Tropical rainforests have some of the largest rivers in the world, like the Amazon, Madeira, Mekong, Negro, Orinoco, and Zaire (Congo), because of the tremendous amount of precipitation their watersheds receive. These mega-rivers are fed by countless smaller tributaries, streams, and creeks. For example, the Amazon alone has some 1,100 tributaries, 17 of which are over 1,000 miles long. Although large tropical rivers are fairly uniform in appearance and water composition, their tributaries vary greatly. Many tropical rivers and streams have extreme high and low water levels that occur at different parts of the year.

In addition to rivers, rainforests have conventional, free-standing lakes and so-called oxbow lakes, formed when a river changes course. These lakes are home to species adapted to the quiet, stagnant conditions.

Tropical waters, whether they be giant rivers, streams, or oxbow lakes, are almost as rich in animal species as the rainforests that surround them. But they, too, are increasingly threatened by human activities, including pollution, siltation resulting from deforestation, hydroelectric projects, and over-harvesting of resident species

Chemically, blackwater rivers are very low in dissolved minerals and often have no measurable water hardness. The very acidic, almost sterile water, with a pH between 3.5-6, keeps parasite and bacterial populations to a minimum. For this reason, blackwater rivers are considered some of the cleanest natural waters in the world, most often compared to "slightly contaminated distilled water." The water chemistry of blackwater also inhibits the proliferation of insect larvae, so the forest around blackwater tends to be less "buggy" in terms of floor-dwelling mosquitoes.

Surrounding some pure blackwater rivers are blackwater forests, which are different than conventional rainforests. The acidity of the water limits the number of tree species that can grow in the area near the river. The low tree diversity is responsible for a lower variety of insect species, because insect species that would otherwise pollinate and feed upon other tree species have no opportunities in blackwater forests. This, coupled with the harsh water conditions of blackwater rivers, results in considerably lower overall insect diversity than in other forests. Accordingly, the blackwater forest areas support fewer numbers of other animal species.

The cause of the soft, acidic conditions of blackwater is the origin of most blackwater streams in the lowland tropical forest, where the ancient soils have no minerals to increase water hardness. Adding to the acidity of blackwater rivers are the tannins released from decaying leaves.
 

Because blackwater rivers are directly fed by run-off from surrounding rainforest, the soils of which are generally nutrient-deficient, these rivers are nutrient poor, and the surrounding floodplain areas are less suitable for cultivation than the floodplains of larger whitewater rivers like the Amazon. The nutrient deficiency of the soils along the shores of the Rio Negro makes the river known by Indians as the River of Hunger. Similarly, blackwater rivers support a lower bio-load than surrounding whitewater rivers, though they tend to have a tremendous diversity of fish species.

Fish have specially adapted to tolerate blackwater conditions. Many of the fish species that inhabit blackwater are best known outside the tropics for their popularity as aquarium fish. Discus, angelfish, arowana, elephantnose fish, many gouramis, and cardinal tetras are a few of the examples of blackwater species kept as aquarium subjects. The brilliant, iridescent colors of many small, schooling species like cardinal tetras, are believed to aid the school in recognition of its members in the dark waters.

The arowana, sometimes kept as an aquarium subject, is also know as the "monkey fish" for its leaping abilities—reportedly up to six vertical feet. It uses these abilities to catch its prey, which includes insects, small animals, and even young monkeys and sloths. The arowana is a large, elongated, even eel-like fish which swims along the surface looking for prey.

The Rio Negro in Brazil is one of the world's largest rivers (five miles at its mouth) and the most famous blackwater river. In contrast to the whitewater Amazon (the Rio Solimones at this point), which has its origins in the mountain valleys of the Andes, the tributaries of the Rio Negro rise in the ancient rock formations of the Guyana shield and flow through white-sand rainforests. The differences between the blackwater of the Rio Negro and the whitewater Amazon are readily apparent where the two rivers meet near Manaus, Brazil. The rivers run side by side, clearly distinct as separate white and black water, before blending together after several miles.