Whitewater river (river type) in the context of Várzea forest

The Meeting of Waters (Portuguese: Encontro das Águas) is the confluence between the dark (blackwater) Rio Negro and the pale sandy-colored (whitewater) Amazon River, referred to as the Solimões River in Brazil upriver of this confluence. For 6 km (3.7 mi) the waters of the two rivers run side by side without mixing. This phenomenon is one of the main tourist attractions of Manaus. The river then flows another 60 km (37 mi) before mixing fully, but the phenomenon persists incompletely for another 30 km (19 mi).

This phenomenon is due to the vast differences in temperature, speed, and amount of dissolved sediments in the waters of the two rivers. The Rio Negro flows at near 2 km/h (1.2 mph) at a temperature of 28 °C (82 °F), while the Rio Solimões flows between 4 and 6 km/h (2.5–3.7 mph) at a temperature of 22 °C (72 °F). The light-colored water is rich with sediment from the Andes Mountains, whereas the black water, running from the Colombian hills and interior jungles, is nearly sediment-free and colored by decayed leaf and plant matter.

The Amazon biome (Portuguese: Bioma Amazônia) contains the Amazon rainforest, an area of tropical rainforest, and other ecoregions that cover most of the Amazon basin and some adjacent areas to the north and east. The biome contains blackwater and whitewater flooded forest, lowland and montane terra firma forest, bamboo and palm forest, savanna, sandy heath and alpine tundra. Some areas of the biome are threatened by deforestation for timber and to make way for pasture or soybean plantations.

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.

Blackwater rivers are lower in nutrients than whitewater rivers and have ionic concentrations higher than rainwater. The unique conditions lead to flora and fauna that differ from both whitewater and clearwater rivers. The classification of Amazonian rivers into black, clear, and whitewater was first proposed by Alfred Russel Wallace in 1853 based on water colour, but the types were more clearly defined by chemistry and physics by Harald Sioli [de] from the 1950s to the 1980s. Although many Amazonian rivers fall clearly into one of these categories, others show a mix of characteristics and may vary depending on season and flood levels.

The Tapajós (Portuguese: Rio Tapajós [ˈhi.u tapaˈʒɔs]) is a river in Brazil. It runs through the Amazon rainforest and is a major tributary of the Amazon River. When combined with the Juruena River, the Tapajós is approximately 2,080 km (1,290 mi) long. Prior to a drastic increase in illegal gold mining and consequent soil erosion it was one of the largest clearwater rivers and currently is an anthropogenic whitewater river, accounting for about 6% of the water in the Amazon basin.

Symphysodon tarzoo, the green discus, is a species of cichlid endemic to Brazil; specifically, to rivers of the western Amazon Basin upriver from the Purus arch, although it occasionally occurs downstream. An introduced population in the Nanay River is based on stock from the Tefé region. The green discus is found in blackwater habitats with a high temperature of 27–30 °C (81–86 °F) and low pH of 4.8–5.9. Although also known from whitewater, its preference for lentic habitats such as floodplains means that the water contain little suspended material (unlike main sections of whitewater rivers).

This species grows to a length of 13.2 centimetres (5.2 in) SL. S. tarzoo has red spots on the anal fin and body, which separates it from other discus species.