More than one million barriers fragment Europe’s rivers
European rivers are fragmented by at least 1.2 million barriers, which can significantly alter their flow and habitat provision, according to an ambitious new study published this week. The results, issued in Nature, suggest that Europe has some of the most fragmented river systems in the world, with an estimated 0.74 barriers per kilometre of river.
River fragmentation is not only the result of large dam and hydropower constructions. Instead, the research team, coordinated by the Horizon 2020 AMBER project, mapped thousands of smaller structures such as weirs, culverts, fords, sluices and ramps in rivers across the European river network.
These structures fragment river systems in different ways, often affecting their flow, course and links with wider floodplains. This variety in impacts extends to ecosystem processes, too: some structures may affect the movement of fish and insect species but barely impact nutrient and sediment flows, or vice versa. Just to add to this complex picture, these barrier impacts can often be highly variable in scale across time and space.
These are key issues for European river managers seeking to conserve and restore their catchments after decades of modifications for energy production, industry, agriculture, urban growth and flood protection. As the authors of the new paper outline, there is a shortfall in knowledge about where river barriers are located in European rivers, and the variable impacts they have on fluvial dynamics and aquatic biodiversity.
“The extent of river fragmentation in Europe is much higher than anyone had anticipated,” says Barbara Belletti, a river geomorphologist who led the study at Politecnico di Milano and is now at CNRS, the French National Centre for Scientific Research.
Belletti and colleagues carried out the mammoth task of mapping European river barriers in three steps. First, they compiled barrier records across the 1.65 million km long European river network from 120 local, regional and national databases. Second, they ‘ground truthed’ this data by walking around 2,700km of the river network in 26 countries during low-flow conditions. This allowed them to record the location and characteristics of river barriers, and to address errors or omissions in the existing data. They found that not one of the 147 rivers they surveyed in person was free of obstructions. Third, the research team extrapolated their data to estimate barrier densities in regions with poor and patchy datasets.
Their study maps around 630,000 river barriers across Europe – largely ramps, bed sills, weirs and culverts. As you might expect, the highest barrier densities are found in the heavily modified river systems of Central Europe, and the lowest densities in more remote and sparsely populated regions. This work is illustrated in the AMBER Barrier Atlas, the first comprehensive river barrier map of Europe.
“Over 100,000 of these barriers are obsolete and negatively impacting freshwater biodiversity and contributing to the poor ecological status of rivers. The AMBER Barrier Atlas provides the push we need to take action, and make the removal of obsolete barriers happen everywhere,” says Herman Wanningen, managing director of the World Fish Migration Foundation.
The research team state that there are still relatively unfragmented river systems to be found in the Balkans, the Baltic states, and parts of Scandinavia and southern Europe. However, many of these systems are threatened by proposed dam developments. The authors suggest that their results can inform the implementation of the EU Biodiversity Strategy, which aims to restore fragmented river systems across Europe.
“Many barriers are obsolete and removing them provides unprecedented opportunities for restoration,” says Carlos de Garcia de Leaniz, AMBER coordinator and Chair in Aquatic Biosciences at Swansea University. “Our results feed directly into the new EU Biodiversity Strategy and will help to reconnect at least 25,000 km of Europe’s rivers by 2030.”
The new AMBER barrier atlas is a highly valuable resource for environmental managers and policy makers in Europe seeking to underpin their conservation and restoration work with accurate data. An important next step may be to extend and join up this study’s approach to global river systems, particularly where datasets on river barriers are currently restricted.
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