DUERO, or DOURO, a river of Spain and Portugal. It rises in the Sierra de Urbion, not far from Durcula, in Soria, and empties itself into the ocean at St João da Foz, below Oporto. Its course through Spain is 306 miles, and through Portugal 185. In the latter kingdom it is not navigable more than seventy-four miles, on account of the rapidity of its course. The streams which flow into it are the Pisuerga, the Esla, the Adaja, the Valderaguary, the Yeltes, the Aguada, the Coa, the Sabor, the Tua, and the Tamaga.