‘Effect of Draining the Enobieta Reservoir (Navarre) on Biodiversity and River Ecosystem Function (DESEMBALSE)’

Aitor Larrañaga Arrizagalaga and Daniel von Schiller Calle, Universidad del País Vasco.

The largest dam to be decommissioned in Spain in the medium future will be dismantled in 2018: the Enobieta Reservoir. This project will analyze the environmental effects of this process, which will take place within an exceptionally well-preserved natural landscape and will focus attention on the high ecological impact caused by reservoirs.

Despite their clear usefulness to mankind, reservoirs cause serious environmental impacts: they transform the river habitat, they flood riverbanks, they hold back the sediment materials carried downstream by the river, they change water quality and prevent the free movement of fish, among other things. Given its geographic and climatic characteristics, Spain has one of the highest dam densities of any country worldwide and therefore faces a problem that is becoming increasingly more common around the world: dam decommissioning.

Many reservoirs built during the 20th Century are now being drained because they are no longer safe or useful due to material aging. However, decommissioning a dam is complicated, expensive and can be socially controversial. Furthermore, there is still a lack of sufficient information on the environmental effects of such processes.

The DESEMBALSE project will analyze these effects at the Enobieta Reservoir, which is scheduled for drainage at the end of the summer 2018. The project intends to dig a tunnel through the base of the dam to thus recover river connectivity without removing the wall, because that would involve moving an enormous volume of waste construction material. The project will study the situation before the process begins; its peak impact; and early recovery.

The reservoir sits in the Artikutza Valley, one of the best-preserved environments on the Cantabrian Coast (an Area of Special Conservation in the Natura 2000 Network) that contains woodlands home to enormous biodiversity. The region is expected to further improve after the Enobieta Reservoir has been drained.