A blackwater river is a river with a deep, slow-moving channel that flows through forested swamps and wetlands. As vegetation decays in the water, tannins are leached out, resulting in transparent, acidic water that is darkly stained, resembling tea or coffee. Most major blackwater rivers are in the Amazon River system and the Southern United States. The term "blackwater" here is an agreed-upon technical one in fluvial studies, geology, geography and ecology/biology. Not all dark-colored rivers are true blackwater rivers in the technical sense. Some rivers in temperate regions, which drain or flow through areas of dark black loam, are colored black due to the color of the soil. These types of rivers can be referred to as black mud rivers; there are also black mud estuaries (see link above to rivers officially named Blackwater River.)

Blackwater rivers are much lower in nutrients than whitewater rivers and have ionic concentrations only slightly higher than rainwater. The unique water conditions lead to a composition of flora and fauna that differs significantly from that found in whitewater sources. Areas where blackwater rivers and whitewater rivers combine are particularly attractive to a diverse group of organisms.