Río Negro (rē´ō nā´grō), principal river of Uruguay, c.500 mi (800 km) long, rising in S Brazil and flowing SW across central Uruguay to the Uruguay River. It traverses a sheep-raising region; there is agriculture along its lower course. On the river is Embalse del Río Negro (c.4,000 sq mi/10,360 sq km), the largest artificial lake in South America. It extends 87 mi (140 km) upstream from Rincón del Bonete, a hydroelectric dam (completed 1949) with a 128,000-kW capacity. Downstream from Bonete is Rincón de Baygorria (1960), with a 108,000-kW capacity.

Río Negro (rē´ō nā´grō), river, c.1,400 mi (2,250 km) long, rising as the Guainía River in E Colombia where it flows NE before turning south to form part of the Colombia-Venezuela border. It then flows SE through Amazonas state, Brazil, to the Amazon near Manaus. The river is filled with islands and has many secondary channels. Its main tributary is the Río Branco. The Río Negro is connected with the Orinoco basin by the Casiquiare, a natural canal. An important commercial channel (rubber and nuts are shipped on it), the Río Negro was discovered (1638) by Pedro Teixeira, a Portuguese explorer. The river was named for its black color, which results from vegetal debris, not sediment.

Río Negro (rē´ō nā´grō), river, c.400 mi (640 km) long, formed in central Argentina by the confluence of the Neuquén and the Limay rivers, and flowing E across Río Negro prov. (N Patagonia) to the Atlantic Ocean. The river is used for irrigation.