The Ebro Basin and its natural constraints

Spain has a wide variety of climatic zones. They range from humid regions in the north to subhumid, semiarid and even small arid zones in south-eastern Spain, and also in interior depressions (as in the Ebro Basin, e.g., Monegros area). Water demand as in some areas triggered seasonally by agriculture and tourism does not coincide with the natural occurrences. So, different strategies for irrigation or water transfer have been developed during the last years [1, 2], generating severe social-political-technical and scientific controversies.

Satellite image mosaic (summer 1995)

Courtesy of CHE

The Ebro river and its main tributaries are more than 12,000 km long. The whole Ebro Basin covers an area of 85,000 km . The highest run-offs are concentrated in the Pyrenees and in the Cantábrica Range. The mean annual discharge is 18,200 million m³ (MCM). The area has a population of 2,800,000 inhabitants. The annual water consumption is 6,000 MCM and may reach 9,000 MCM in the future. To preserve the existing Ebro delta, an annual minimum outflow of 3,100 MCM into the sea is necessary.

The Ebro Basin is the southern foreland basin of the Pyrenees, with an asymmetrical (in its western parts a symmetrical) fill of Tertiary sediments thickening to the north. It is bounded by the Iberian Ranges to the south and the Catalan Coastal Ranges to the east.

Digital elevation model of the Ebro Basin (courtesy of CHE)

Both, the Pyrenees and the Iberian Ranges have produced thrust sheets at its margins reducing mainly during the late Oligocene the basin width by about 70%. Pre-Oligocene successions include shelf carbonates and marls, coastal and deltaic detrital facies, and evaporites. Post-Oligocene sediments of the Ebro Basin sediments are continental and reach thicknesses of more than 1,000 m. They include a wide variety of alluvial deposits, fresh-water lacustrine carbonates and saline playa lake deposits. The alluvial sediments were deposited in relatively localized alluvial fans (<15 km long) as well as in larger systems up to 100 km long.

At the end of the Tertiary, the basin changed from a closed interior basin to one drained by the Ebro River to the Mediterranean Sea. The respective incision and sedimentation of the river dominates the present geomorphology. Quaternary deposits, like pediments and fluvial terraces commonly crowned by calcretes, cover about one-third of the depression. Subsurface dissolution of evaporites caused local thickening in some places.

[1] Confederación Hidrográfica del Ebro (1996): Plan hidrológico de la cuenca del Ebro [available at www.chebro.es].

[2] Ministerio de Medioambiente, Secretaría de Aguas y Costas, Dirección General de Obras Hidráulicas y calidad de las aguas (2000): Libro Blanco del Agua en España.

[3] Alonso-Zarza,A.M. et al. (2002): Tertiary. In: W. Gibbons & T. Moreno; The Geology of Spain, Geol. Soc. London.