Eleven culvert removals, Belgium

Introduction

The Forêt d’Anlier is one of Belgium’s largest forests. With some 7,000 hectares of deciduous forest on the borders of the communes of Habay, Léglise, Fauvillers and Martelange, it covers the dividing ridge between the Rhine and Meuse basins. Fully included in the Natura 2000 network, it is home to many species and habitats that are fragile on a European scale, if not global. The freshwater pearl mussel (Margaritifera margaritifera) is currently listed as “endangered” on IUCN’s world redlist (EU “critically endangered”). Streams flowing through the Forêt d’Anlier are home to over 90% of the remaining individuals in Belgium. Pearl mussels require excellent water quality, rivers with a natural flow, and healthy populations of brown trout (Salmo trutta fario) to carry out their life cycle, which includes a parasitic stage on a host fish. 

Project context and opportunity

Since 2002, restoration efforts are implemented to protect Pearl Mussels populations, leading to the gradual recovery of habitats and water quality, and the recent restoration of the mussels reproductive cycle. However, the density of individuals remains low, resulting in too little probability of contact between larvae and host fish. Recent fish surveys have shown that trout populations are not at optimal levels, which we have recently linked to significant fragmentation of the small tributaries used as spawning grounds. While actions taken in the past decades have specifically targeted water quality (wastewater treatment, reduction of sediment inputs, etc.) or riparian habitat (removal of coniferous trees, restoration of riparian strips and hay meadows, etc.), ecological continuity has so far not been addressed. In this respect, the project will make it possible to mitigate continuity issues that complements ongoing conservation actions for pearl mussels. 

Project aims

The project will directly address this continuity issue by completely removing 11 barriers and replacing them with new ecologically transparent bridges, opening up a cumulated 28 km of river of significant ecological importance. By restoring access to high-quality spawning ground, the project aims to increase abundance in brown trout, especially in young individuals (most susceptible to parasitism by pearl mussel glochidia). Fish diversity in the target streams is also expected to increase since complete barrier removal will allow poor-swimming protected species (European Sculpin Cottus gobio and brook Lamprey Lampetra planeri) living downstream to colonize new river sections.

Moreover, the project does not focus solely on fish migration but aims to achieve an overall free-flowing river objective. Other ecological issues will be addressed, like restoration of natural sediment transport, river bed self-adaptation process, and climate resilience through access to refuge areas with cooler water, therefore responding to the Water Framework Directive’s objectives of reaching and maintaining the “very good” ecological status on both target waterbodies. In the longer term, we hope to achieve the overall aim of the project to improve the conservation status of critically endangered freshwater pearl mussel populations. 

Project achievements

Led by Parc naturel Haute-Sûre Forêt d’Anlier, this project has achieved remarkable ecological, economic, and social outcomes, contributing to the restoration of the Rulles and Anlier waterbodies in Belgium. The removal of 11 impassable culverts and their replacement with ecologically transparent bridges has reopened 28 km of free-flowing rivers in the Rulles and Anlier waterbodies. Fish passage and sediment transport have been fully restored, as confirmed by post-removal ICE analysis and direct observations of species like Brown trout, Bullhead, Minnow, and Stone loach utilizing the new bridges. These outcomes support access to critical spawning areas and thermal refuges, laying a strong foundation for increased fish diversity and abundance over time.

Serving as a model for reproducible techniques in barrier removal, it influenced the Nature & Forests Administration, which has already initiated similar projects in other areas. Knowledge-sharing events, such as field visits and discussions with colleagues from other nature parks, demonstrated the project’s capacity to catalyse a broader movement toward river restoration across the region. The project also gained interest from stakeholders beyond Belgium, including a road manager from Luxembourg, who explored ways to replicate these efforts.

The initiative attracted significant media attention, resulting in interviews, television features, and radio broadcasts, amplifying awareness of the importance of river restoration.

The restoration efforts have enhanced the overall landscape quality and ecotourism appeal of the protected area. Encounters with local walkers, cyclists, and horse riders during the works highlighted the community’s positive reception of the project.

Finally, it demonstrated that river restoration can align with forestry operations. The new bridges (built outside the scope of the ORP-funded project) were designed to support logging trucks and forestry machinery, improving forest road quality and reducing risks of damage compared to the old culverts. The project was completed in November 2024.