DOURO (Span. Duero, Port. Douro, anc. Durius), a river of the Iberian Peninsula. The Douro rises south of the Sierra de la Demanda, in the Pico de Urbion, an isolated mountain mass 7389 ft. high. It describes a wide curve eastwards past Soria, then flows westward across the Castilian table-land, passing south of Valladolid, with Toro and Zamora on its right bank; then from a point 3 m. E. of Paradella to Barca d’Alva it flows south-west and forms the frontier between Spain and Portugal for 65 m. It crosses Portugal in a westerly direction through a narrow and tortuous bed, and enters the Atlantic 3 m. below Oporto at São Jõao da Foz. The length of the Douro, which is greater than that of any other Iberian river except the Tagus and Guadiana, is probably about 485 m.; but competent authorities differ widely in their estimates, the extremes given being 420 and 507 m. In Spain the Douro receives from the right the rivers Pisuerga, Valderaduey and Esla, and from the left several small streams which drain the Sierra Guadarrama, besides the more important rivers Adaja, Tormes and Yeltes; in Portugal it receives the Agueda, Côa and Paiva from the left, and the Sabor, Túa and Tamega from the right. The area drained by the Douro and its tributaries is upwards of 37,500 sq. m., and includes the greater part of the vast plateau of Old Castile, between the watersheds of the Cantabrian Mountains, on the north, and the Guadarrama, Gredos, Gata and Estrella ranges, on the south. The lower stream is beset with numerous rapids, called pontos, and is subject to swift and violent inundations. On this account navigation is attended with difficulties and risks between its mouth and Barca d’Alva; but a railway, running for the most part along the right bank, skirts the river during the greater part of its course through Portugal. The mouth of the river is partly blocked by a sandy bar; only ships of light draught can enter, while those of greater burden are accommodated at the harbour of Leixões, an artificial basin constructed about 3 m. N. On its way through Portugal the Douro traverses the Paiz do Vinho, one of the richest wine-producing territories in the world; large quantities of wine are conveyed to Oporto in sailing boats. The Douro yields an abundance of fish, especially trout, shad and lampreys.