According to a study, there is an obstacle every 1.4 kilometers in Europe’s rivers, which is not only a problem for fish /.
Hundreds of thousands of salmon once swam up the Rhine every year. Until overfishing and water contaminated with chemicals put an end to the salmon in the Rhine in the 20th century. In order for the migratory fish to recapture the river, millions of young salmon have now been released there – with moderate success. Only occasionally do the fish, which spend their lives mainly in the sea, now reach their spawning waters on the upper reaches of the Rhine and its tributaries. Because even if the water has become cleaner, a number of dams block their way upstream and prevent a stable salmon population from developing.
By cutting up flowing waters, human structures not only threaten fish stocks or have already led to their extinction. They also affect water quality and hold back large amounts of sediment. Scientists recently described the extent of this problem in Europe in an article in the journal Nature. According to this, there are at least 1.2 million barriers in the course of European rivers – on average one every 1.4 kilometers.
As part of the study, which was headed by the Center for Sustainable Aquatic Research at the University of Swansea in Wales, the team led by Barbara Belletti and Carlos Garcia de Leaniz walked more than 2,700 kilometers on 147 rivers and noted all structures perpendicular to the direction of flow. A good 60 percent of them were missing from the existing, in any case not comprehensive, overviews. They then extrapolated the data to the European river network. The number of barriers determined in this way is a conservative estimate, as it does not take smaller source flows into account, according to Wouter van de Bund from the Joint Research Center of the European Commission, one of the authors.
In addition to large dams, the 1.2 million obstacles mainly include low hydraulic structures such as weirs and sluices or those that reduce river bed erosion. More than two thirds of the transverse structures are less than two meters high. Even if viewed individually, these represent a less massive encroachment on rivers than huge hydroelectric power plants – according to the researchers, they all cause “death by a thousand cuts”.
“Even pipes that lead brooks under a street or a field path are an obstacle for many fish,” says Martin Pusch from the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB) in Berlin, who participated in the study. “They don’t dare to swim through there.”
In the hurdles of the rivers of Europe, Germany ranks third after Switzerland and the front runner Netherlands. Almost 225,000 obstacles interrupt the migration routes of salmon, eel and sea trout every 500 meters on average. Many of our rivers and streams have been dammed or straightened in the past two centuries. For example, for efficient cargo shipping and in the course of land consolidations in order to increase food production. “If a river is straightened and thus shortened, you have to pull in transverse structures because the flow speed and gradient increase,” explains Martin Pusch. Filled stones, for example, prevent a body of water from digging deeper and deeper into its bed and the banks from collapsing.
Unregulated rivers that follow their natural course, such as the Peene in Mecklenburg-Western Pomerania, can hardly be found in Germany. According to the Federal Environment Agency, the heavy construction of many bodies of water – in addition to the nutrient pollution from agriculture – is the main cause of their often poor ecological condition.
Structures that cut rivers are not only a major problem for migratory fish that commute between fresh and salt water, says Martin Pusch: “Fish always have to swim upstream to spawn because the current drives their larvae towards the mouth.” Barriers can collapse their stocks in the upper reaches. Downstream there is a risk that fish will be chopped in power plant turbines.
Another aspect is the transport of sediment: sand and gravel banks, for example, which are easily flowed through, play an important role in the self-cleaning of rivers. Pusch calls it the “liver of rivers”. When transverse structures hold back the supply of material, the sandbanks erode, as do estuaries or beaches. In addition, dammed water often leads to algae blooms, a lack of oxygen and the river silting up.
Built-up rivers that direct their water straight into the sea also ultimately weaken the resistance of the water supply to climate change. An original river with a comparatively high water level, on the other hand, ensures that the groundwater reserves are adequately filled in times of drought, such as the one Germany experienced in the recent past.
So there are many reasons to remove some of the hurdles. Especially those who no longer serve their purpose or cause particularly large environmental damage. This is also one of the goals of the new EU biodiversity strategy: by 2030, 25,000 kilometers of rivers in Europe should be able to flow freely again. The many small transverse structures that could be removed with reasonable effort offer potential for demolition.
If countries like Germany do not make rivers more accessible, they will still not be able to comply with the European Water Framework Directive (WFD). This prescribes an at least good chemical and ecological status for waters. Not even one in ten rivers currently meets these criteria.
This is also due to a type of energy generation that is generally considered sustainable: around 400 larger power plants in Germany generate more than 80 percent of electricity from hydropower. The remainder is accounted for by 7,300 small hydropower plants, which cover around 0.5 percent of the total electricity demand – in view of the muddy waters and countless fish that perish in turbines, albeit at a high price. With small hydropower plants, the costs for nature are disproportionate to their benefits, says Martin Pusch.
Scientists and environmentalists alike are concerned about plans to build new dams in regions of Europe that have not been built up so far. According to the current study, mostly free-flowing rivers can still be found in the Balkans as well as in parts of the Baltic States, Scandinavia and southern Europe. In the Balkans in particular, however, to the horror of environmentalists, thousands of new hydropower plants are to be built in the coming years. “Even building small dams can have serious consequences for the environment here,” warns Wouter van de Bund.
On other continents as well, both existing and planned hydropower plants threaten the biodiversity and ecological integrity of many bodies of water. Hundreds of new dams are to be built on the Congo, Mekong and Amazon rivers, whose fish feed millions of people. According to a study published in 2019, every second river worldwide is impaired in its permeability. Even more than three-quarters of the largest rivers on earth do not make it into the sea without a break.
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