, located in the southern arm of the Guadiana, is responsible for Western Europe's largest reservoir. The Guadiana flows east to west through Spain and south through Portugal, then forms the Spanish-Portuguese border; it flows into the
Gulf of Cádiz, part of the Atlantic Ocean, between
Vila Real de Santo António (Portugal) and
Ayamonte (Spain). It is long, of which are within Spanish territory, within Portugal, while are shared between the two nations. About 82 percent, , of its basin is in Spain, while about 17 percent, is in Portugal.
Sources The exact source of the river in Castilla-La Mancha is disputed, but it is generally believed to spring in the ,
Villarrubia de los Ojos municipal term,
Ciudad Real Province, Castile–La Mancha, about in elevation. A classic theory introduced by
Pliny the Elder was that the river originated from the
Lagunas de Ruidera and divided into two branches: the Upper Guadiana () and the Guadiana, while separated by a subterranean course. This legend developed from a misguided belief (which persisted until the 19th century) that the river appeared and disappeared over time, because of its subterranean tributary. In fact, no subterranean course exists, and the belief that the Lagunas de Ruidera is the source is also controversial. Toponymically and traditionally, the Upper Guadiana, which runs from
Viveros (
Albacete) until
Argamasilla de Alba (
Ciudad Real) had been identified as the main branch of the Guadiana. Even hydrogeological characteristics, though, indicate that the Upper Guadiana may not be the principal river within the system. Another of the origin theories, postulated that the
Cigüela and
Zancara Rivers were the sources of the Guadiana. Today, they are considered integral parts of the river's headwaters and important tributaries, but not necessarily the exact origin. The Ciguela's source is in Altos de Cabreras (
Cuenca) and pertains to the
Sistema Ibérico, at an elevation of . Its course is long, receiving contributions from the rivers Jualón, Torrejón, Riánsares,
Amarguillo, and
Záncara. The union of the rivers Ciguela and Záncara permits the replenishment of the waters in the
Tablas de Daimiel National Park, a wetland that was designated for protection by the Spanish government in 1973 (situated in the municipalities of Villarrubia de los Ojos and
Daimiel, in the province of Ciudad Real).
Course From its origin/spring, it runs from the southern Iberian plain in a direction east to west, to near the town of
Badajoz, where it begins to track south leading to the Gulf of Cádiz. The Guadiana marks the border of Spain and Portugal twice as it runs to the ocean: First, between the River
Caia and
Ribeira de Cuncos, then later from the
River Chança until its mouth. The river is not used to completely mark the boundary between the two states; between the Olivença ravine and the Táliga ravine, the border still remains a disputed section claimed
de jure by both countries and administered
de facto by Spain (as part of the Spanish
autonomous community of
Extremadura). For the most part, the Guadiana is navigable from the Atlantic Ocean until
Mértola, a distance of . North of Mértola on the Guadiana is the highest waterfall is Southern Portugal called
Pulo do Lobo. The ecosystem has Mediterranean hydrological characteristics, including high variation in intra- and interannual discharge, large floods, and severe droughts. This variability is a consequence of considerable variation in rainwater supply averaging around an annual level of . The climate is
semiarid with an average annual temperature of . ==Estuary==