Sénia River Nature · la Sénia, Montsià, Tarragona The Sénia river, formerly called the Ulldecona river, rises in the mountains of els Ports de Beseit, in the municipality of la Pobla de Benifassà, at about 1,200 metres of altitude, and runs about 49 kilometres before flowing into the Mediterranean Sea, at the southernmost point of Catalonia. It gives its name to the town of la Sénia and forms the historic border between the Kingdom of Valencia and the Principality of Catalonia, as established by James I in 1240. Its waters are collected in the Ulldecona reservoir and, downstream, flow through steeply sloping areas that favour good oxygenation and abundant vegetation, suitable for fish life; further on they feed the area's irrigated farmland, about 2,360 hectares, before entering the Plana de Vinaròs and reaching the sea near Alcanar. Photo: Diego Molina (CC BY-SA 4.0) What to see and do in la Sénia