The Douro or Duero (Latin: Durius, Spanish: Duero, Portuguese: Douro, pron. IPA: ['doɾu]) is one of the major rivers of the Iberian Peninsula, flowing from its source near Duruelo de la Sierra in the province of Soria across northern-central Spain and Portugal to its outlet at Porto. Its total length is 897 km, of which only sections on the Portuguese river are navigable by light rivercraft.
Its name must have come from the Celtic tribes that inhabited the area before Roman times: dwr is a Celtic word meaning "water".
In its Spanish section, the Duero crosses the great Castilian meseta and meanders through five significant provinces of the autonomous community of Castile-Leon: Soria, Burgos, Valladolid, Zamora, and Salamanca, passing through the towns of Soria, Almazán, Aranda de Duero, Tordesillas, Valladolid and Zamora.
In this region there are few large tributaries of the Duero. The most important are the Pisuerga, passing through Valladolid, and the Esla, which passes through Benavente.
This region, for the most part, is one of semi-arid plains planted with wheat and in some places, especially near Aranda de Duero, in wine grapes, in the Ribera del Duero wine region. Sheep rearing is also still important.
Then, for 112 km, the river forms part of the national border line between Spain and Portugal, in a region of narrow canyons, making it an historical barrier for invasions and a linguistic dividing line. This isolated area has now a protected status: the International Douro Natural Park.
Once the Douro enters Portugal, major population centres are less frequent. Except for Porto and Vila Nova de Gaia, at the river mouth, the only population centres of any note are Foz do Tua, Pinhão and Peso da Régua. Tributaries are small and flow into canyons to enter the larger river. The most important are the Côa, the Tua, and the Tâmega. None of these small, fast flowing rivers are navigable.