Rivers are important habitats, connecting routes and transport routes. But in Europe, more than 1.2 million barriers block the course of rivers, a study has revealed. According to this, there is a dam, sluice or other barrier every 108 meters along European rivers. These barriers follow one another particularly closely in densely populated Central Europe. The rivers in Scandinavia, Scotland, Iceland and some remote alpine regions, on the other hand, are somewhat less obstructed. Overall, however, this makes Europe the region with the highest degree of river fragmentation worldwide.
Streams, rivers and streams are an important part of nature. Because they transport water, nutrients, sediments and other components of the global material cycle into the sea. Rivers are also an important habitat for fish and other aquatic animals and an important source of food for birds and other animal groups. “Rivers contain some of the most biodiverse ecosystems in the world, but also some of the most threatened,” explain Barbara Belletti from the Polytechnicum in Milan and her colleagues. But humans are also highly dependent on rivers: thousands of years ago they preferred to build their settlements on river banks, used the water to drive mills, or shipped goods on the waterways. “Rivers provide essential services to our society, but our use of this resource has almost always been linked to its fragmentation,” say the researchers. Over time, dams, weirs and other structures have increasingly obstructed the rivers.
More river barriers than anywhere else
Belletti and her team have now determined how far this fragmentation of rivers in Europe has progressed in the most comprehensive survey to date. To do this, they collected data on weirs, locks and dams in 36 European countries from 120 regional, national and global data collections. They compared this data with each other and classified the barriers into six categories. In addition, they checked many entries through on-site visits. In each of these, the researchers walked a 20-kilometer stretch of river and mapped all the barriers they could see in the water. During these excursions, the scientists recorded a total of 147 rivers and a length of 2715 kilometers, which corresponds to around 0.16 percent of the entire European river network. Based on all the data, they created a model that shows the fragmentation for the entire European river network of at least 1.65 million kilometers in length.
“This is the first comprehensive estimate of river fragmentation in Europe,” say the researchers. According to this, there are a total of around 1.2 million barriers in the European river network – not a single river on our continent still flows unhindered into the sea along its entire length. The median value for the barrier distances at around 108 meters. Overall, the density of barriers in European rivers is higher than anywhere else, as the scientists emphasize. “That makes Europe probably the most fragmented river network in the world,” state the scientists. Smaller barriers such as barrages and bed sills account for the largest share with almost 32 percent, followed by weirs with a good 30 percent, dams and sewers under streets with a good 17 percent. Dams, on the other hand, only make up a good nine percent, locks even only 1.8 percent.
Rivers in Central Europe are the most fragmented
The extent to which the rivers are fragmented, however, varies greatly within Europe: the highest density of weirs, dams and the like can be found in densely populated Central Europe. For example, the rivers in the Netherlands have an average of 19.4 barriers per river kilometer. In contrast, the rivers in remote, less populated regions such as Scandinavia, Scotland, Iceland or some alpine areas are significantly less built up. Some of the upper reaches of rivers in the Baltic or Southeastern Europe also still flow relatively unhindered. “What is worrying, however, is that these are the areas in which the construction of hydropower plants is often already planned,” say Belletti and her colleagues.
The results also show that previous mappings have greatly underestimated the true fragmentation of rivers in Europe. The new value is 61 percent higher than earlier. One possible reason for this: “For example, many weirs were built at the end of the 18th century, sometimes even much earlier,” said Belletti and her team. Therefore, most of them are not recorded. Another factor that plays a role is the fact that 68 percent of the river barriers are less than two meters high and sometimes hardly or not at all above the water surface. “In Switzerland, for example, the fragmentation is mainly caused by around 100,000 small thresholds in the river bed, which are intended to slow down the erosion-related cutting of the rivers into the subsoil,” the scientists explain. Such barriers and low weirs are easily overlooked. “We therefore call for better mapping of these small barriers in particular, because they are the most common and the main cause of fragmentation,” they say.
This survey is also important for the EU’s biodiversity strategy. Because according to this, 25,000 kilometers of European rivers should be made permeable and barrier-free again by 2030. “To achieve this, however, a paradigm shift in river restoration is necessary, which also takes into account the effects of the small barriers,” state Belletti and her team. “In addition, to reconnect the rivers, it will not be enough to remove old barriers if new barriers are being erected elsewhere at the same time.”
Source: Barbara Belletti (Polytechnicum, Milan) et al., Nature, doi: 10.1038 / s41586-020-3005-2