Urrutienea Dam, France

Reconnecting the River Nivelle

The River Nivelle

The Nivelle is a small, cross-border coastal river that runs for 52 km and has a catchment area of 279 km2. It springs in Spain and crosses the boarder with France after 12 km, and eventually flows into the Bay of Biscay. The River Nivelle, classified as Natura 2000 (FR7200785) area, is home to numerous threatened species, including the Atlantic salmon (Salmo salar), the European eel (Anguilla anguilla), the Allis shad (Alosa alosa), the Twaite shad (Alosa fallax), three lamprey species, the freshwater pearl mussel (Margaritifera margaritifera), the endemic Pyrenean desman (Galemys pyrenaicus), the European otter (Lutra lutra) and the White-clawed crayfish (Austropotamobius pallipes). The Atlantic salmon population in particular is well monitored, since the River Nivelle is part of the European «Salmon Index Rivers» network. The River Nivelle used to be fragmented by 4 dams, associated with former mills. The largest of them was Urrutienea Dam and constituted an unsurpassable obstacle to all fish species. The other three barriers along the river are smaller and equipped with functioning fish passes and have minimum to none impact on fish migration.

Figure 1. The River Nivelle catchment area. The location of the Urrutienea Dam is also shown © FDAAPPMA 64

The Urrutienea Dam

The Urrutienea Dam was a 4.9-m tall and 35.3-m long barrier (Figure 2) associated with an old fish farm that has been abandoned for more than 10 years (Figure 3). The dam was preventing fishes from reaching the most upstream 12 km of River Nivelle and 8 km of one of its tributaries (the River Lapitxuri). For Atlantic salmon the dam constituted the upstream colonization limit, despite the fact that the river stretches above it are very favorable to salmonids in general. Multiyear monitoring showed that salmonid reproduction is much more effective on such upper reaches, where, for example, the density of juvenile trout is 2 to 5 times higher than downstream of the dam.

Figure 2. The Urrutienea Dam © Charlie Pichon

Figure 3. Ponds at the abandoned Darguy fish farm © FDAAPPMA 64

The Urrutienea Dam was also altering the natural sedimentation. Fine sediment was being entrapped behind the dam, causing significant sediment deficit downstream of it. Occasionally the riverbed was totally lacking fine sediment and only bedrock was visible (Figure 4). The retention of pebbles and gravels was detrimental for the freshwater pearl mussel, but also for all fish species. Water temperature had also been affected by the presence of Urrutienea Dam. During summer temperatures upstream of the dam were up to 2 °C higher than downstream of it.

Figure 4. Sediment deficit downstream of the Urrutienea Dam © Charlie Pichon

The dam’s previous owner, despite their obligations imposed by regulations, never complied with the law. In 2022, the entire site (former Darguy fish farm) along with the dam was up for sale. Fédération des Pyrénées-Atlantiques pour la Pêche et la Protection du Milieu Aquatique, with the help of the Fondation des pêcheurs, the AAPPMA de la Nivelle and the Agence de l’Eau Adour-Garonne, acquired the site (the former fish farm, the canal and the dam). In the same year, an initial grant from the Open Rivers Programme enabled technical feasibility studies to be carried out for the Urrutienea Dam. The final project aimed to:

• restore the alluvial mattress downstream the dam by freeing 6000 m3 of pebbles
• improve the quality of aquatic habitats downstream of the dam for all species (fish, insects, freshwater mussels, etc.)
• enable salmon and trout to access the best spawning areas in the catchment area, which were inaccessible due to the dam
• improve the quality of aquatic habitats upstream of the dam, which were clogged and homogeneous (Figure 5)
• eliminate the warming effect of the water upstream of the dam

Figure 5. Habitat upstream of the Urrutienea Dam before its removal © FDAAPPMA 64

Along with the restoration of the river connectivity, another ambitious project was taking place, the restoration of the freshwater pearl mussel population in River Nivelle. To do so, a conservation farm was launched in 2022 to strengthen the wild population.

Dam removal
The project was launched in 2021 by contacting the dam owner. Fish rescue was conducted prior to the onset of the in-river work (Figure 6). In July 2023, the initial phase of the removal took place, involving the mid-level threshold (2.5 m) levelling (Figure 7, Video 1). During the next couple of months, mild floods enabled the mobilization of sediments previously trapped behind the dam.

Figure 6. Fish rescue before the removal of Urrutienea Dam © FDAAPPMA 64

Figure 7. Mid-level threshold (2.5 m) levelling of Urrutienea Dam in July 2023 © FDAAPPMA 64

In October 2023, the remaining part of the dam was removed (Figures 8-10, Video 1), but the overall site restoration continues in 2024. The project was among the final three nominees for the Dam Removal Europe Award 2023.

Figure 8. Removal works of Urrutienea Dam © Charlie Pichon

Figure 9. The removal site at River Nivelle after the removal of Urrutienea Dam © Charlie Pichon

Figure 10. River Nivelle downstream of the removal site after the demolition of Urrutienea Dam © Charlie Pichon

The total budget for the project, including land purchase, preliminary studies, removal of the dam, dismantle of the former fish farm and monitoring over 4 years was 260000 €. Various partners contributed financially to the project, sometimes by financing a specific part of the project. The funding partners were: Open River Programme, Agence de l’Eau Adour-Garonne, Fondation des Pêcheurs, Fédération de pêche des Pyrénées-Atlantiques and AAPPMA de la Nivelle.

Video 1. The removal of Urrutienea Dam © FDAAPPMA 64

Challenges
As usually happens with such projects, the dam removal faced some opposition. It often stems from misinformation spread by mill preservationists or people wanting to develop the hydroelectric industry. The biggest challenge during the project was convincing the local community, since they wanted to develop a new hydroelectric plant on the site. The main advantage in the discussions was the fact that the local community was already engaged in a vast 5-year freshwater pearl mussel conservation program on River Nivelle. In that respect, the development of a hydroelectric facility on the site was incompatible with their commitment.

Ecological and community benefits

The main ecological benefit from the Urrutienea Dam removal will be the colonization of the recently opened river stretches by salmonids. Natural sedimentation processes are expected to be re-established and the temperature differences between upstream and downstream stretches of the river will be eliminated. Freshwater pearl mussel (which depends on salmonids to complete its life cycle) will be re-establish in the newly accessible parts of River Nivelle where it has no presence at the moment.

The benefits of the dam removal are already visible. Only two months after the works were completed the first salmon were spotted 6 km upstream of the removal site, in the Spanish section of River Nivelle for the first time in centuries. Even though it’s quite early to assess the reaction of the local community towards the project, the news of the return of the salmon were well received. Monitoring will continue for the next 5 years to record the precise benefits of the project. Actions to promote the village’s historical heritage are also in place.

Before & After photo pair

The removal site at River Nivelle: (left) before and (right) after the demolition of the Urrutienea Dam © Charlie Pichon

The River Nivelle downstream of the Urrutienea Dam: (left) before and (right) after its demolition © Charlie Pichon

With many thanks to Charlie Pichon for providing the information and the visuals presented herein.

Written by Foivos A. Mouchlianitis