A whitewater river is classified based on its chemistry, sediments and water colour. Whitewater rivers have high levels of suspended sediments, giving the water a pH that is near-neutral, a high electric conductivity and a pale muddy, café au lait-like colour. Whitewater rivers are of great ecological importance and are important to local fisheries. The major seasonal Amazonian floodplains known as várzea receive their water from them.
The best-known whitewater rivers are Amazonian and have their source in the Andes, but there are also whitewater rivers elsewhere in South America and in other continents.
Amazonian rivers fall into three main categories: whitewater, blackwater and clearwater. This classification system was first proposed by Alfred Russel Wallace in 1853 based on water colour, but the types were more clearly defined according to chemistry and physics by 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.
Location
In South America
The best-known whitewater rivers are Amazonian and have their source in the Andes. The main whitewater rivers are Solimões–Amazon, Caquetá–Japurá, Putumayo, Marañón, Ucayali, Javary, Juruá, Acre, Purus, Madre de Dios, and Madeira. Although the Branco River traditionally is considered whitewater, it has a number of characteristics (some of them varying with season) that do not fit clearly into the classification and some refer to it as clearwater.
Outside the Amazon, a small number of South American rivers are considered whitewater, most notably certain tributaries of the Orinoco such as the Guaviare, Meta and Apure Rivers, and of the Paraná—Paraguay such as the Bermejo and Salado Rivers, which have their source in the Andes.
In other continents
Outside South America, this system of classification is not widely used, but there are several rivers with mainly whitewater characteristics. In Africa, these include the Niger main stem and its floodplain, Orashi, the Nile (notably the Blue Nile), the middle and lower Zambezi, and the Cross, Mungo, Sanaga, and Wouri rivers. In Asia, examples are the Mekong mainstream (especially in the rainy season), and several upland streams in large river basins in the southern and southeastern part of the continent. In Europe, sections of the Danube have whitewater characteristics.
Chemistry and sediments
In South America, most whitewater rivers originate in the Andes where they collect high levels of nutrient-rich sediments, notably illite and montmorillonite. They have a near-neutral pH (typically 6.5–7), high levels of dissolved solids (especially alkali earth metals and carbonate), and high electric conductivity. The water is turbid, with a low visibility that is usually between . In the main stem of the Amazon River, about 82% of the total suspended solids and 90–95% of the suspended load of sediments originate from the Andes. Along their course, whitewater rivers often become diluted due to the inflow of black- and clearwater tributaries. For example, the Rio Negro, the largest blackwater tributary, accounts for 14% of the total Amazon basin water and Tapajós, the largest clearwater tributary, accounts for 6%. Consequently, although the Amazon River is whitewater throughout its course, the electric conductivity is 120–200 μS/cm in the Andes, but by the time it reaches Santarém (after the inflow of Rio Negro, Tapajós, and some smaller black- and clearwater tributaries), it has fallen to 40-70 μS/cm. At high elevations in the Andes near the headwater, the pH of whitewater rivers can be above 8.
In some parts of the Amazon where the rivers are not naturally whitewater, "pseudo-whitewater" exists because of soil erosion from human activities.
Ecology
The difference in chemistry and visibility between the various black, white and clearwater rivers result in distinct differences in flora and fauna. Although there is considerable overlap in the fauna found in the different river types, there are also many species found only in one of them. Many blackwater and clearwater species are restricted to relatively small parts of the Amazon, as different blackwater and clearwater systems are separated (and therefore isolated) by large whitewater sections. These "barriers" are considered a main force in allopatric speciation in the Amazon basin.