In its Spanish section the Douro crosses the great Castilian meseta passing through Soria, Almazán, Aranda de Duero, Tordesillas, and Zamora. There are few large tributaries of the Duero before it enters Portuguese territory. The most important are the Pisuerga, passing through Valladolid, and the Esla, which passes through Benavente.
Once the Douro enters Portugal major population centers practically disappear. The Portuguese Douro is a region of canyons and tiny villages, which until recently were relatively isolated from the rest of the country. Except for Porto, at the river mouth, the only population centers of any note are Foz de Tua, Pinhão and Peso da Régua. Tributaries are small and flow into canyons to enter the larger river. The most important are the Coa, the Tua, the Tâmega, the Balsemão, and the Sousa. None of these small, fast flowing rivers are navigable.
Entering Portugal the river passes through a region of narrow canyons making it a historical barrier for invaders from the north and a linguistic dividing line. This isolated area is now a protected area.
These upper reaches of the Douro have a microclimate allowing for cultivation of olives, almonds, and especially grapes important for making the famous Port wine. The region around Pinhão and São João da Pesqueira is considered to be the center of Port wine with its picturesque quintas or farms clinging on to almost vertical slopes dropping down to the river. Many of these quintas are owned by multinational beverage companies and are worth a visit.
Traditionally, the wine was taken down river in flat-bottom boats called rabelos to be stored in barrels in cellars in Vila Nova de Gaia, just across the river from Oporto. Quite often these boats were lost in accidents and seasonal fluctutions in the river made transport precarious. In the eighties dams were built along the river ending this river traffic. There are nine dams on the Portuguese Douro alone making the flow of water uniform and generating hydroelectric power. Now Port wine is transported in tanker trucks.
Recently, a prosperous tourist industry has developed based on river excursions from Oporto to points along the upper Douro. Boats pass through the dams by way of locks.