 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).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.