The Douro is one of the major rivers of the Iberian Peninsula, flowing from its source near Duruelo de la Sierra in Soria Province across northern-central Spain and Portugal to its outlet at Porto.
It is the third longest river in the Iberian Peninsula after the Tagus and Ebro; its total length is 897 kilometres (557 mi), of which only sections of the Portuguese extension are navigable, by light rivercraft.
In its Spanish section, the Douro crosses the great Castilian meseta and meanders through five provinces of the autonomous community of Castile and León: Soria, Burgos, Valladolid, Zamora, and Salamanca, passing through the towns of Soria, Almazán, Aranda de Duero, Tordesillas, and Zamora.
In this region, there are few tributaries of the Douro. The most important are the Pisuerga, passing through Valladolid, and the Esla, which passes through Zamora. This region is generally semi-arid plains, with wheat and in some places, especially near Aranda de Duero, with vineyards, in the Ribera del Duero wine region. Sheep rearing is also still important.
Then, for 112 kilometres (70 mi), the river forms part of the national border line between Spain and Portugal, in a region of narrow canyons, making it a historical barrier for invasions and a cultural/linguistic divide. In these isolated areas, in which the Aldeadávila Dam impounds the river, there are protected areas: the International Douro Natural Park (on the Portuguese side) and the Arribes del Duero Natural Park (on the Zamoran margin).
The Douro fully enters Portuguese territory just after the confluence with the Águeda River; 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 here are small, merging into the Douro along the canyons; the most important are Côa, Tua, Sabor, Corgo, Tavora, Paiva, Tâmega, and Sousa. None of these small, fast flowing rivers are navigable.