Encyclopædia Britannica, Ninth Edition/Tagus

TAGUS (Span. Tajo, Portug. Tejo), the longest river of the Iberian Peninsula. Its length is 566 miles, of which 192 are on or within the frontier of Portugal, and the area of its basin, according to Strelbitsky, is 31,864 square miles. 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 at a considerable depth below the general level of the adjacent country. The source of the river is at the height of 5225 feet above sea-level, on the western slope of the Muela de San Juan, in the south-west of the province of Teruel. Thence it flows at first north-westwards, but, after receiving the Rio 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. The rocky gorges which occur in its course (the principal being where the river is overhung on the right bank by the ancient city of Toledo, and again at the Puente del Arzobispo, near the frontier of Estremadura) all belong to the Spanish section of the river, and in this section the stream is frequently encumbered by sandy shallows or broken by rocky rapids, and is not navigable except for short distances. The Portuguese section has a quieter current, and Villavelha, the highest point to which boats can ascend, lies within the Portuguese frontier. 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 Estrelha. 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, and these arms enclose a deltaic formation, a low tract of marshy alluvium known as the Lezirias, traversed by several natural canals or minor branches of the river. Both these arms enter the upper end of the fine Bay of Lisbon (111/2 miles long by about 7 broad), and the Tagus leaves this bay in the form of a channel 41/2 miles long by 2 wide (see vol. xiv. p. 692), communicating with the ocean, but having unfortunately a bar at its mouth. On the north side of this channel stands the city of Lisbon. Only slight traces are still to be found of the gold for which the sands of the Tagus were anciently celebrated.

The narrower part of the Tagus basin lying to the south, the tributaries on the left bank are almost all mere brooks, most of which dry up in summer. The principal exception is the Rio Zatas or Sorraya, which, rising in the Serra d'Ossa, flows westwards across the plateau of Alemtejo, and joins the Mar de Pedro. The principal tributaries on the right bank, besides the 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 Credos and Gata.