant local challenges.

Project context and opportunity

The Ressègue River falls under List 2 of Article L214-17 of the French Environment Code, requiring owners of such structures to adhere to ecological continuity regulations. Given its importance for the reproduction and habitat of pearl mussels, removing the structure on this river is deemed a priority by the expert unit of the Auvergne-Rhône-Alpes National Pearl Mussel Action Plan. This action would yield significant ecological benefits for the river and potentially connect two pearl mussel populations. The Open Rivers programme has provided financial support to enable owners to pursue this solution and ensure compliance with French legislation. A pre-demolition grant for this demolition was completed in 2023.

Project aims

The project aims to restore the ecological and hydromorphological continuity of the Ressègue river while enhancing the population of pearl mussels and maintaining the fario trout population, crucial for mussel reproduction. Precision in relocating mussel individuals, alongside weir removal and habitat diversification, will be key in achieving these goals.

Project achievements

The demolition of the Pont de Rhodes dam was successfully completed in September 2024. Prior to the work, a naturalist was called in to capture and relocate the pearl mussels present on the site, with a view to preserving them. Electric fishing was then used to drain the riverbed and create a diversion channel. The demolition of the weir took three days. After this phase, the diversion channel was emptied, revealing the presence of pearl mussels, which were immediately recovered. The cofferdams were then removed and the channel gradually filled in. The materials from the structure were reused on site to revitalize the flow of the watercourse.

This project has provided a better understanding of the Natura 2000 site and enriched data on pearl mussels. Fish populations will be able to navigate more freely in the watercourse, reducing the isolation of fish and mussels. The works also helped create habitats and diversify the river flow on the former section of the weir. The entire project was carried out in close coordination with the partners.