Which river system contains XXX?
Douro
The Douro (Portuguese: Douro (ˈdo(w)ɾu); Spanish: Duero (ˈdweɾo); Latin: Durius) 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.