Running the length of Portugal’s south-eastern border with neighbouring Spain, the River Guadiana rises up from the heart of the Spanish province of Albacete at an altitude of 1,700 metres.
After meandering its way through some of the most picturesque landscapes in the whole of the Iberian Peninsula, the river eventually spills out into the Atlantic Ocean at the border town of Vila Real de Santo António in the Algarve.
Its fertile banks and vast commercial potential attracted merchants and traders for centuries, starting with the Phoenicians in 8 BC and subsequently the Greeks two hundred years later.
The Carthaginians, Romans and Moors also frequented the calm waters of the Guadiana to trade gold, silver, copper, wheat, leather, olive oil, honey, salt, fish and other useful commodities.
After the Christian occupation of the Algarve in the 13th century, the Guadiana became a natural boundary between the kingdoms of Portugal and Spain, and today the frontier towns of Castro Marim, Alcoutim and Mértola are good examples of the kind of fortified trading posts that flourished along the river during that period, particularly the latter which was already a bustling commercial centre long before the Romans arrived.
Mértola was the capital of the once-powerful Moorish empire and the town’s rich Muslim heritage is still very evident, most notably inside the parish church which was originally a 12th-century mosque and retains its Moorish prayer niche behind the altar on the eastern wall.
With its fine views sweeping across the Guadiana Valley, Mértola Castle (indicated on the map below) dates from the 13th century and was built at the time the town was the national seat of the Order of St James.
Further downriver lies Alcoutim, without doubt one of the most picturesque places along the Guadiana. Despite being smaller than Mértola, it was a major river port in the times of the Phoenicians and on the 31st of March 1371 the town witnessed the signing of a rare peace treaty between King Fernando I of Portugal and King Enrique II of Castile, albeit very short-lived.
Built in the 14th century, Alcoutim Castle looms over the river on a site which archaeological excavations suggest was inhabited during the Iron Age and at the time of the Roman occupation.
For the more adventurous visitor, a cross-border zip line (the only one of its kind in the world) is on hand to transport visitors from Alcoutim to Sanlúcar on the Spanish side of the river. This unique opportunity to travel between the two countries takes less than a minute but is an exhilarating trip over the River Guadiana.
The journey south towards Castro Marim promises some of the most enchanting scenery in the whole of the Algarve, particularly in early spring when the almond trees are in full blossom.
With its great fortress looking out across the Guadiana frontier towards Spain, Castro Marim became the first headquarters of the Military Order of Christ in 1319 and for centuries the town was an important centre for fishing, salt production, agriculture and ship-building.
Built by King Afonso III, the powerful sandstone walls of Castro Marim’s medieval castle enclose the ruins of an even older castle, while on the other side of town stands the fortress of São Sebastião built by King João IV, father of Catherine of Bragança.
After weaving a gentle and very peaceful passage from its source (indicated on the map below) through the heart of the southern Iberian countryside, the River Guadiana finally gushes out into the Atlantic Ocean at the eastern end of the Algarve to complete its 829 km journey.
Standing guard over the estuary is Vila Real de Santo António, a pleasant seaside resort set around a lovely main square named after the Marquês de Pombal, the man who built the town to a very strict plan in as little as five months in 1774 to both intimidate the Spanish and boost the Algarve fishing industry. All the stone window and door frames and other fittings were brought down from Lisbon ready made as an extension of the classical grid system he devised for Lisbon after the great earthquake of 1755.