The Tagus (Tajo in Spanish, Tejo in Portuguese) is the longest river in the Iberian Peninsula, which it crosses in its central part in Spain, following an east-west course, with a slight inclination towards the southwest, which becomes more pronounced when it reaches Portugal, where it receives the name of Tejo.
The river’s Latin name is Tagus.
It is born in the Montes Universales, in the Sierra de Albarracín (Albarracín, Teruel province), on the western branch of the Iberian system and, after travelling 1007 km, it reaches the Atlantic Ocean in the city of Lisbon. At its mouth it forms the estuary of the Mar de la Paja, into which it pours an average flow of 456 m³/s. In its first 816 km it crosses Spain, where it runs through four autonomous communities (Aragon, Castilla-La Mancha, Community of Madrid and Extremadura) and a total of six provinces (Teruel, Guadalajara, Cuenca, Madrid, Toledo and Cáceres).
After forming a border between Spain and Portugal for 47 km, it enters the latter country. Here it runs for 145 km, through the traditional regions of Beira Baixa, Alto Alentejo, Ribatejo and Extremadura, which include the districts of Castelo Branco, Portalegre, Santarém and Lisbon. The most important cities it passes through are Aranjuez, Toledo and Talavera de la Reina, Alcántara in Spain; and Abrantes, Santarém and Lisbon, in Portugal.
Its basin has a total area of 80,600 km². It is the most populated in the Iberian Peninsula, with more than ten million inhabitants. It includes the metropolitan area of Madrid and the region of Lisbon. The Tagus basin has a total reservoir capacity of around 14,500 hm³. As regards the Spanish territory, it is regulated by the Tajo Hydrographic Confederation, an agency dependent on the Ministry of Agriculture, Fisheries, Food and Environment. The Tagus-Segura Water Transfer diverts part of the river’s flow towards the southeastern part of Spain.
Its upper course is included within the Alto Tajo Nature Reserve (Guadalajara province). Its middle-lower course is also legally protected by the Monfragüe National Park, located in the province of Cáceres. Further along is the Tajo International Natural Park, between the province of Cáceres and Portugal. At its mouth is the Tagus Estuary Nature Reserve.
The most important dam on its course is the Entrepeñas reservoir and the José María Oriol-Alcántara II.
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