Tagus River, Portuguese: Rio Tejo, Spanish: Río Tajo. It rises in the Sierra de Albarracín of eastern Spain, at a point about 90 miles (150 km) from the Mediterranean coast, and flows westward across Spain and Portugal for 626 miles (1,007 km) to empty into the Atlantic Ocean near Lisbon. Its drainage basin of 31,505 square miles (81,600 square km) is only exceeded on the peninsula by that of the Ebro River, to the northeast. The Tagus covers the heart of Portugal and Spain and has been of vital importance to the modern economic development of the two nations.
The upper Tagus cuts into limestone rocks and flows generally southwestward through narrow, sinuous valleys with deep canyons and abundant ravines. Near Trillo (Guadalajara province) it runs more peacefully, and just before the town of Bolarque it is held back by the dams of Entrepeñas and Buendía, forming an artificial lake known as the Sea of Castile, which covers an area of 51 square miles (132 square km).