Dalloz Weir, River Grosdar, France

River Tacon is the main tributary of River Bienne in Jura Province in Eastern-central France (Figure 1). It’s a typical and representative example of freshwater watercourse in this karstic French region. River Tacon is of great ecological importance and since 1995 is included in the Natura2000 Network (site number: FR4301331). Several valuable fish species inhabit this river, such as Bullhead (Cottus gobio) and Brown trout (Salmo trutta). However, the populations of these fish species have been decreased dramatically during the last decades, mainly due to manmade infrastructure, like dams and other river barriers, and the industrial usage of freshwater. The River Bienne watershed is highly fragmented, since more than 300 river barriers have been identified by the Parc Naturel Régional du Haut-jura. All these barriers have been classified based on their degree of connectivity relative to the existing hydrological regime using a river connectivity index (ICE).

Dalloz Weir had also been considered as having a substantial negative impact on fish migration. Targeted studies suggested that this barrier was unsurpassable to more than 70% of the resident trout population. This result was based on the analysis of fish movements through Radio Frequency Identification (RFID) tracers aiming to evaluate the impact of this low-head dam on the trout population.

Barrier Removal Process

Parc Naturel Régional du Haut-jura follows a manifold approach in designing and implementing its projects. Their rationale is to solve many problems at once in a single area. The removal of the Dalloz Weir was part of the “Work together to restore our rivers” project that also incorporated solutions to change the industrial processes of Dalloz Créations and enhancement of the stability of the bridge over the barrier.

In 2019, Parc Naturel Régional du Haut-jura prepared the engineering design for the project, which was then presented to Dalloz Créations and the State. The implementation phases, including the barrier removal (Figures 4-6), the remodelling of the riverbanks (Figures 7-9) and the changes in the industrial processes of the company were successfully carried out from 2020 to 2021. The project costed 110.000 euro and was co-financed by the French Water Agency (70%) and the Region (10%).

The Dalloz Weir demolition reopened 3 km of the river stretch that are now accessible to all resident fish species, regardless of their size or the hydrological conditions (Figure 10). Additionally, the water has returned to its original course, and it’s no longer redirected to the artificial channel to serve industrial purposes. The unhindered passage of trout has already been documented through surveys using RFID tracers (Figure 11) and such monitoring will continue up to one year after the removal of the barrier. Dalloz Créations is also satisfied with the newly implemented processes and is now paying more attention to the river status, the water quality and the local biodiversity.