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Guadiana (gwäŧħyä´nä), river, 510 mi (821 km) long, rising in the La Mancha Plateau, E Spain. It flows west through central Spain, then south, forming part of the Spanish-Portuguese border (except for a swing into Portugal), to the Gulf of Cádiz in the Atlantic Ocean. Although one of the longest rivers of the Iberian Peninsula, it is only navigable to Mértola, c.40 mi (60 km) upstream. The Guadiana has been dammed to create several large reservoirs; these waters are used to irrigate the fertile area around Badajoz and to generate hydroelectric power.