DOURO (Span. Duero, Port. Douro, anc. Darius), a river of the Iberian peninsula. The Douro rises south of the sierra de la Demanda in the Pico de Urbion, an isolated mass 7,389 f t. high. It curves past Soria and then flows westward across Old Castile. It passes south of Valladolid and proceeds westward to the Portu guese frontier. The numerous tributaries have been most im portant influences in the growth of Castile's power in the middle ages. The northern tributaries, especially the Pisuerga, Valdera duey and Esla, come from the region of heavy rainfall. The tributaries from the south, of which the Adaja, Tormes and Yeltes are the most important, come from the drier Meseta.
From the great highland basin the Douro turns south-west, forming from a point east of Paradella to Barca d'Alva a section of the boundary between Spain and Portugal. This marked north-east to south-west line of the middle Douro has some relation to the general direction of the old mountain ribs of the Meseta. The river leaves the ancient plateau in a series of deep gorges in the difficult country of northern Portugal. In this country it receives the Aqueda, Coa and Paiva from the left, and the Sabor, Tua and Tamega from the right. The river enters the Atlantic ,3 m. below Oporto at Sao Joao da Foz. The probable length of the river is about 485 m., and the area of its basin upwards of 37,500 sq. miles. The rapids (pontos) and gorges of the Portuguese section of the river seriously impede naviga tion. The lower reaches of the river are also liable to heavy floods. The difficulty of navigation is compensated for by the railway that follows the valley through most of the Portuguese section. The mouth the river is partly blocked by a sand-bar, which only allows small ships to enter. The large vessels have to be accommodated at Leixoes, an artificial harbour to the north of the estuary. The Douro is the outlet for the important wine producing region of the Paiz do Vinho. The Douro yields an abundance of fish.