Guadiana
The Guadiana River (, , , ), or Odiana, is an international river defining a long stretch of the Portugal-Spain border, separating Extremadura and Andalusia (Spain) from Alentejo and Algarve (Portugal). 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 (Portugal) and Ayamonte (Spain), where it flows into the Gulf of Cádiz. With a course that covers a distance of , it is the fourth-longest in the Iberian peninsula , and its hydrological basin extends over an area of approximately (the majority of which lies within Spain). Etymology. The Romans referred to the river as the , the "River of Ducks". During the Moorish occupation and settlement, the name was extended and referred to as "Wadi Ana" ("wādī" being the Arab term for "river"), later passed on to Portuguese and Spanish settlers as the Ouadiana, and later just Odiana. Since the 16th century, due to Castilian influences, the name has slowly evolved to take on the form Guadiana, a cognitive variation that developed from many Moorish-Arab river place-names using  the prefix "guad-" (such as the rivers Guadalquivir, Guadalete, Guadalajara or Guadarrama). Watershed. 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 Ojos del Guadiana, 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. But even hydro-geological characteristics indicate that the Upper Guadiana may not be the principal river within the system. Another of the origin theories, postulated that the Cigüela and Záncara 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 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 Olivenza 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 inter-annual discharge, large floods and severe droughts. This variability is a consequence of considerable variation in rainwater supply averaging around an annual mean of . The climate is semiarid with an average annual temperature of . Estuary. The river empties into the Gulf of Cadiz between Ayamonte and Vila Real de Santo António, the two highly touristic regions  of the Algarve and the sea-side of Andalusia There it forms a saltmarsh estuary. The estuary has a maximum width of , and its depth ranges from . Tides are semi-diurnal, ranging from ; their upriver propagation is limited by falls situated from the mouth at Moinho dos Canais. In the lower estuary there are nature reserves covering a total of ; in Spain, the Marismas de Isla Cristina and, in Portugal, the Reserva Natural do Sapal de Castro Marim e Vila Real de Santo António (English: "Castro Marim and Vila Real de Santo António Marsh Natural Reserve"); they give a valued  nature conservation character to the region. Human impact. In Spain, three autonomous communities, (Castilla-La Mancha, Extremadura and Andalusia) (comprising the provinces of Ciudad Real, Badajoz, Huelva and to a small extent Albacete) are crossed by the Guadiana. Meanwhile, in Portugal the river crosses the regions of Alentejo and Algarve, and the districts of Portalegre, Évora, Beja and Faro. Dams. There are over 30 dams on the river basin. The following are the dams on the Guadiana river itself: