From a useful website with lots of Douro Valley information.

The Douro Valley is home to the River Douro, the third largest river in the Iberian Peninsula, which runs for eight hundred and ninety seven kilometres from the Spanish town of Duruelo de la Sierra to Porto, Portugal’s second city. For one hundred and twelve kilometres the river forms part of the border between Spain and Portugal. In the past, impassable canyons formed a barrier against marauding armies. Today this wild and beautiful frontier landscape is protected by the authority of the Douro International Park.

Source: Douro Valley – History, photos, accommodation & tourist attractions | Travel in Portugal