Douro (Port. dōˈrŏ) [key] or Duero Span. dōāˈrō, river, c.475 mi (765 km) long, rising in the Sierra de Urbión, N central Spain. It flows W across N Spain, then southwest to form part of the Spanish-Portuguese border before flowing W across N Portugal to the Atlantic Ocean at Pôrto. One of the longest rivers of the Iberian Peninsula, it drains most of the northern portion of the central plateau; the Tormes, Esla, and Pisuerga rivers are its chief tributaries. Silting, rapids, and deep gorges combine to make the Douro unnavigable. The middle Douro is extensively used for irrigation. Several hydroelectric power plants are along the river, and through an international agreement the power is used for irrigation and development. Grapes are the chief crop of the Douro valley, and the Douro estuary is the center of Portuguese wine trade.