Douro (the Roman Dttrius), a river of Spain andi Portugal, rising in the Pico d'Urbion in the Spanish province of Soria, flowing S.E. and S. past the town of Soria, through the provinces of Burgos, Valladolid, and Zamora, then S.W. and forming the frontier between Spain and Portugal, and falling - after a course of 400 miles, in which it receives several tributaries - into the Atlantic a little below Oporto. Running for much of its course in a deep narrow valley, it is rocky and has rapids and eddies, but is navigable for more than 80 miles from the mouth. It has plenty of fish.