The lower course of the Guadiana River forms the eastern border of the Algarve with Spain. Near Vila Real d.S.A. and Ayamonte the river flows into the Gulf of Cadiz.
The Guadiana river is the southernmost of the three main rivers (as well as the Tejo and Douro), flowing from Spain to Portugal. It rises in Spain near Lagunas de Ruidera, flows for a while underground and rises again near Ojos del Guadiana. Its length is - depending where you consider its origin - 744 km or 788 km.
Up until the time of the Roman Empire the river was an important transportation route. In the surrounding areas of Alcoutim copper and iron have been mined since the Copper Age (Chalcolithic). The ore was smelted locally and then shipped downriver. The Romans called the river Anas, later altered by the Arabs to Uádi Ana ( arab. Wadi meaning river).
Today the Guadiana is only navigable up to Mértola in the Alentejo and is used only by smaller boats and pleasure cruises.
From the Serra de Caldeirão to the Atlantic Ocean in Portimão
The Rio Arade originates in the Serra do Caldeirão and its mouth is at Portimão where it flows into the Atlantic. During the Moorish period the lower part of the river was navigable, connecting Xelb (today Silves) with its harbour, Portimão. Nowadays the river is silted, and in its delta various species of water birds, including flamingos, nest.
The confluxes of the Rio Arade are:Ribeira do Arade, Ribeira de Boina (originating in the Serra de Monchique) and Ribeira de Odelouca
The Arade provides water to two reservoirs: the Barragem do Funcho near São Bartolomeu de Messines, and the smaller Barragem de Arade.
From the Serra de Caldeirão to the Ria Formosa near Tavira
The river's source is located in the Serra do Caldeirão mountains of the eastern Algarve, at the confluence of the Asseca, Zimbral and Alportel rivers. It changes its name to Rio Gilão at the ancient bridge named Ponte Romana in the center of Tavira and waters into the Atlantic at Quatro Aguas.
Ribeira de Aljezur
From the Serra de Monchique to the Costa Vicentina.
The rivulet rises in the hills of the Serra de Monchique and empties into the Atlantic Ocean west of Aljezur. This little town, built below the castle, used to be an important trading place, because the river had been navigable up to the 16th Century and the estuary was the only access to the open sea in the nearby rugged coastline.
Ribeira de Odeleite
A tributry stream of the Rio Guadiana between Castro Marim and Alcoutim
The Ribeira de Odeleite is dammed near Odeleite by the Barragem de Odeleite. This reservoir is linked by a tunnel to the nearby Barragem de Beliche, ensuring the water supply for the entire eastern Algarve. A few kilometres to the east the Ribeira de Odeleite flows into the Rio Guadiana at a place called Foz de Odeleite. The name Odeleite goes back to an Arabian name for the stream meaning Rio de Leite (i.e. stream of milk).
Ribeira de Quarteira
From the hills of the Serra de Caldeirão to the lowlands of Vilamoura.
The rivulet is formed by the junction of the two creeks Ribeira de Alte and Ribeira de Algibre near Paderne. It flows into the Atlantic Ocean west of Vilamoura Marina.
Ribeira de Seixe
The river forming the boundary of the Algarve with the Alentejo.
The river rises in the mountains of the Serra de Monchique and flows into the Atlantic Ocean near Odeceixe. Praia de Odeceixe has one of the most beautiful beaches on the entire western coast, the Costa Vicentina. The Seixe forms the natural boundary of the Algarve with the Alentejo region in the north.