Negro, Río (rēˈō nāˈgrō) [key], 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.