noun - A blackwater river is a river with a deep, slow-moving channel flowing through forested swamps or wetlands. As vegetation decays, tannins leach into the water, making a transparent, acidic water that is darkly stained, resembling tea or coffee. 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.
Blackwater rivers are lower in nutrients than whitewater rivers and have ionic concentrations higher than rainwater. The unique conditions lead to flora and fauna that differ both from whitewater and clearwater rivers.
The classification of Amazonian rivers into black, clear and whitewater was first proposed in 1853 by Alfred Russel Wallace (8 January 1823 – 7 November 1913), a British naturalist, explorer, geographer, anthropologist, and biologist.