Paraguay, river, c.1,300 mi (2,090 km) long, rising in the highlands of central Mato Grosso state, Brazil. Flowing generally southward, it forms the border between Brazil and Paraguay in the pantanal, then crosses the center of Paraguay, dividing the Gran Chaco from E Paraguay. Two large tributaries, the Pilcomayo and the Bermejo rivers, join it from the west. Below the Pilcomayo, the Paraguay River flows SW to the Paraná River, forming part of the Paraguay-Argentina border. Navigable for most of its course, the Paraguay River is one of the major arteries of the Río de la Plata system, with its chief port at Asunción, Paraguay.