Rio Negro, (1) one of the principal affluents of the Amazon River, rises as the Guainia in south-eastern Colombia, and flows east into Venezuela, then south into Amazonas in Brazil, and again east and south-east until it empties into the Marañon, after a course estimated at 1350 miles. Its chief tributary on the right is the Uaupés; on the left it receives the Cassiquiare (q.v.), by means of which communication is established between the Orinoco and the Amazon, and also the Cababuri, Branco, and other streams. It is over a mile broad when its clear, inky-black stream enters the yellow, muddy
Amazon. A few miles from its mouth is Manaos (q.v.), on the left bank; and higher up the river opens into great lagoons, nearly choked with numerous islands. See A. R. Wallace's Travels on the Amazon and Rio Negro (1853).—(2) A river of Argentina, which rises in the Andean lake of Nahuel-Huapi, flows north-east as the Cunnonilla and afterwards the Limay, and receives the name of Rio Negro at its junction with the Neuquen, after which it flows east and south-east into the Atlantic Ocean. It is over 500 miles long, and for small steamers it is navigable all the way. Near its mouth there are great saline lakes, from which immense quantities of salt are collected. It bounds on the north and gives its name to a national territory, formerly part of Patagonia, and now containing an area of 81,895 sq. m. of for the greater part level but barren soil. The chief town is Viedma (pop. 1500), 20 miles from the mouth of the Rio Negro.