MADRID – Spain, with 1,200 large dams, is one of the countries with the most dams per million inhabitants. But it also has many outdated, old dams and is working on the removal of disused reservoirs.
Reservoirs are important for, among other things, the drinking water supply, for irrigation and for generating energy. In addition, they also regulate the water level in the rivers. This reduces the risk of flooding and provides a water buffer in case of drought. However, reservoirs also cause problems. A reservoir can flood fertile land. Moreover, sometimes an entire village or villages have to move. Whole stretches of river have to be turned into artificial ecosystems. Some dams discharge the cold water from the bottom of the reservoir, often with a low oxygen content and many metals. That has an effect on life in the river.
Consequences for landscape and biodiversity
Ecoavant.com writes that dams have a major impact on the entire landscape of the river. Dams trap sediment that carries the river water with the current. As a result, sand and gravel banks with the accompanying vegetation disappear into rivers. In the Ebro Delta, an area of great economic and natural value on the Spanish east coast, the sea is receding the coast due to a shortage of sediments. Fish can no longer migrate upstream because they cannot pass the dams, while migration is important for their reproduction.
The signal for animals to start migrating
The artificial water management as a result of dams also impoverishes river life. A dam changes the size and frequency of high and low water. This is especially noticeable with reservoirs for the generation of electricity. The flow is strong at times when energy consumption is high and low at other times. Life in the rivers, from small microbes to fish, is used to a natural water balance and is very sensitive to changes. A change in the water balance is often the signal for animals to start migrating, for example.
Dams were not built for eternity and many are now obsolete. People across Europe are calling for the removal of disused dams. The Dam Removal Europe project gathers knowledge and experiences about the removal of dams and brings together initiatives of local groups. Based on data from official authorities, including the Spanish Ministry for Ecological Transition, a map has been made with dams removed in Europe.
There are many old dams in Spain. In the Basque province of Gipuzkoa on the Bay of Biscay, for example, most of the nearly 1,000 dams are outdated. You cannot just leave them, because without maintenance they pose a danger. They either need to be modernised or removed.
Spain is removing many disused small flood defenses or dams. Some examples of relatively large removed are the dams of Robledo de Chavela, Retuerta, Yecla de Yeltes, Gotera and Inturia.
Enobieta dam is one of the largest in Europe
The department of fluvial ecology at the University of the Basque Country studies the effect of human intervention on the ecosystem of rivers. The effect of the removal of the Enobieta dam, one of the largest reservoirs removed in Europe, is also being investigated. This 42 meter high dam is located in the valley of Artikutza (Navarra), a Natura 2000 area. The dam was built in the 1950s. Due to geotechnical problems, the design had to be adjusted and the dam became much smaller than originally planned. Twenty years later, the reservoir of Añarbe was built downstream in the same river, rendering the Enobieta dam useless.
The municipality of Donostia-San Sebastián started the removal of the reservoir in 2018. It took two years to drain the lake. Researchers studied the nature in the river valley at the time of the reservoir, during the years when it emptied and during the restoration of nature. The river quickly returns to its natural course, life in the river is increasing and forest vegetation is once again growing on land that was under water by the reservoir. Nature takes possession of the valley again.