The Ebro is the longest river entirely within Spain. It rises at Fontibre in Cantabria, only 42 km from the Bay of Biscay, and flows about 930 km east-southeast through Castile and León, La Rioja, Navarre, Aragon and Catalonia to the Mediterranean, where it ends in the Ebro Delta in the Terres de l'Ebre region of Tarragona province. On the Iberian Peninsula it ranks second in length after the Tagus, and its basin of 85,550 km² is second only to the Douro's. The Ebro matters for the water it gives Zaragoza and the farms of Aragon, and for its delta, a wetland of roughly 320 to 340 km² that is a Ramsar site, a natural park and the breeding ground of the world's largest colony of Audouin's gulls. The animals and plants that live along it tell the story of how the river has changed.
Kermes oak, tamarisk and salt lagoons of the dry Ebro depression
Below the green Cantabrian headwaters and the gorges of Burgos province, the Ebro enters a broad basin walled off from sea air by the Pyrenees to the north and the Iberian System to the south. Rainfall on the valley floor is only 300 to 600 mm a year, summer days pass 35 °C and winter nights drop below freezing. The vegetation follows that dryness. Holm oak and the shrubby kermes oak dominate the scrub, mixed with pine, myrtle, the dwarf fan palm Chamaerops humilis, juniper, rosemary and thyme. Farther from the river the cover thins to dwarf juniper and then to treeless steppe, as on the Llanos de Belchite and the so-called Calanda desert.
The valley was once a sea floor, and its soils are thin, stony and in places salty. Where water collects in closed basins it forms endorheic salt lagoons fringed with tamarisk, home to specialist halophytes, salt-tolerant plants, including endemic mosses and members of the goosefoot, sea-lavender and sedge families. The badlands of the Bardenas Reales in southeast Navarre, about 42,000 ha of clay, chalk and sandstone eroded into canyons, mesas and isolated hills called cabezos, lie in the middle of this depression at 280 to 659 m above sea level, with streams that stay dry for most of the year.
Lost meanders between Logroño and Zaragoza
On its middle course, past Logroño, Tudela and into Aragon, the Ebro widens and slows as Pyrenean tributaries add their spring snowmelt. This was once a river of shifting channels, oxbow lakes, scroll bars and dense riparian forest. Most of that habitat has gone. Lagoons, salt ponds and freshwater marshes were drained or filled, and almost all of the riverside forest was cleared for crops or for pulpwood plantations. Because dams and embankments now regulate the flow, the river no longer cuts new meanders or leaves new oxbow lakes, and the old ones are drying out or silting up. Only small fragments are protected.
The water that feeds this stretch comes mostly from the north. The Ebro's left-bank tributaries drain about two thirds of the southern slopes of the Pyrenees, and all of Andorra drains into the Ebro through the Segre. That snowmelt is why the river runs highest in spring, and mountain tributaries beyond the reach of the invaders described below are where native barbels have held on.
Zaragoza shows the setting. The river crosses the city from west-northwest to east-southeast, entering the municipality at 205 m and leaving at 180 m. Near its meanders, the gypsum-rich ground has collapsed into sinkholes that fill with irrigation water to form ponds, and a seasonal salt lagoon, la Sulfúrica, appears on the moors in the south of the municipality. The city sits where the Huerva and the larger Gállego, which rises in the Pyrenees, join the Ebro. Our guide to Spain's geography places the basin among the country's other great valleys.
The Ebro barbel and the wels catfish released at Mequinenza in 1974
Where the river reaches the Aragon–Catalonia border it is held back by three large dams: Flix, finished in 1948, and Mequinenza and Riba-roja, about 35 km apart, which the state power company ENHER was mandated to build in 1955. Mequinenza's reservoir, known as the Mar de Aragón, began filling in December 1965; it is almost 110 km long, covers 75.4 km² and holds 1.53 billion m³ at full level. It was in this reservoir, in 1974, that the wels catfish was introduced. The species, native to the Baltic, Black and Caspian Sea basins and prized as a sport fish, can exceed 2 m in length, and some of the largest specimens on record have been caught in the Ebro.
From Mequinenza the catfish spread through the lower Ebro and its tributaries, especially the Segre. The fish it displaced include the Iberian barbels of the genus Barbus and its relatives, which had once been abundant in the river. The Ebro barbel, Luciobarbus graellsii, is native to northeast Spain from the Ebro to the Ter basins; it eats aquatic invertebrates and algae, swims upstream to spawn on gravel between May and August, and can live 16 years. Competition with and predation by the catfish caused barbels to disappear completely from the middle channel of the Ebro around 1990. Populations in mountain tributaries the catfish has not reached were unaffected. The IUCN lists the Ebro barbel as near threatened.
Zebra mussels, red swamp crayfish and mercury in the lower river
The catfish is not the only newcomer. The zebra mussel, native to the Black and Caspian seas, is spreading through the delta and upstream. It breeds fast, competes with native shellfish and clogs underwater machinery at ports, dams and hydroelectric plants. The American red swamp crayfish, Procambarus clarkii, has caused economic losses; it thrives in disturbed habitats that the native Iberian crayfish cannot use. In the far upper basin the coypu, a South American rodent, is expanding along northwestern tributaries and damaging crops and protected areas in the Basque Country.
Pollution is the other legacy. Fish caught along the whole lower Ebro carry high levels of mercury, and the European Union prohibits their sale, including the large wels catfish. The river also has a growing problem with dense aquatic vegetation, seaweed and algae.
Audouin's gulls and 95 breeding bird species on the Ebro Delta
At its mouth the Ebro has built a rounded delta from sediment carried down over centuries. Amposta, a seaport in the 4th century, now stands well inland. Spain first classed the delta as a wetland of international interest in 1962, and a law introduced in phases from 1983 to 1986 created the Ebro Delta Natural Park, today 7,802 ha of lagoons, beaches, marshes, salt pans and estuaries. In 1993, 7,736 ha were added to the Ramsar list of wetlands of international importance, and the wider Terres de l'Ebre became a UNESCO Biosphere Reserve in 2013. Read more about the area in our Ebro Delta guide.
The park is of international importance for 8 plant species and 69 vertebrate species. Some 95 bird species breed there, and more than 300 species use it, including large numbers of wintering waterfowl and migrants for which the delta is an essential stopover. The emblem is Audouin's gull, a fish-eating gull of the Mediterranean that was among the world's rarest in the late 1960s, with only about 1,000 pairs in total. The delta now holds the largest colony on Earth; in 2006 it reached a record of more than 15,000 pairs. River-mouth islands give nesting birds some shelter: the Isle of Buda covers 1,231 ha, the Isle of Garxal 280 ha and the Isle of Sant Antoni 170 ha.
Much of the delta that is not park is rice. The rice cooperative of the Ebro Delta farms about 22,000 ha and can produce 45,000 tonnes a year from 14 varieties, and the delta has held a Protected Designation of Origin for rice since 1992, covering six varieties including bomba and sènia grown in towns such as Amposta, Deltebre and Sant Jaume d'Enveja. A network of canals and irrigation ditches, maintained by both farming and conservation groups, keeps water moving through paddies and lagoons alike, so the delta's agriculture and its wildlife depend on the same managed water.
How a 99 percent loss of sediment endangers the delta's habitats
The delta exists because the river once delivered more sediment than the waves could remove. That balance has broken. During the 20th century the Ebro's mean annual flow fell by about 29 percent, because of dams, irrigation and evaporation from reservoirs, and studies estimate that the sediment load fell by more than 99 percent as reservoirs trapped it. With no new silt arriving, the coastline is eroding.
Lower flow also lets seawater push up the riverbed as a salt wedge beneath the fresh water, changing the habitat for fish and plants in the estuary.
Storms show how exposed the low ground has become. In January 2020 Storm Gloria drove a surge that swept up to 3 km inland across the delta's rice paddies. Climate change is adding to the pressure through stronger erosion from the sea and a rising saltwater table, damaging both biodiversity and the farmers who share the delta with its birds. The fate of the gull colonies, the lagoons and the rice fields now depends on how much water and sediment the dams upstream let through.
High water still reaches the sea in some years: between 27 March and 11 April 2007 the river delivered 1,874 million m³ into the delta, an average of 117 million m³ a day. Flood pulses of that size, many times the discharges in the chart above, are a reminder of the floods that once built the delta and fed its lagoons.