Guadiana, river in the Iberian peninsula, rising as the Upper Guadiana, or Guadiana Alto, on La Mancha Plateau in central Cuenca Province, Spain. After flowing south through a series of small lakes, the Upper Guadiana turns north-west and, about 5 km (3 mi) from the Záncara River, disappears underground, reemerging when it reaches the Záncara River. At Ciudad Real the combined streams form the Guadiana, which from there flows circuitously south-west to Badajoz. From Badajoz, the river forms the boundary between Spain and Portugal; it enters Portugal near Mourão. The Guadiana again forms the Spanish-Portuguese boundary near Mértola, Portugal, and remains a frontier stream until it empties into the Gulf of Cádiz, an arm of the Atlantic Ocean, through a large estuary. In its total course of 829 km (515 mi), the river is navigable for a distance of 68 km (42 mi) from its mouth. It drains an area about 82,880 sq km (32,000 sq mi). The controversial Alqueva Dam, which, when completed, will create the largest artificial lake in Europe, was inaugurated in February 2002.