The Portuguese Rio Douro is the third longest river of the Iberian Peninsula. Rising near Madrid, the river flows westward across Spain and northern in Portugal to the Atlantic Ocean. It is navigable in Portugal between Peso da Régua and Porto (Oporto).Spain
In its Spanish section, the Duero rises in the Sierra de Urbión and crosses the great Castilian meseta and meanders through five provinces of Castile and León: Soria, Burgos, Valladolid, Zamora, and Salamanca, passing through the towns of Soria, Almazán, Aranda de Duero, Tordesillas, and Zamora.
Borders
The river forms part of the border between Spain and Portugal for 70 miles (113 km) where it plunges about 1,250 feet (380 m) in a series of gorges and rapids, making it an historical barrier for invasions and a linguistic dividing line. This isolated area, where the Aldeadávila Dam captures the river, now has protected status: the International Douro Natural Park on the Portuguese side, los Arribes del Duero Natural Park on the Zamoran bank. The Douro fully enters Portugual just after the confluence with the Águeda River.
Portugal
Below Vega de Terron in Spain you can can take a wonderfully scenic journey through deep-cleft gorges terraced with Port wine-producing vineyards, past sleepy villages and a tranquil countryside of almond, olive, cherry and citrus trees. The Douro passes through Regua and Pinhao en route to the sea where Oporto and Vila Nova de Gaia occupy opposite banks. Explore quaint villages and towns; visit magnificent pousadas (country estates) and delightful quintas (wine estates). The Douro vinhateiro, an area of the Douro Valley in Portugal, has been classified by UNESCO as a World Heritage Site.