Douro, Or Duero, one of the largest rivers of the Iberian peninsula, rises near Monte Ur-bion, on the northern frontier of the province of Soria in Spain, about 20 m. N. W. of the city of Soria. After following a southeasterly course for about 50 m. it makes an abrupt bend and flows westward; this continues to be its general direction, in which it passes Aranda, Toro, Zamora, and Miranda, until it empties into the Atlantic at Oporto. Its current is rapid, and its course for the most part through narrow valleys. For about 50 miles it forms the boundary between Spain and Portugal. It is navigable for small vessels as far as the Spanish frontier, and receives the waters of the Pisuerga, Sequillo, Esla, Sabor, Tua, and Ta-mego from the north, and the Adaja, Tormes, Agueda, Coa, and Tavora from the south, besides many smaller streams. Navigation is often interrupted by freshets, and the river is but little used for commerce. On its banks are the vineyards which produce the celebrated wines of Oporto. Its length is estimated at 450 to 500 m.