Yesa Reservoir

The dam is equipped with a spillway system consisting of four shafts with independent tunnels of an average length of 600.5 m ending in a common damper for all of them. The four spillways are equipped with concrete gates with a floating sector of 20 m in span and 6 in height. They are located on the left side of the dam. The height of the fixed lip is 482,61 m and they allow outflows of:

• In normal regime, 2240,00 m³/s (560,00 m³/s each span).
• In flood regime, 3060,00 m³/s (765,00 m³/s each span).

• Intermediate: it is located on the intake of the central one foreseen at the foot of the dam at 448,40 m level. They are two conduits of 1 m of diameter. It uses two Howell-Bunger type valves with fixed cone. The capacity of drainage is 38,5 m³/s (19,25 m³/s each one of them).
• At the bottom: they are located at the foot of the dam at an elevation of 428.0 m on the left side, on what was the old bed of the Aragón river. They are two pipes of 1.50 x 2.00 m governed by valves. The total capacity of the outfall is 172 m³/s (86.0 m³/s each).

• Intake of the Bardenas canal: at 453.35 m elevation, located on the left side. It consists of three 2 m diameter pipes governed by ring-seal valves and Tainter type gates. The total normal flow is 90 m³/s (30 m³/s each).
• Intake for power plant at the foot of the dam: it is located at 448.4 m and consists of two pipes of 2.5 m diameter that are governed by wagon gates from the upper part, in the lower part they are closed by a blind flange. The dimensions of the gates are 2,5 x 2,5 m.
• Intake for the power plant next to the Bardenas intake: of the same type as the previous one, consisting of two 3 m diameter pipes. The dimensions of the gates are 3 x 5 m[6]

View of Tiermas over the Yesa reservoir.

The use of the Aragón river has been considered since the Middle Ages: as early as 1498 a proposal was made to the King of Aragón Ferdinand the Catholic to divert water from the river for use as irrigation by the Council of Tauste. In 1622 it was proposed to build a canal to Bardenas to irrigate the lands of an abbey that was planned to be built there. In 1702 the engineer Josef Estorguia and a group of hidalgo knights launched the proposal to divert the river so that it would run through Aragonese lands, thus avoiding the payment of tariffs and tolls to the Navarrese. 54 years later, in 1756, a project was drawn up for the construction of a canal that, rising in Tiermas, was to irrigate the Aragonese region of the Cinco Villas. Soon after, another project was presented, this time by Captain J. A. Monroy, who proposed a canal (which was born at a similar level to the current Bardenas) that would irrigate the region of Cinco Villas and Bardenas, with a total of 15 228 ha of irrigated land, as well as the construction of 18 new urban settlements.

In 1865 a company was created for the construction of an irrigation canal to go down to the Bardenas. The project was carried out by Antonio de Lesarri. The society would be in charge of the construction in exchange for the use of the water and the irrigators paying the corresponding alfardas.[note 1] In 1880 the concession was declared null and void, since the works had not begun.

In 1902 a Royal Decree of 25 April proposed a National Plan of Hydraulic Works in which a hydraulic complex appeared, consisting of the Bardenas de Yesa, Foz de Biniés, Salvatierra, and Usún canals, each of them with a reservoir. This complex would generate an irrigated area of 30 000 ha in several towns of Navarre and Zaragoza. The reservoirs would be located as follows: Yesa, where the current one is located; Foz de Biniés in the waters of the Veral river; Salvatierra, on the Ezka River, Usún would be located in the Salazar Valley, 6 km from Lumbier. Along with this proposal were also those of the Canal Imperial de Aragón and the Lodosa Canal, which made the need to dam the Aragón and its tributaries more evident.

On March 2, 1909, the Ebro Hydraulic Division was commissioned to study the project for the Yesa Dam. This project was carried out by the engineers Cornelio Arellano, first, and then Manuel Abascal, who completed it in 1912. The canal project was not approved until 1917. The dam designed was a 53-meter-high gravity dam.

In 1923 a new project was commissioned, drafted by the engineers Félix de los Ríos, Mariano Vicente, and Antonio Colom, who carried out the study based on an irrigable area of 130 000 ha, to contribute to the Ardisa dam with 400 000 million cubic meters per year and to supply drinking water to Zaragoza. This proposal was included in the linking of the three large rivers on the left side of the Ebro, the Aragón, the Gállego and the Cinca.

On May 7, 1926, a Royal Order approved the Yesa Dam project, making it compulsory to respect the uses and exploitations downstream of the dam. The same team that conceived the dam also planned the Bardenas canal, presenting their work on 29 December 1924. This would be approved eight years later, on June 30, 1932, by a Resolution of the General Directorate of Hydraulic Works. In 1926 it was estimated that Yesa reservoir would only be used for irrigating of the Bardenas and the Cinco Villas region, and its possible contribution to the Imperial Canal of Aragón was rejected. In the 1st General Plan of Works and Miscellaneous Works of the Ebro Hydrographic Confederation, in 1926, the construction of Yesa reservoir was proposed. This would have a capacity of 470 hm³; destined for the Bardenas canal and the irrigation of the farming area of the basin itself, some 15 000 ha, although the possibility of contributing to the Imperial canal was still being discussed so that this would free up resources for the Lodosa canal. The forecasts for the Bardenas canal, which, starting from Yesa, was to reach Ardisa, were for a dam outlet of 60 m³;/s, of which 39 would reach Ardisa.

In the II Works Plan of the Confederation, which was published the following year, the works were delayed as it was considered that the studies on the regime, flow and layout of the canal and the capacity of the reservoir had not been carried out properly. That same year the Agronomic Service of the Hydrographic Confederation prepared and published a study in which it foresaw the conversion of 80 000 ha to intensive irrigation and another 30 000 ha to semi-intensive irrigation. For this purpose, it estimated that 619 952 hm³;/year of water were needed, with incidence of use in the summer months.

On November 13, 1928, the foundation works for the dam, which had been put out to tender in the third Works and Works Plan, began. With respect to the Bardenas canal, it was decided to reconsider the route.

In 1931 the Yesa swamp project was carried out, which was a complement to the 1924 project. It defined the body of the dam and everything related to it, such as the outlets and intakes. By then, the problems that had arisen with the settlement of the right abutment and the doubts about the capacity of the inflows had already been solved. This project was approved on June 30, 1934. In 1933, the contract with the builders was terminated due to different problems. By that date the Bardenas Canal was already under construction.

After the Civil War, during which the works were halted, they were resumed with the construction of the variant of the race in 1940 and the excavation of the right slope. In 1945, under the direction of René Petit, two reforms of the project were carried out, the first one was approved by Ministerial Order on February 6 and the second and definitive one on September 15. The uses to be given to the Yesa water were definitively designated, which were the same as those foreseen in the Félix de los Ríos project, but the irrigable surface area was reduced to 110,000 hectares. The reservoir was inaugurated in 1959 and in 1960 came into service.

The closure of the gates of the Yesa dam caused the abandonment of several villages, Ruesta, Tiermas, and Escó, with an affected population of more than 1500 people and flooded the valley leaving 2400 ha of arable land under water, of which 1000 were of excellent quality. The 8528 ha that were expropriated in the affected municipalities were distributed among the municipalities of Sigüés, with 3752; Urriés, with 1498 ha, Undués de Lerda, with 1754 ha and Los Pintanos, with 1524 ha, the latter belonging to the Cinco Villas region. A collateral effect was the purchase by the Patrimonio Forestal del Estado of the valley of La Garcipollera, which leads to Castiello de Jaca, by means of a decree of forced repopulation[2][7] to plant laricio pine to prevent erosion and silting of the reservoir. This led to the abandonment of the villages of Larrosa, Villanovilla, Bescós de Garcipollera, Bergosa, Acín de Garcipollera, Cenarbe, and Yosa de Garcipollera.

Area of sulfurous water baths in the old spa of Tiermas.

The climatology of the area where the reservoir is located is mild, with more sunny days than in the Cuenca de Pamplona. It is worth mentioning the north wind (cierzo) which enters in gusty form from the mountains, reaching force 5, as well as the more frequent south wind. This wind characteristic makes it very favorable for sailing, especially with light boats and windsurfing.

The richness of fish in the waters of the Aragón river, with abundance of trout, carp, and American perch makes the banks of the reservoir a very popular place for fishing enthusiasts.

The location of the reservoir, at the foot of the Leire Range and the fact that its right bank is bordered by the N-240 road makes hiking and cycling activities highly recommended. The Camino de Santiago passes along its banks and nearby there are many places with relevant buildings such as the monastery of Leyre or the Castle of Xavier or the bridge of the Roncaleses of Roman origin and rebuilt in the 11th century.

Some of the villages on the banks of the Aragón river have been abandoned since the construction of the dam and the subsequent filling of the reservoir flooded the valley's farmland. In Tiermas, among the ruins, an alternative social experience has arisen as a result of the concern of a group of young people. The same happens in Ruesta, which has been ceded for exploitation to the trade union Confederación General del Trabajo de España.

Before the filling of the reservoir, there was a famous spa that took advantage of the sulfurous water springs there and that had been used since times before the Roman Empire conquest. The waters gush forth at a temperature of 42 °C. The installations, whose level is below the filling level of the reservoir, reappear during periods of time when the level of the reservoir is low. Many people take advantage of this circumstance to continue using these waters and the mud that is formed, which has therapeutic properties for the skin.

In the area of the reservoir, agricultural activity continues to be one of the pillars of the economy. The farms are, in general, of cereal monoculture (90%) which is complemented with leguminous crops and vegetable gardens dedicated almost entirely to their own consumption. The Confederación Hidrográfica del Ebro owns about 20% of the arable land, which it managed unilaterally until 1983, when it ceded these lands to the municipalities of Artieda, Mianos, and Sigüés for a period of ten years.

Livestock farming is focused on the sheep sector with approximately 5,000 head distributed in six flocks. The livestock is fed in the large areas of scrubland and forest that occupy almost 60% of the land. A very high part of the labor force employed is foreign, since the population of the area is highly aged and there has been a very high rate of management by state agencies, the Confederación Hidrográfica del Ebro and the ICONA, which has inhibited the initiative of the native settlers.

A new activity is that derived from reforestation, carried out to protect the land from erosion. In the forests that have been created, the exploitation of hunting preserves and for mushroom hunting is taking place.

In the regions of Cinco Villas and Bardenas, where the water from the Aragón river is transferred through the Bardenas Canal, it irrigates some 70,000 hectares. This irrigated agriculture is mainly cereal. Currently, it is possible to plant fruit trees and olive trees due to a change that took place in the ordinances in 2007.

Although an industrial use of the Yesa reservoir was foreseen for electricity production at the foot of the dam, the power plant has never been built. However, in the Bardenas Canal there are several mini-power plants, managed by the Bardenas Irrigation Community, which use the water transferred from the Aragón to generate electricity.