River, southeastern South America.
Rising in southern Brazil, it forms the border between Argentina and Brazil and between Argentina and Uraguay. Above Buenos Aires , it combines with the Paraná River to form the great estuary of the Río de la Plata . Its 990-mi (1,593-km) course is interrupted by rapids, but it is navigable by ocean vessels for about 130 mi (210 km) upriver from its mouth.