Eleven culvert removals, Belgium

Removing several culverts in Forêt d'Anlier to protect Belgium's last pearl mussel populations.


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. 

At a glance

Project typeDam removal: demolition
Project statusLive
RiverRulles and Anlier
Km to be opened28
Focal speciesFreshwater pearl mussel, brown trout, European sculpin, brook lamprey
Removal dateJune - October 2024

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.