Tens of thousands of dams block two thirds of the largest water courses on the planet.

Europe, the United States, China, Mexico, the Middle East, most of India, southern Africa, Australia, Southeast Asia and the southernmost portion of Latin America no longer have large rivers free of human constructions that stand in the way of your way to the sea A study of millions of kilometers of river has found how tens of thousands of reservoirs, dams, dams, roads … have quartered the course of the waters. The consequences go far beyond the fact that some salmon can not go upstream. And for some the demolition operation can not be postponed.

Bigest rivers in the world free of obstacles

A large group of researchers from a dozen countries and the World Wide Fund for Nature has analyzed the situation of the rivers of the planet, especially those of more than 500 kilometers. What they were looking for was their degree of connectivity, what and how many obstacles the water had every few kilometers (an average of 4.2 km per stretch) until reaching the sea. They have managed to map 12 million kilometers of river and the results, published in the journal Nature, are dramatic.

Of the 500 rivers of more than 500 kilometers analyzed, only 37% can be considered rivers free of obstacles (in English the expression free-flowing rivers is used). And of the hundred courses of more than 1,000 kilometers, only 21 connect their headwaters with the ocean without a significant human construction. Almost all are concentrated in the Arctic regions of America (such as the Yukon) and Asia (the Lena) or in the Congo Basin and the Amazon. The authors of the study have created a page to show their world map of rivers and their degree of connectivity.

Only in the arctic regions and the basins of the Congo and the Amazon do large free rivers remain

“The rivers of the world form an intricate network with vital connections to the earth, the subsoil and the atmosphere,” recalls the researcher at McGill University (Canada) and the study’s lead author, Günther Grill. “Rivers that flow freely are important for both humans and the environment, but economic development around the world is making them increasingly rare,” he adds.

Of the six elements that most damage rivers, dams and reservoirs are the main agent of disconnection in two thirds of them. According to the International Commission of Large Dams, there are about 60,000. But if all kinds of dams are included, even for a small reservoir, the figure jumps to 2.8 million. And here do not enter dams, channels for irrigation and other types of barriers. Another 20% of the connectivity loss is due to the regulation of water flow and 5% to sediment retention. The extraction of water, roads, urbanizations and other infrastructures in the riparian zones complete the threat picture.

“When you build a reservoir, you can store a large part of the river’s flow and release it when you need it,” McGill’s co-author of the study, Bernard Lehner, said in an e-mail. “But after the dam, the river could have a seasonal behavior, with little flow in the dry season and floods in the wet, now the reservoir can hold the water and release it arbitrarily (regulated) depending on what it is used for. For example, if it is a hydroelectric dam, the objective is probably always to produce the same amount of energy, so operators will always release the same amount of river flow, so there will be no dry and wet season, “he concludes.

The engineer of the Fish Migration Foundation Pao Fernández, not related to this study, summarizes why the rivers should run freely: “They allow the transport of nutrients and sediments downstream and to the sea, enrich the riverbanks and the areas that flood. sediments, water uses the energy it spent in transporting them to erode the soil.With reservoirs, you change the temperature of the water, warmer in a lower flow, and the circulation of oxygen.Not to mention the migratory fish. literally, the veins of our ecosystems.The difference is that our body reacts immediately to an obstacle, while the ecosystem will need centuries “.

In Europe there is a barrier for every kilometer of river and, at least, 30,000 abandoned dams

Fernández participates in the AMBER project, an initiative promoted by the European Union to inventory fluvial barriers. Although the inventory is not yet finished, the figures show a provisional result of one million barriers identified in European rivers. “There is a dam for every kilometer of river,” says the Spanish engineer. And at least 30,000 of the dams are abandoned.

In them the Dam Removal Europe initiative is putting the focus, which aims for rivers to recover their course and connectivity, starting with the demolition of disused dams. According to data provided by Fernández, there are already thousands of concrete removal projects: 251 in Spain, 450 in Finland, 1,600 in Sweden, 2,300 in France … But there is no general plan promoted by the administrations, which act mostly in emergency situations, as in the case of the Cenicientos dam or pushed by initiatives such as the Dam Removal Europe.

“We need dams, but in good conditions,” insists Fernandez. “We need them to obtain energy, to have water, but to be as efficient as possible, multipurpose and with a localized and limited impact”, recalls the engineer. Only then could the rivers recover their place and function.