The Tagus is the longest river in the Iberian Peninsula. The river rises in the Montes Universales in mid-eastern Spain, flows 1,007 km, generally west with two main south-westward sections, to empty into the Atlantic Ocean near Lisbon. Its drainage basin covers 80,100 square kilometers – exceeded in the peninsula only by the Douro. The river is highly used. Several dams and diversions supply drinking water to key population centres of central Spain and Portugal; dozens of hydroelectric stations create power. Between dams it follows a very constricted course, but after Almourol, Portugal it has a wide alluvial valley, prone to flooding. Its mouth is a large estuary culminating at the major port, and Portuguese capital, Lisbon.
The source is specifically: in political geography, at the Fuente de García in the Frías de Albarracín municipality; in physical geography, within the notably high range, the Sistema Ibérico, of the Sierra de Albarracín Comarca. All the major tributaries are right bank, which is locally to the north. The river flows 716 km in Spain, 47 km along the two countries' border and 275 km in Portugal.