Guadiana

The Guadiana River (, also, , ), 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 la Mancha and 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 829 km, it is the fourth-longest in the Iberian peninsula, and its hydrological basin extends over an area of approximately 68000 km2 (the majority of which lies within Spain).

Etymology
The Romans referred to the river as the Flumen Anas, 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 valley"), later passed on to Portuguese and Spanish settlers as the Ouadiana, and later just Odiana. Since the 16th century, the name 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 hydronyms Guadalquivir, Guadalete, 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 818 km long, of which 578 km are within Spanish territory, 140 km within Portugal, while 100 km are shared between the two nations. About 82 percent, 55,444 km2, of its basin is in Spain, while about 17 percent, 11560 km2 is in Portugal.

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 68 km. 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 400 to 600 mm. The climate is semiarid with an average annual temperature of 14 to 16 C.

Estuary
The river empties into the Gulf of Cádiz 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 550 m, and its depth ranges from 5 to 17 m. Tides are semi-diurnal, ranging from 0.8 to 3.5 m; their upriver propagation is limited by falls situated 76 km from the mouth at Moinho dos Canais. In the lower estuary there are nature reserves covering a total of 2089 ha; 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, and Huelva) 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:
 * Alqueva Dam, the largest dam, located near Moura, in the Beja District, responsible for the largest reservoir in Western Europe. (There are several larger ones in Russia and Ukraine.) The Alqueva reservoir, occupies an area of 250 km2, with a capacity for 4150 hm3.
 * García Sola Reservoir
 * Cíjara Reservoir
 * El Vicario Reservoir
 * Orellana Reservoir