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 Caroní River is the second most important river of Venezuela, the second in flow, and one of the longest, 952 kilometres (592 mi) from the Kukenan tepui through to its confluence with the Orinoco River. The name "Caroní" is applied starting from the confluence of the Kukenan with the Yuruaní River at 182 kilometres (113 mi) from the source of the Kukenan and 770 kilometres (480 mi) from its discharge in the Orinoco. The confluence takes place in Bolivar State. The Caroní River is the longest river located entirely within Venezuela.