The Tagus is the longest river in the Iberian Peninsula. It is 1,007 km long, 716 km in Spain, 47 km along the border between Portugal and Spain and 275 km in Portugal, where it empties into the Atlantic Ocean near Lisbon. It drains an area of 80,100 square kilometers. The Tagus is highly utilized for most of its course. Several dams and diversions supply drinking water to places of central Spain and Portugal, while dozens of hydroelectric stations create power. Between dams it follows a very constricted course, but after Almourol it enters a vast alluvial valley prone to flooding. At its mouth is a large estuary on which the port city of Lisbon is situated.
The source of the Tagus is the Fuente de García, in the Frías de Albarracín municipal term, Montes Universales, Sistema Ibérico, Sierra de Albarracín Comarca. All its major tributaries enter the Tagus from the right bank.