Caia (river)

The Caia () is a river in the Iberian Peninsula, a tributary to the Guadiana. It is one of the main water courses in the Portalegre District, Portugal. Portugal does not recognise the border between the Caia and Ribeira de Cuncos River deltas, since the beginning of the 1801 occupation of Olivenza by Spain. This territory, though under "de facto" Spanish occupation, remains a "de jure" part of Portugal, consequently no border is henceforth recognised in this area.
Course.
It has its sources in the Serra de São Mamede and for the lower of its course it forms the international Portugal-Spain border. Finally it joins the Guadiana River southwest of the city of Badajoz.