GUADIANA (anc. Anas, Moorish Wadi Ana), a river of Spain and Portugal. The Guadiana was long believed to rise in the lowland known as the Campo de Montiel, where a chain of small lakes, the Lagunas de Ruidera, are linked together by the Guadiana Alto or Upper Guadiana. This stream flows north-west ward from the last lake and vanishes underground within 3 m. of the river Zancara or Giguela. About 22 M. S.W. of the point of disappearance, the Guadiana Alto was believed to re-emerge in the form of several large springs, which form numerous lakes near the Zancara and are known as the "eyes of the Guadiana" (los ojos de Guadiana). The stream which connects them with the Zancara is called the Guadiana Bajo or Lower Guadiana. It is now known that the Guadiana Alto has no such course, but flows underground to the Zancara itself, which is the true "Upper Guadiana." The Zancara rises near the source of the Jucar, in the east of the tableland of La Mancha ; thence it flows westward, assuming the name of Guadiana near Ciudad Real, and reaching the Portuguese frontier 6 m. S.W. of Badajoz. In piercing the Sierra Morena it forms a series of foaming rapids, and begins to be navigable only at Mertola, 42 m. from its mouth. From the neighbourhood of Badajoz it forms the boundary between Spain and Portugal as far as a point near Monsaraz, where it passes into Portuguese territory, with a southerly direction. At Pomarao it again becomes a frontier stream and forms a broad estuary 25 m. long. It enters the Gulf of Cadiz between the Portuguese town of Villa Real de Santo Antonio and the Spanish Ayamonte, after a total course of s i o m. Its mouth is divided by sandbanks into many channels. The Guadiana drains an area of 31,940 square miles. Its principal tributaries are the Zujar, Jabalon, Matachel and Ardila from the left ; the Bullaque, Ruecas, Botoa, Degebe and Cobres from the right.