The Río Negro, the largest blackwater river in the world, is a major tributary of the Amazon. The source of the Río Negro lies in eastern Colombia's rainforests. At Manaus, the Río Negro joins the Solimões River to form the Amazon River. At this point, the "meeting of the waters" phenomenon occurs.
Río Negro: Amazonia
The Río Negro is a major tributary of the Amazon River, accounting for approximately 14% of the water in the Amazon Basin. It is the largest blackwater river in the world and one of the world's ten largest rivers by average discharge.
Its total length, including its headstreams, is approximately 2,250 km (1,400 mi), of which 1,370 km (850 mi) are in Brazil. It is navigable for about 725 km (450 mi) above its confluence with the Amazon, where, although population settlement is sparse, the river is a major transportation artery.
The source of the Río Negro lies in eastern Colombia's rainforests, where it is known as the Vaupés and Guainía. After approximately 400 km (250 mi), the Guainía begins to form the border between Colombia and Venezuela.
About 120 km (75 mi) further downstream, the Guainía joins the Bravo Casiquiare, a natural waterway from the Orinoco River. It is here, near San Carlos de Río Negro, where it becomes the Río Negro and soon after enters Brazil.
The Río Negro generally meanders east-southeastward, picking up various tributaries that include the Vaupés, Içana, Marié and Branco rivers. Here, in what is known as the Anavilhanas Archipelago, the river becomes almost 30 km (18 mi) wide during the wet season.
The Anavilhanas Archipelago was created by the buildup of sediments and debris resulting from the erosion of the Guyana Highlands that was brought down by the Río Branco and deposited here in the dark acidic waters of the Río Negro in a spectacular delta-like formation.
Meeting of the Waters
Below the archipelago, at Manaus, the Río Negro joins the Solimões River (or "upper Amazon") to form the Amazon River. At this confluence, the "meeting of the waters" phenomenon occurs.
The black waters, caused by the decomposition of organic matter and low silt content, contrast with the yellowish silt-laden waters of the Branco and Amazon rivers.
Map of the Amazon River drainage basin with the Rio Negro highlighted