Tagus Basin

The Tagus Basin is the drainage basin of the Tagus River, which flows through the west of the Iberian Peninsula and empties into Lisbon. It covers an area of 78,467 km2, which is distributed 66% (55,645 km2) on Spanish territory and 34% on Portuguese land (22,822 km2). It is the third largest basin in the Iberian Peninsula, after the Douro Basin, with 98,258 km2, and the Ebro Basin, with 82,587 km2. The Tagus basin is one of the most important in the Peninsula, due to its extension and its flow, being the one with the highest population density in Spain and the peninsula. It is formed by an elongated surface with an east–west orientation, the Tagus River flows from the Sierra de Albarracín, where it has its source, to the estuary, Mar de la Paja, next to Lisbon, through the center of the Hesperian Massif with a length of 910 km, in the Spanish area, 1092 km in total length. The basin is wedged between the Central System, to the north, the Montes de Toledo and Sierra de Montánchez, to the south and the Iberian System, (Serranía de Cuenca and Sierra de Albarracín), to the east; bordering to the north with the Ebro and Douro basins; to the south with the Guadiana basin and to the east with the Ebro and Júcar basins. The western limit, as far as the Spanish area is concerned, is delimited by the Erjas and Sever rivers, which form the border with Portugal. In the interior of the area defined by these mountains and by the minor reliefs of the Hercynian massif, which completes the closure to the west, it is structured in a graben filled by Cenozoic materials, sands, clays, marls, gypsum and some limestone in the upper levels, which constitute horizons of silting up of the ancient lake that occupied the original depression. The mountain ridges of the Tagus basin only reach high altitudes in the Central System, especially in the middle and eastern sectors (Sierra de Béjar, Sierra de Gredos and Sierra de Guadarrama), where they frequently exceed 2000 meters above sea level; in the Iberian System, only some peaks of the Montes Universales exceed 1800 m.