Cuiabá River, river, central Mato Grosso state, Brazil, rising northeast of Rosário Oeste, between the basins of the Amazon and Paraguay rivers, and flowing for 300 miles (480 km) south-southwest to join the Saõ Lourenço River. These two rivers’ combined courses, sometimes called the Cuiabá, continue across the Paraguay floodplain in a braided fashion to enter the Paraguay River, north of Corumbá. The river forms part of the boundary between Mato Grosso and Mato Grosso do Sul states. Gold deposits are found in the Cuiabá’s headwaters.