Guadiana

The Guadiana river, sometimes known as the Odiana, is a lengthy section of the Portugal-Spain border that separates Extremadura and Andalusia (Spain) from Alentejo and Algarve (Portugal). The river's basin stretches from Extremadura's east coast to the Algarve's southern provinces. The river and its tributaries flow from east to west across Portugal, eventually flowing into the Gulf of Cádiz through the border towns of Vila Real de Santo António (Portugal) and Ayamonte (Spain). It is the fourth-longest river in the Iberian Peninsula, covering 829 kilometers, and its hydrological basin encompasses an area of around 68,000 square kilometers (the majority of which lies within Spain).

The Guadiana crosses three autonomous communities in Spain (the provinces of Ciudad Real, Badajoz, Huelva, and to a lesser extent Albacete). In Portugal, the river runs through the Alentejo and Algarve regions, as well as the districts of Portalegre, Évora, Beja, and Faro.

Length: 508 miles (shared with Portugal)