Guadiana

The Guadiana River, or Odiana, is an international river defining a long stretch of the Portugal-Spain border, separating Extremadura and Andalusia from Alentejo and Algarve. The river's basin extends from the eastern portion of Extremadura to the southern provinces of the Algarve; the river and its tributaries flow from east to west, then south through Portugal to the border towns of Vila Real de Santo António and Ayamonte, where it flows into the Gulf of Cádiz. With a course that covers a distance of 829 kilometres, it is the fourth-longest in the Iberian peninsula, and its hydrological basin extends over an area of approximately 68,000 square kilometres.

Source: Wikipedia