The Yecla de Yeltes D﻿am in Spain is being demolished in the largest project of its kind ever in Europe.﻿Credit: Dam Removal Europe

The Yecla de Yeltes Dam in western Spain supplied drinking water to local communities for half a century, until newer projects rendered it obsolete. Its demolition this month is the biggest dam-removal project in the European Union so far — and is being hailed by ecologists as a milestone for river-restoration efforts in the continent.

Such efforts are ramping up in many European countries — although some, notably those in the Balkan Peninsula, are on a dam-building spree. An initiative has begun to take the first continent-wide census of all dams. And although dam removal is generally welcomed by most scientists, some call for more research into potential ill effects.

Hundreds of thousands of dams and weirs, most small and many no longer in use, fragment Europe’s rivers. The structures, some of them thousands of years old, have provided irrigation, energy and other benefits. But their presence also threatens the habitats of endemic fish and wildlife.

“Dams alter the natural characteristics of a river system,” says Jeroen van Herk, a project manager with Dam Removal Europe, a group that promotes river restoration in the continent. “Long stretches of rivers, which once flowed freely from source to outlet, become a series of pools, hindering migrating fish from reaching spawning grounds in the upper reaches.”

The Yecla de Yeltes is on the Huebra River, a 122-kilometre-long tributary of the Duero, which is one of the Iberian Peninsula’s main rivers. Ecologists suspect that the 22-metre-tall dam, built in 1958, is partly responsible for the observed decline of the small freshwater fish called the sarda (Achondrostoma salmantinum), along with that of other endemic species, including otters and black storks (Ciconia nigra), which were once abundant in the area. Scientists in Spain are set to monitor whether the animals come back after the dam is removed.

Across much of Europe, rivers unfettered by artificial barriers are exceedingly rare. However, over the past 20–25 years, at least 5,000 small dams, weirs and culverts have been removed from rivers in France, Sweden, Finland, Spain and the United Kingdom, according to Dam Removal Europe. (There are few reliable records from other countries in Europe.)

Dam removal gained momentum after the EU adopted the Water Framework Directive in 2000. This legislation requires member states to improve the ecological protection of rivers and lakes, but as yet, only about half of rivers in the EU meet its environmental objectives, says Wouter van de Bund, an aquatic ecologist with the European Commission’s Joint Research Centre (JRC) in Ispra, Italy.

In the United States, about 1,200 barriers have been dismantled in recent decades, with generally positive effects on local ecosystems, says Laura Wildman, a fisheries engineer with eco-consultants Princeton Hydro in South Glastonbury, Connecticut.

But restoration projects need to be monitored for negative effects, too, experts say. Decommissioning existing river barriers might mobilize toxic sediment, or affect buildings or bridges downstream. And existing dams could help prevent the spread of invasive species such as the North American signal crayfish (Pacifastacus leniusculu) or the Asian topmouth gudgeon (Pseudorasbora parva). There are also historic dams, such as the Roman-built, 22-metre-high Prosperina Dam near Mérida in Spain, which need to be preserved as cultural heritage.

Dams were built with little regard for the impacts they might have on ecosystems, says Carlos Garcia de Leaniz, an ecologist at Swansea University, UK, who coordinates the US$6.2-million, EU-funded Adaptive Management of Barriers in European Rivers (AMBER) project. “We must not make the same mistake when dams are being removed.”

In collaboration with the JRC, AMBER is managing an exercise to map the location of all registered dams and weirs in 38 countries across the continent, including some that aren’t EU members. The project database currently holds information on 230,000 river barriers in 13 EU countries. Drawing from nine case studies, the project also aims to develop tools to help water authorities assess the costs, benefits and damage potential of dam-removal projects.

Inventories are important for planners and policymakers to understand the scale of issues caused by river fragmentation, says Wildman.

A number of small dams in the Netherlands, Denmark and Spain are scheduled for removal later this year. And starting in 2019, French scientists plan to systematically monitor the impacts of a removal project even larger than Yecla de Yeltes: the demolition of two hydropower dams in the Sélune Valley in Normandy, one 35 metres tall and the other 15 metres.

But while old barriers are being removed, new dams are built elsewhere. Some 2,800 hydropower plants are currently being planned across the Balkans — a threat, says van de Bund, to many of the continent’s last untouched rivers.