Reservoirs are part of the everyday landscape of Spain, although they often go unnoticed until a... prolonged drought or a spell of heavy rain And they're making headlines again. Behind every sheet of water held back by a dam lies engineering, water resource management, safety, and also a great deal of history, from Roman constructions to major 20th-century projects.
In this tour we will see in detail How are reservoirs distributed in Spain, which are the largest, what are their uses, and what problems do they face? and how to check their levels in real time. We'll also review some basic concepts of water geography and interesting facts about iconic dams, always using accessible language designed to be understood by anyone, from those preparing for a geography exam to those who simply want to know what's happening with their tap water.
What is a reservoir and how does it differ from a swamp, a tank or a pond?
In everyday language, we often use words like reservoir, swamp, or artificial lake interchangeably, but in geography and engineering there are nuances. Simply put, a A reservoir is an accumulation of water retained by a dam built in the course of a river. or stream, closing the valley totally or partially to store that resource.
According to the definition of the National Geographic Institute, a reservoir is a “accumulation of water produced by a construction in the bed of a watercourse”This always involves a man-made structure (the dam) and a preceding river or ravine. The water is impounded upstream of the dam, forming an artificial lake whose size varies depending on the dam's capacity, the topography, and the volume of water stored.
The term is also used a lot in Spain. swampTechnically, swamp is associated with a depression where water naturally stagnates, with muddy bottoms and shallow depthHowever, in popular language, the term "swamps" is used interchangeably to refer to artificial reservoirs, hence the frequent confusion.
Below the reservoirs, in size and function, we find the reservoirs and pondsWater reservoirs typically store enough drinking water for one or two days of consumption for a population. They serve to regulate the operation of pumps and prevent water from flowing directly from the source into the pipe network.
Elevated reservoirs are the most characteristic: they are built in flat areas to raise the water and give pressure to the networkFor every 10 meters of height, approximately 1 atmosphere of pressure is gained, and a typical urban network needs between 2 and 3 atmospheres to function normally. That's why it's not uncommon to see water tanks around twenty meters high in towns and cities.
The rafts, for their part, are open-air water reservoirs, usually small or medium-sizedThese tanks are primarily used to regulate agricultural irrigation. They typically have capacities that rarely exceed 1 cubic hectometer (hm³) and allow for the management of irrigation schedules or the storage of water from transfers or wells.
Total water storage capacity in Spain and distribution by territory
Spain is one of the European countries with the highest number of dams and reservoirs. Throughout the country there are more than 350 large-capacity reservoirscapable of storing around 54.000-56.000 cubic hectometers of water (hm³), which is approximately equivalent to 50% of the river flow that flows through Spanish rivers.
This enormous storage capacity is not a whim. In a country with a very irregular Mediterranean climateWith alternating wet and dry years, reservoirs are the great "water store" that allows the population to be supplied, crops to be irrigated, industry to be sustained, hydroelectric power to be generated and the peaks of floods to be reduced.
Looking at the territorial distribution, the community with the greatest capacity to store water in Spain is Estremadura, with approximately 14.225 hm³. It is closely followed by Andalucía, with approximately 11.385 hm³, and Castilla y Leonwhich has a capacity of around 8.305 hm³. At the opposite extreme, the community with the smallest reservoir capacity is La Rioja, which barely reaches 136 hm³.
At the provincial level, it stands out especially Badajoz, which is the province with the most stored water in Spain, with around 6.939 hm³. It is followed by Cáceres, with about 5.890 hm³, and Córdoba, which exceeds 3.200 hm³. In contrast, Valladolid It barely accumulates 6 hm³ of its own capacity, which shows a huge geographical inequality in terms of storage infrastructure.
Overall, it is a system that It covers about half of the river flows and which has allowed the development of large irrigated areas and guaranteed drinking water in populations that, without these reservoirs, would suffer frequent cuts during drought episodes.
Main uses of reservoirs: water, energy and flood control
Reservoirs serve several functions simultaneously, which are often combined within a single system. The most obvious use is the supply of drinking water to cities and townsMany of the large reservoirs are connected to distribution networks that supply drinking water to millions of people.
Another essential use is the agricultural irrigationModern irrigation systems depend on a relatively stable water supply, which is only possible when the natural flow of rivers is regulated by dams and canals. In many inland and southern areas of the Iberian Peninsula, intensive agriculture would not exist without this network of reservoirs.
Thirdly, numerous reservoirs are exploited for generate hydroelectric powerThe drop in water level between the reservoir and the power plant allows turbines to be driven, producing renewable electricity. In some cases, several dams in a row on the same river form veritable hydroelectric "steps."
We must not forget the function of flood defenseMany reservoirs are designed to regulate floods: when a sudden rise in water level occurs, the dam retains most of the peak flow and releases it gradually. In this way, the risk of downstream overflows is reduced, protecting populations, infrastructure and cultivated fields.
Finally, some reservoirs play a decisive role in the nuclear power plant coolingFor example, the reservoirs that cool the Almaraz nuclear power plant on the Tagus River act as large water "lungs" that absorb heat from the cooling circuit. In contrast, Vandellós II is an exception in Spain, as it is cooled directly with seawater and not by an internal reservoir.
How to check the current level of reservoirs in Spain
In recent years, checking reservoir levels has become very popular, especially during droughts. If you want to know What percentage of water does each reservoir or each watershed have?There are several official sources and specialized portals where you can see the information quite clearly.
A basic reference is Datos.gob.esThe Spanish government's open data portal publishes, among many other datasets, those relating to national reservoirs. It is a very comprehensive and rigorous source, designed primarily for users with some data handling skills or technical interest, so it may be somewhat dense if you only want a quick overview.
The website is also very useful. RTVE, which periodically offers reports with Interactive maps of reservoir levelsThese include charts showing trends and comparisons with the average of recent years. They are usually visual and easy to read, ideal for understanding the overall situation at a glance.
Another well-known portal is Reservoirs.netThis website specializes in collecting and displaying the status of Spanish reservoirs. It presents data by river basin, by autonomous community, and by individual reservoir, including filling percentages, volumes in hm³ and evolution graphsIt is one of the clearest options for the average user.
The site iAgua.es It also offers aggregated information on the state of reservoirs in Spain, albeit in a more general way. It usually focuses on analysis and news from the water sector, including summaries of the water situation and available reserves.
Step by step: practical example with Embalses.net
Although there are several useful pages, one of the most intuitive for the general public is Reservoirs.netUpon entering their homepage, you will see, in the center, a large summary with the total water stored in reservoirs in Spain, expressed both in hm³ and as a percentage of the maximum capacity.
On the left side there is a panel with menus to expand the information, but you only need to scroll down a little to find a Map of Spain by autonomous communitiesEach community is colored according to the percentage of water it has stored at that moment, so you get a quick overview: darker shades indicate fuller reservoirs, lighter shades indicate relative scarcity.
If you click on the autonomous community you are interested in, a page will open with the detailed data from that region: percentage of water stored, comparison with the previous week, with the average of the last 10 years, and even with the value of last year on these same dates.
From there you can go down a little further and you'll find a list by provinces within that region. Clicking on a province opens a new page with a specific map showing the different reservoirs and rain gauges in the area. Reservoirs are represented by droplet-shaped icons.
Clicking on the drop of a specific reservoir opens a card with the exact fill percentage, water volume, and trend graphs over the last few monthsBy repeating this process with different provinces or reservoirs, you can get a very accurate idea of the water situation in any corner of the country.
Recent situation of the water reserve: increases and contrasts
When several rainstorms occur, or when there's a rainy spring, the effect on reservoirs is spectacular. There have been cases where, in just three weeks of continuous rainfallThe water reserve has experienced an almost “miraculous” recovery.
In one of these recent episodes, the overall level of reservoirs in Spain was around 71,2% of its total capacityAfter gaining 5,4 percentage points in just seven days. If we look only at reservoirs for consumptive use (those intended for human consumption and agriculture), the reserve reached approximately 66,6% of its capacity.
That data was especially significant because it was located 12 points above the average of the last decadewhich was around 54,4%. If compared with the same period of the previous year, the difference was even more striking: the consumptive reservoirs were then around 46,5%, that is, about 20 points less.
In the midst of a period of abundant rainfall, around the 68% of the 374 reservoirs on the peninsula They showed levels higher than the average of the last ten years. In the case of reservoirs dedicated to human supply (some 281 reservoirs), the percentage that was above the historical average rose to 71%.
This situation allowed for the easing of restrictions in several basins. For example, the Segura Hydrographic Confederation It decided to significantly reduce water cuts for irrigation after verifying that the volume of water stored in reservoirs had increased by about 100 hm³. The limitations for traditional irrigation went from around 40% to just over 20%, and in non-traditional irrigation they were reduced from around 57% to about 33%.
Specific examples: from Entrepeñas and Buendía to Seville and Iznájar
Among the reservoirs that attract the most attention in these situations, the reservoir of EntrepenasIn Guadalajara. During one of these periods of abundant rainfall, it stored approximately 691 hm³ out of a possible maximum of 813, meaning it was 45 points above its average level of the last decadeA radical change from the years of drought, when its image was much more worrying.
The level at Entrepeñas reached such a high point that, for the first time in almost 28 years in the MakingThe canal connecting it to the reservoir was opened Good daywhich is usually more in need of supplies. The transfer between the two was around 3,5 hm³ of water per day, that is, more than 100 hm³ per month, which helped to balance the situation between the two large reservoirs of the system known as “Sea of Castile”.
Another striking example occurred in the western half of Andalusiaespecially in the reservoirs that supply Seville. Since mid-February, these reservoirs have accumulated some 623,6 hm³ of water, a volume equivalent to consumption of the entire metropolitan area during a full year, according to the public company Emasesa.
Thanks to those rains, the reservoirs that supply Seville and the eleven municipalities connected to its network reached close to 97% of its maximum capacityThe company estimated that, with that water, the supply could be guaranteed for approximately six years, a scenario radically different from the previous year, when at that time the stored volume barely reached 48%.
However, the situation is not uniform throughout the Guadalquivir basin. The reservoir of IznajarThe largest reservoir in Andalusia and the sixth largest in Spain in terms of water consumption, was at around 28,2% of its capacity, about six points above the previous year but still about 16 points below its average for the last decadeThe Segura basin also showed contrasts, with reservoirs like Cenajo below the levels that would be expected in an average year.
Reservoirs with water deficits and particularly vulnerable areas
Despite occasional improvements, even during rainy periods there are usually more than a hundred reservoirs left in deficit situationA recent count revealed more than 100 reservoirs below the historical average, of which 81 were primarily used for drinking water.
The areas with the most problems tend to be concentrated in the eastern half of Andalusia (including the upper Guadalquivir and Segura basins) and in the internal basins of CataloniaIn this last region, although some reservoirs have improved compared to the previous year, they are still far from the average levels of the decade.
An illustrative case is the swamp of Your questThe largest of the Catalan internal basins, with a capacity of around 233 hm³, at one point reached approximately 42,9% of its capacity, double the amount of water it held at the same time the previous year, but it still remained... more than 24 points below its historical average of the last ten years.
These inequalities highlight that, although the national balance may be positive, Water management is very uneven depending on the watershed and the climatic characteristics of each area. That's why it's so important to have monitoring tools and policies for saving and efficiency, even when the maps appear "green" in most of the country.
The ten largest reservoirs in Spain
If we focus on the giants of the system, Spain has a handful of reservoirs that stand out above the rest. They are veritable inland seas that, due to their size and capacity, play a strategic role in water and energy management.
The reservoir of La SerenaThe [unclear] in the province of Badajoz (Extremadura) tops the list. It entered service in 1989 and can store around 3.220 hm³ of waterThis makes it not only the largest reservoir in Spain but also one of the largest in Western Europe. To give you an idea, that volume could supply the entire region of Extremadura for more than 30 years.
Very close in capacity is the reservoir of AlcantaraAlso in Extremadura, but in the province of Cáceres. It was completed in 1969 and has a capacity of approximately 3.162 hm³At the time of its construction, it was the second largest water reservoir in Europe, and it is still a key element in the regulation of the Tagus River and in hydroelectric production.
The third great colossus is the reservoir of Almondsin Salamanca. In addition to its enormous capacity, which is around 2.648 hm³It is notable because its dam is the tallest in Spain, at approximately 202 meters high. The reservoir covers about 8.650 hectares, including the natural course of the Tormes River and extensive flooded areas.
Following these, we find the reservoir of Good day, in Guadalajara, whose dam was inaugurated in 1958. It can store around 1.638 hm³ of water and, together with Entrepeñas, Bolarque, Zorita and Almoguera, it forms part of the system popularly known as "Sea of Castile", fundamental for the Tajo-Segura water transfer and for local uses.
The fifth position is usually occupied by the reservoir of Mequinenza, in the province of Zaragoza, also known as “Sea of Aragon”Built in 1966, its volume is around 1.530 hm³, and it has also become an important destination for sport fishing and domestic tourism.
Behind them come other big names: the reservoir of cijara, on the border between Cáceres and Badajoz, with around 1.505 hm³; the reservoir of Valdecanas, in Cáceres, with an area of approximately 7.300 hectares and a capacity of 1.446 hm³; Ricobayo, in Zamora, completed in 1935 with approximately 1.200 hm³; and AlarcónIn Cuenca, which is around 1.112 hm³ and has at its tail the canal where the waters of the Tagus-Segura mix with those of the Júcar.
The reservoir closes this list IznajarThe reservoir in Córdoba, completed in 1969 and with a capacity of approximately 981 hm³, holds the title of the largest reservoir in Andalusia. Beyond the list, the collection of these large reservoirs illustrates to what extent has the Spanish geography been transformed by the construction of dams in the last century.
Reservoirs by region: Extremadura, Meseta, Andalusia and northern peninsula
If we look at some specific territories, the map of reservoirs in Spain becomes even more interesting. EstremaduraFor example, reservoir water is a central element of the economy. In addition to La Serena and Cijara, the region has the aforementioned [dam/reservoir]. Valdecanas, one of the largest reservoirs in the Tagus basin with its 7.300 hectares of water surface, used for irrigation, electricity production and recreational uses.
In the Meseta region, in addition to Buendía, other notable reservoirs include the one at Alarcón, located in the province of Cuenca. Its capacity is around 1.112 hm³, and at its furthest end from the dam is the point where the Tagus-Segura water transfer canal It mixes its waters with the flow of the Júcar River, a key node in inter-basin water management.
Continuing along the Meseta, we find the reservoir of NavacerradaLocated in the Community of Madrid, at an altitude of approximately 1.200 meters and very close to the well-known ski resort of the same name. Although not one of the largest in volume, it is of great importance to the supply of the mountains and part of the capitalas well as being a place frequented by hikers and nature lovers.
In the northern part of the Iberian Peninsula, the reservoir of Ullíbarri-GamboaThe reservoir in Álava is one of the most emblematic. It supplies water to both Vitoria-Gasteiz and, in part, the Bilbao area. Furthermore, it has become a very popular leisure area, with inland beaches in Landa and Garaio and ample natural spaces for swimming and water sports.
If we look northeast, in Catalonia, the embassament d'Ulldecona (on the Sénia River) is another interesting example. Its dam owes its name to the fact that its construction was financed by farmers from the town of Ulldecona, and its surroundings have become a very peaceful place, with old settlements such as the abandoned village of Mangraner on its northwest bank.
Andalusian reservoirs and unique landscapes
Andalusia has a long list of reservoirs that, in addition to their hydrological function, have been transformed into first-rate landscape spacesMany of them have been revalued as destinations for domestic tourism, water sports and outdoor activities.
One of the most striking is the NegratínLocated in the province of Granada, within the municipality of Freila, this lake boasts a striking blue surface that stands out against a semi-desert landscape of gullies and badlands, making it a highly photogenic spot. Visitors can enjoy canoeing, sailing in small boats, or simply relaxing at its viewpoints and designated swimming areas.
In the province of Malaga, the reservoir of La Viñuela It was created in the mid-80s after the Guaro, Vélez, and Seco rivers were dammed. It is a key point for the water supply and irrigation of the Axarquía region. Furthermore, it has become popular as a place for cool off in summer and practice water sportsCarp were introduced into its waters to help keep aquatic weeds under control, an example of biological management that is debated but common in these types of environments.
the already mentioned Iznájar reservoirStraddling Córdoba, Málaga, and Granada, though administratively within the province of Córdoba, it is the largest in Andalusia. Its size and location on the Genil River make it a key component of the Guadalquivir basin, in addition to offering inland beaches and recreational areas very crowded in summer.
In the province of Zamora, although outside the Andalusian region, the reservoir of Ricobayo It offers another example of a historic reservoir. Completed in 1935, it can store about 1.200 hm³, and its hydroelectric exploitation marked the energy development of the region for much of the 20th century, with a landscape profoundly transformed by the water's surface.
On the other hand, the little one Cervera reservoirWith a capacity of just 10 million m³ (0,01 hm³), it is often cited as the smallest on the list of Spanish reservoirs. This is a stark contrast to giants like La Serena or Alcántara, and demonstrates that Not all reservoirs are intended to be large-scale hydraulic engineering projects.Many fulfill very local but equally important functions.
History and curiosities of emblematic dams
Beyond the numbers, many Spanish dams hide unique stories. One of the most surprising is that of the reservoir of Proserpina, in Mérida (Extremadura), considered one of the Europe's oldest functioning reservoirsIt was built by the Romans more than 2.000 years ago to supply the ancient city of Augusta Emerita and is still used today, adapted to modern regulations, as a water resource.
On the opposite side of the timeline, dams like La Serena, Alcántara or Almendra They represent the great wave of hydraulic engineering projects in 20th-century Spain. Many of them were built in the mid-20th century, coinciding with irrigation and electrification plans. The Almendra Dam, with its height of over 200 meters, is a symbol of this large-scale engineering.
Nor should we forget that, in addition to their functional role, numerous reservoirs hold islands, submerged villages and heritage remains that have been submerged. In times of extreme drought, some of these remains resurface and become, for a few months, unexpected destinations for curious onlookers and photographers, although they also spark debates about historical memory and heritage.
In the educational field, reservoirs have become a common resource in classrooms. Geography of Spain for ESO and BachilleratoMaps and infographics explain concepts such as flood, flood attenuation, overflow, river basin, and water transfer. Applications have even emerged, such as APPangea, focused on learning to locate rivers, landforms, countries or capitals, with specific units on topics such as water (SDG 6), the history of Rome or the Middle Ages in Spain.
This blend of technology, landscape, history, and education makes reservoirs much more than just "giant water tanks." In practice, they constitute one of the most crucial infrastructures for understanding how the Spanish territory worksfrom everyday supplies to responding to extreme weather events.
The official inventory of dams and safety management
Behind every reservoir lies a vast amount of technical information that doesn't usually reach the general public but is essential for its safe management. In Spain, there is a Inventory of dams and reservoirs where, for each infrastructure, its geographical location, basin characteristics, storage capacities, status of security documents, design data, photographs and plans are collected.
This inventory, managed at the state level, serves as a tool for control and planningIt allows you to know which dams need inspections, which reservoirs have operating restrictions, how they are distributed across river basins, or what effective capacity they maintain after years of sediment buildup.
For general users, much of this data translates into simple maps, charts, and summaries, but it's worth remembering that behind every fill percentage figure, there's more to it. security protocols, emergency plans and very strict regulations to ensure that the dams function correctly even in extreme situations.
In the context of climate change, with more irregular rainfall and more concentrated intense rain events, this information becomes even more critical. The capacity of reservoirs to mitigate floods and ensure reserves during periods of drought will depend largely on how these infrastructures are updated and maintained and how its use is combined with savings and efficiency measures in all sectors.
This entire network of dams, reservoirs, ponds and tanks paints a picture of a Spain where water is everywhere. carefully regulated, stored and transported to try to compensate for an erratic climate and increasing demands. Knowing where the main reservoirs are located, what their capacity is, how to check their level in real time and what problems they face in different basins allows us to better understand why some maps appear tinted dark blue and others, on the contrary, maintain much more muted tones despite episodes of rain, and helps to raise awareness that every cubic meter stored is the result of a complex combination of nature, technology and political decisions.
