A blackwater river is a type of river with a slow-moving channel flowing through forested swamps or wetlands. Most major blackwater rivers are in the Amazon Basin and the Southern United States. The term is used in fluvial studies, geology, geography, ecology, and biology. Not all dark rivers are blackwater in that technical sense. Some rivers in temperate regions, which drain or flow through areas of dark black loam, are simply black due to the color of the soil; these rivers are black mud rivers. There are also black mud estuaries.

The Amazon basin is the part of South America drained by the Amazon River and its tributaries. The Amazon drainage basin covers an area of about 7,000,000 km2 (2,700,000 sq mi), or about 35.5 percent of the South American continent. It is located in the countries of Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru, Suriname, and Venezuela, as well as the territory of French Guiana.