THE FOREST RIVER—RIVER TYPES
First-time visitors to the Amazon or other large tropical rivers are often shocked to see the muddy brown, almost
polluted-looking water. However, this color results not from sewage or pollution, but from the heavy sediment load
of the water. Each day, tons of sediment are washed into rainforest rivers from the mountains and from run-off of
surrounding forest areas due to heavy tropical rains. The sediment load is even greater where deforestation has
left the soils unprotected and massive amounts of topsoil are eroded by the rains.
Despite their cafe-au-lait appearance, such tropical rivers are generally known as "whitewater" or brown-water rivers.
Because whitewater rivers are often fed by a large number of acidic tributaries, they are relatively soft in terms
of water hardness due to their relatively low mineral content, and they have a slightly acidic to neutral pH (6.3-7.0).
Whitewater tropical rivers are the typical form of large rivers in lowland tropical rainforests. Because such rainforests are generally flat with little elevation, large tropical rivers have little gradient and flow
relatively lazily through them. The Amazon, for example, falls only 345 feet (105 m) from the Peruvian
river port of Iquitos, a full 2,300 miles from the ocean. Thus the river descends at a rate of only 1.8 inches
per mile (2.8 cm/km).
Sone of the best-adapted animals to the low visibility of the muddy whitewater are river dolphins, which are found
in the Amazon, Ganges, and Indus among rainforest rivers. River dolphins have very poor eyesight, and like oceanic dolphins rely on sonar for navigation and location of prey. River dolphins are most abundant in the large
open river channels, although during Amazonian floods they will range through the flooded forest areas.
Smaller tropical rainforest rivers are not so uniform in composition and in water flow as large tropical-forest rivers,
which tend to be whitewater. There are two other water types found commonly in the tropical rainforest besides
whitewater: blackwater and clear- or blue-water rivers.
More common in tropical lowland forests than clearwater rivers are blackwater rivers. The term blackwater describes
the appearance of the water of such rivers, which is a dark coffee color. This color results from the leaching of tannins from the decaying leaves of adjoining vegetation. Blackwater rivers are also characterized by striking water clarity; so clear that visibility may exceed 30 feet (9 meters). However, after rainstorms, blackwater rivers can lose their typical
clarity and color while sediment runs off from the surrounding forest. Within a few hours to a few days, the normal
Rio Negro, Brazil 1999
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.
"Mixing of the waters" near Manuas, Brazil. The division between the waterwater Amazon and blackwater Rio Negro can be clearly seen for miles. The bottom-most satellite image is courtesy of DigitalEarth 2005.
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.
Clearwater river in the upper Amazon in Peru
Clearwater or bluewater rivers are so named for their clear water. Such waters are fairly common as creeks and
rivers flowing through ancient rock, but are not abundant in lowland tropical rainforest. Clearwater rivers are
mostly found in the highlands like the Guyana and Brazilian shields of South America where mountain and cloud forests
grow. Because of their elevation, and tendency to run over rock, clearwater rivers are often rapid or fast-flowing. The Xingu river is one of the biggest and best known clearwater rivers in the Amazon.
Clearwater rivers have a higher pH and tend to have some dissolved minerals, making the waters harder than both
blackwater and whitewater rivers. There is not much suspended matter because the rock formations are ancient and
no longer erode in the current.
Due of their clarity and mineral content, some clearwater rivers support abundant plant growth. Additionally, algae
grow vigorously on the rocky substrate supporting a variety of sucker-mouth catfish, another popular aquarium
subject commonly known as plecos.
More information on tropical freshwater fish habitats is available at Freshwater Biotopes and Tropical Freshwater Aquarium Fish
Lowland rainforest rivers joining in Peru. (Photo by R. Butler)
- What are three major water types in the rainforest?
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Continued / Next: Rivers, Streams, and Creeks
Selection of information sources
The River of Hunger is a name used by Davis, W. in One River (New York: Touchstone, 1996) for the Rio Negro in Brazil. The name originated from indigenous people living around this blackwater river. Davis goes on to contrast the blackwater Rio Negro with the whitewater Amazon.