Bidassoa, a river of Spain, 45 m. long, the last 12 m. forming the boundary between France and Spain. It rises in Navarre, and falls into the bay of Biscay near Fuenterrabia, watering the Bastan and other beautiful valleys. The river is locally called Bastan Zubi in the upper part of its course, the name of Bidassoa being generally applied to it only after its entrance into the valley of San Esteban. It has many small affluents. Near Irun, where the French had a tete-de-pont constructed in 1813, is the Pheasants' island, a sort of neutral ground, also called the Conference island from the many Franco-Spanish conferences held there. The treaty of the Pyrenees was negotiated at this place in 1659. The Spanish crossed the Bidassoa thrice in 1793, defeating the French, who in July were finally victorious. A French army of 16,000 men under Soult was defeated on the banks of the Bidassoa at San Marcial, Aug. 31, 1813, by 8,000 British and Spanish troops under Wellington. On Oct. 7 of the same year Wellington drove the French from their strong intrenchments at the same point.