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The Douro is one of the major rivers of the Iberian Peninsula, flowing from its source near Duruelo de la Sierra in Soria Province across northern-central Spain and Portugal to its outlet at Porto.

The name, Latinized Durius, may have come from the Celtic tribes that inhabited the area before Roman times: the Celtic root is *dubro-. In modern Welsh, dŵr is "water," as well as dour in modern Breton with cognate dobhar in Irish. In Roman times, the river was personified as a god, Durius.

The Douro vinhateiro , an area of the Douro Valley in Portugal long devoted to vineyards, has been designated by UNESCO as a World Heritage Site. Traditionally, the wine was taken down river in flat-bottom boats called rabelos, to be stored in barrels in cellars in Vila Nova de Gaia, just across the river from Porto. In the 1950s and 1960s, dams were built along the river, which ended this river traffic from the upper regions in Spain and along the border. Now Port wine is transported in tanker trucks.

In 1998, Portugal and Spain signed the Albufeira Convention, an agreement on the sharing of trans-boundary rivers to include the Douro, Tagus and Guadiana. The convention superseded an original agreement on the Douro, signed in 1927, that was expanded in 1964 and 1968 to include tributaries.