Tagus (Span. Tajo, Portug. Tejo), the longest river of the Iberian Peninsula. Its length is 565 m., of which 192 are on or within the frontier of Portugal, and the area of its basin is about 31,850 sq. m. The basin is comparatively narrow, and the Tagus, like the other rivers of the Iberian tableland, generally flows in a rather confined valley, often at the bottom of a rocky gorge below the general level of the adjacent country. The river rises on the western slope of the Muela de San Juan (5225 ft.), a mountain which forms part of the Sierra de Albarracin, 88 m. E. of Madrid. Thence the Tagus flows at first northwestwards, but, after receiving the Gallo on the right, it flows west, and then south-west or west-south-west, which is its general direction for the rest of its course. Regular river navigation begins only at Abrantes, a few miles below which the Tagus is greatly widened by receiving on its right bank the impetuous Zezere from the Serra da Estrella. Passing Santarem, the highest point to which the tide ascends, and the limit of navigation for large sailing vessels and steamers, the river divides below Salvaterra into two arms, called the Tejo Novo (the only one practicable for ships) and the Mar de Pedro. These branches enclose a deltaic formation, a low tract of marshy alluvium known as the Lezirias, traversed by several minor channels. Both branches terminate in a broad tidal lake immediately above Lisbon. The Tagus estuary, though partly blocked by a bar of sand, is one of the chief harbours of south-western Europe.
The narrower part of the Tagus basin lies to the south, and the left-hand tributaries which drain it are almost all mere brooks, dry in summer. The principal exception is the Zatas or Sorraia, which, rising in the Serra d'Ossa, flows westwards across the plateau of Alemtejo, and joins the Mar de Pedro. The principal right-hand tributaries, besides the Gallo and Zezere, are the Jarama, descending from the tableland of New Castile a little below Aranjuez, the Alberche and the Tietar, which collect their head waters from opposite sides of the Sierra de Gredos, and the Alagon, from the rough and broken country between the Sierras de Gredos and Gata.
