Icon: enlarge
Raccoon Mountain pumped storage power plant: Reservoir on the Tenessee River in the USA
Photo: Voith Hydro / dpa
The Selous Game Reserve in Africa is an animal paradise.
Large elephant populations live here, as well as rhinos and lions on around five percent of the entire national territory of Tanzania.
The park is also one of the last refuges for the endangered African wild dog.
Now largely untouched nature is to be sacrificed for a large dam project on the central Rufiji River.
An area twice the size of Lake Constance would be flooded.
River forests, savannahs and wetlands would disappear.
Dams are used for irrigation and more importantly: They deliver extremely climate-friendly electricity from hydropower - and thus contribute to the fight against climate change and the end of fossil fuels.
But they often contradict the natural cycles of water.
Nevertheless, more than 500 dams are currently planned in protected areas or are already under construction.
That corresponds to about a seventh of all current dam projects, researchers working with Michele Thieme from the environmental organization WWF recently reported in the journal »Conservation Letters«.
In addition to WWF, the study paid for the United States Department of State and Conservation International, another environmental organization.
The effects of such projects are diverse, explains Boris Lehmann from the Technical University of Darmstadt, who was not involved in the investigation.
Sediments are deposited in the reservoir because there is no current.
This "silts up" the ecologically important space at the bottom of the water and thus destroys the habitat for the aquatic fauna.
It is true that the sludge can be flushed into the downstream watercourse by flushing the reservoir.
But that also has negative consequences in terms of water ecology, says Lehmann.
Fish, for example, suffocate if the concentration of fine sediments is too high.
According to the study, there are currently more than 58,000 large dams worldwide - structures that are at least 15 meters high.
Large dams are also those of five meters or more with a capacity of three million cubic meters.
Almost 24,000 of these are in China and around 370 in Germany.
Only a third of all rivers that are more than 1,000 kilometers long still flow unhindered from the source to the mouth.
According to the European Environment Agency (EEA) from 2018, there are hundreds of thousands of obstacles such as barrages, weirs or dams in European rivers.
According to the Federal Environment Agency, Germany alone had around 200,000 such transverse structures in 2015.
Dams would have the most significant influence on the migratory movements of certain fish species.
When river fish stand in front of a wall
The fact that fish can move freely in their habitat is important for another reason.
It is a way for them to adapt to climate change.
Because global warming affects temperature and water quality, says Christian Wolter from the Berlin Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB).
However, river fish cannot develop new habitats if they are hindered by dams.
Dams change the dynamics of rivers not only in the direction of flow, but also in exchange with the floodplain, as Christiane Zarfl from the Center for Applied Geosciences at the University of Tübingen explains.
And because the dams also hold back the supply of sediments into the valley, delta regions could sink in the long term.
This is particularly problematic in view of rising sea levels due to climate change.
"Intact river systems fulfill many functions for an ecosystem," says Zarfl.
"They are a means of transport, living space or provide food."
Hydropower farmers oppose this
Ricarda Bohn from the Voith Hydro Group, one of the world's leading suppliers of hydropower plants, refers to sustainability assessments in order to make hydropower plants and their construction as environmentally friendly as possible.
"They are the basis for assessing whether a project can, for example, be built in an environmentally friendly way and whether it is even approved."
But especially when it comes to power generation, the economic benefit is often questionable for some experts.
In Germany, around 85 percent of electricity is generated from hydropower by just 146 plants.
However, there are a total of 7,700 hydropower plants.
"That means: the 7,500 small plants produce a total of only 15 percent of all electricity from hydropower," says IGB expert Christian Wolter.
Electricity from hydropower has a share of just three percent in the German electricity mix.
Does that justify such a large number of assets?
The plant manufacturers at Voith Hydro are pointing out climate change: Even smaller hydropower plants save CO₂.
In addition, there are very high ecological requirements in Germany, such as minimum water flow, continuity of water or fish protection.
"In hydropower projects, the aim is to keep an eye on sustainability as a whole, that is, to balance the economic, social and ecological aspects," says Bohn.
Opponents of hydropower, however, rely on a study from 2019. In it, scientists found that the same amount of electricity that around 2,600 dams in the USA produce could be achieved using photovoltaics on 13 percent of the area of these reservoirs.
After all, not all dam projects are implemented in protected areas.
In Brazil, the environmental authorities refused to grant an environmental license to the construction of the “São Luiz do Tapajós” project on the Tapajós River in the state of Pará in 2016.
A mega dam over 7.6 kilometers in length was planned.
The planned hydropower plant should deliver more than 8,000 megawatts of power, which corresponds to about six nuclear power plants.
And researchers and environmentalists recently celebrated a partial success in Albania too.
A planned dam to tame the Vjosa, one of the last wild rivers in Europe, will probably not be built.
Icon: The mirror
joe / dpa
