A South American river 1800 m. long, the chief tributary of the Paraná, rises in some lakes near Matto Grosso, Brazil, and flows southward through marshy country till it forms the boundary between Brazil and Bolivia, then traversing Paraguay, it becomes the boundary between that State and the Argentine Republic, and finally enters the Paraná above Corrientes; it receives many affluents, and is navigable by ocean steamers almost to its source.