The ecological value of the Tajo River basin

“The Tajo River received its name from a king of all the Spain’s; it is born in that place and dies in the Ocean Sea, kissing the walls of the famous city of Lisbon, and it is thought that its sands are of gold”.

From the English translation of El Quijote

 Miguel de Cervantes, 1605

Among the river basins of the Iberian Peninsula, the Tajo (or Tejo in Portuguese) basin is the third largest, covering 80.151 km² with a length of 1007 km. Born in the Sierra de Albarracín (Teruel, Spain), part of the Iberian System, the Tajo flows westwards towards the Atlantic. Along its course, it traverses the autonomous communities of Aragón, Castilla la Mancha, Madrid and Extremadura, and then through the Portuguese regions of Beira Baixa, Alto Alentejo and Ribatejo, until it meets the Atlantic Ocean in the Portuguese capital of Lisboa. The Tajo basin hosts the largest population among Iberian basins. In 2019, over 8 million people lived in the Spanish side, more than 6.5 million of whom were in the autonomous community of Madrid. On the Portuguese side, nearly 4 million people lived in the basin in 2018, the majority in the Lisbon metropolitan area.

Medieval bridge crossing the Tajo River in Spain, highlighting the historic and cultural heritage of the area, as well as the diversity of landscapes. (https://commons.wikimedia.org/wiki/File:Puente_medieval_sobre_el_R%C3%ADo_Tajo.JPG)

Topographically, the basin is bounded by the Central System in the North, where the Sierra de Gredos and Guadarrama represent the regions of highest elevation of the basin, surpassing the 2000 m; the Montes the Toledo to the South, reaching about 1600 m; and the Iberian System to the East. This mountainous enclosure creates an asymmetrical tributary network. Right-bank tributaries, which originate in the Central and Iberian Systems, are longer and carry more water, while left-bank tributaries are shorter and contribute less flow.

The basin’s climate is mostly Mediterranean, marked by its yearly variation with warm and dry summers and cool, wet winters. Towards the northwest, where Atlantic influence is higher, climate becomes more temperate. Elevation and Atlantic influence constitute two key factors in the Tajo basin that interact to define important gradients in temperature and precipitation. Moisture-laden Atlantic fronts are blocked by the Central System, preventing rainfall from reaching the central depression of the basin, resulting in uneven water availability, exacerbating the challenges of managing the basin’s water resources, especially under climate change.

The large population of the basin, along with extensive agricultural activity and energy production, generates a significant water demand. As a result, numerous reservoirs and dams have been built for water supply and hydroelectric power generation. Additionally, the Tajo serves as the source of a major water transfer to the Segura River, which is essential for agriculture in southeastern Spain. These factors make the Tajo one of the most regulated basins in the Iberian Peninsula, requiring complex management to address diverse socio-economic and ecological challenges.

Tajo basin map courtesy of Miguel Cánibe Iglesias

The Tajo basin’s vast expanse and climatic diversity support a wide range of ecosystems, hosting a high proportion of Iberian biodiversity. At higher altitudes, coniferous forests dominate, with species such as Pinus sylvestris and Pinus nigra. In harsher, frost-prone areas, these forests are interspersed with “sabinares” of Juniperus thurifera and Mediterranean high-mountain shrubs, including “piornales” of Cytisus oromediterraneus and “enebrales” of Juniperus communis. Below the 1500 m, forest is dominated by Quercus species, with evergreen species such as the holm oak (Q. ilex) and cork oak (Q. suber) in the sun-exposed slopes and marcescent or deciduous oaks like Q. pyrenaica and Q. petraea in the shady and more humid hillsides. In the lowlands, the “dehesas” characteristic of inner plains combine open forest dominated by Q. ilex with the typical Iberian “jarales” of Cistus ladanifer. As the Tajo approaches its mouth in Lisboa, wetlands and marshes appear providing refuge for numerous aquatic and bird species.

This rich diversity of landscapes and vegetation supports a remarkable variety of fauna, including species of national and international conservation importance. The Tajo hosts the largest European population of the Black vulture (Aegypius monachus) and the endemic Iberian imperial eagle (Aquila adalberti), as well as population of many species included in the European Habitat and Bird directives, subjected to conservation regulations.

The ecological significance of the Tajo basin has led to the designation of numerous protected areas along its course. Notable examples include the Alto Tajo Natural Park in Guadalajara (Castilla-La Mancha), the Monfragüe National Park in Cáceres (Extremadura), the Tajo International Natural Park along the Spain-Portugal border, and the Tajo Estuary Natural Reserve in Portugal. Additionally, the Tajo basin contains extensive Natura 2000 network sites, including 99 Special Conservation Areas (SCAs), 80 Important Bird Areas (IBAs), and three Sites of Community Importance (SCIs) in Spain, covering 19,831 km²—almost a quarter of the basin’s total area. The Portuguese side includes 16 SCAs and 11 IBAs. Wetlands are also key habitats, leading to the designation of three RAMSAR sites in Spain and five in Portugal to safeguard wetland biodiversity and ecosystem functions.

Despite its natural wealth, the Tajo basin faces growing pressures. Water management is heavily challenged by the high demands, with droughts exacerbated by climate change. Competing land uses, from urban expansion to agriculture and the fast-growing renewable energy sector add further complexity. Preserving its biodiversity and habitats will require both Spain and Portugal to collaborate in managing the catchment. Within INSPIRE, we aim to work together with stakeholders to develop a management plan able to tackle the potential challenges and conflicts that can arise when managing all these resources, within a framework that allow us to identify key areas for conservation and other uses that ensure and effective management for the catchment in the present and the future. In INSPIRE we aim to prioritize the allocation of management zones for this multiplicity of objectives, trying to harmonize biodiversity conservation and human uses in the catchment, through an integrated spatial planning exercise.