Here's what Wikipedia has to say about blackwater rivers:
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.
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.