Summer of Science

Field Trip Monday#1


The pictures are of Enobieta Reservoir (43º 12’ N, 1º 48’ W), in Artikutza Valley, Navarre, northern Spain. it is among the rainiest areas of the Iberian Peninsula, with over 3000 mm y-1. In the late 19th century, the coastal town of San Sebastian used the stream from this valley to supply drinking water to its population. In the early 20th century they had an epidemics of tiphoid fever, which killed several people and scared the Spanish royalty, who used to spend the summers in the cool coast of the Bay of Biscay. Because water treatment schemes were mostly unknown at that time, the municipality of San Sebastian decided to buy the entire valley, almost 4000 hectares of land, in 2019. They closed the main activities (iron mining, charcoal making, livestock, forest activities…), and decided to manage the entire valley for water. They concentrated all people (over 100 persons) in a single neighborhood, most of them were hired as municipal workers, new forests were planted, old ones were left by their own, and in general, the conservation status of the valley improved notably. 


Although a very humid place, the rainfall patterns can be quite erratic, what caused frequent periods of scarce water supply. Therefore, in the late 1940’s the municipality decided to build a reservoir. Initially planned as a 42 m-tall dam storing 2.5 Hm3 of water, when the building operations started, it was discovered that the place was geologically unstable. So they were forced to change the dam design and to create a smaller reservoir, of only 1.6  Hm3. Event his was problematic, as low oxygen concentrations in the hypolimnion caused metal concentration far above the legal thresholds. Therefore, they decided to build a larger dam further downstream: Añarbe reservoir (over 60 m tall, 44 Hm3). After the closure of the Añarbe reservoir in the late 1970’s, Enobieta became useless and its management was neglected. It was on the brink of dangerous overflowing during the 1983 floods, and thereafter it status deteriorated swiftly. For instance, the bottom floodgate, which was the main structure to control the water level in the reservoir, ceased to work sometimes in the 1990’s.


In 2016 the municipality of San Sebastian had to decide whether to spend a considerable amount of money in repairing Enobieta dam, or whether to decommission it, despite it being useless for water supply. They voted for decommissioning, as they considered it as the main black dot in an otherwise extraordinary valley. It will be the tallest dam decommissioned in Europe. It was decided not to dismantle the whole dam, as it would be expensive, noisy and cause lots of dust. Instead, the dam will be breached or a tunnel dug across its base -the final decision has still not been taken. The reservoir was totally emptied in June 2019 and that the small weir was taken down in October 2019.


So far, the dam has been emptied very slowly (it took over 1.5 years) to avoid the sediments being scoured away and impacting the Añarbe Reservoir, and a small weir, older than the dam, that emerged when it was emptied, has been taken down. There were some episodes of turbidity, but nothing extraordinary compared to other rivers in the region, and there was no impact associated, either to fish or to other organisms. Now the water is extremely clear, there is not trace of metal pollution, and the stream is recovering its ancient channel, which is also being colonized by algae, invertebrates, and even the Pyrenean desman, a highly endangered water mole. Below the dam, the stream was severely starved of sediments, but now things have changed. The emerged sediments are being colonized naturally by an increasing variety of plants, and the riparian forests start to recover. Finally, although most people, either locals or visitors, initially opposed the idea of removing the reservoir, now they seem to support the idea. 





Arturo Elosegi

Faculty of Science and Technology

University of the Basque Country

PO Box 644

48080 Bilbao, Spain

Office: +34 946015514

Cell phone: +34 664391189

Fax: +34 946013500

e-mail: arturo.elosegi@ehu.eus



Field site location:

Enobieta Reservoir (43º 12’ N, 1º 48’ W), Artikutza Valley, Navarre, northern Spain

Description: Photo time series of a large, decommissioned dam contributed by Arturo Elosegi.

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