Awhitewater river is classified based on its chemistry,sediments and water colour. Whitewater rivers have high levels of suspended sediments, giving the water apH that is near-neutral, a highelectric conductivity and a pale muddy,café au lait-like colour.^[1] Whitewater rivers are of greatecological importance and are important to local fisheries. The major seasonalAmazonian floodplains known asvárzea receive their water from them.^[2][3]

The best-known whitewater rivers are Amazonian and have their source in theAndes, but there are also whitewater rivers elsewhere in South America and in other continents.^[1][4][5][6]

Amazonian rivers fall into three main categories: whitewater,blackwater andclearwater. This classification system was first proposed byAlfred Russel Wallace in 1853 based on water colour, but the types were more clearly defined according to chemistry and physics byHarald Sioli [de] from the 1950s to the 1980s.^[7][8][9] 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.^[8][10]

The best-known whitewater rivers areAmazonian and have their source in theAndes. The main whitewater rivers areSolimões–Amazon,Caquetá–Japurá,Putumayo,Marañón,Ucayali,Javary,Juruá,Acre,Purus,Madre de Dios, andMadeira.^[8] Although theBranco River traditionally is considered whitewater,^[11][12] 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 asclearwater.^[13]

Outside the Amazon, a small number of South American rivers are considered whitewater, most notably certain tributaries of theOrinoco such as theGuaviare,Meta andApure Rivers, and of theParaná—Paraguay such as theBermejo andSalado Rivers, which have their source in the Andes.^{[4][5][14][15][16]}

Outside South America, this system of classification is not widely used, but there are several rivers with mainly whitewater characteristics. In Africa, these include theNiger main stem and its floodplain,Orashi,^[17] theNile (notably theBlue Nile), the middle and lowerZambezi,^[6] and theCross,Mungo,Sanaga, andWouri rivers.^[18] In Asia, examples are theMekong mainstream (especially in the rainy season),^[19] and several upland streams in large river basins in thesouthern andsoutheastern part of the continent.^[6] In Europe, sections of theDanube have whitewater characteristics.^[20]

In South America, most whitewater rivers originate in the Andes where they collect high levels of nutrient-rich sediments, notablyillite andmontmorillonite.^[9] They have a near-neutralpH (typically 6.5–7), high levels ofdissolved solids (especiallyalkali earth metals andcarbonate), and highelectric conductivity.^[3][8] The water isturbid, with a low visibility that is usually between 20 and 60 cm (0.7–2.0 ft).^[8] In the main stem of the Amazon River, about 82% of thetotal suspended solids and 90–95% of thesuspended load of sediments originate from the Andes.^[21] Along their course, whitewater rivers often become diluted due to the inflow ofblack- and clearwater tributaries. For example, theRio Negro, the largest blackwater tributary, accounts for 14% of the total Amazon basin water andTapajós, the largest clearwater tributary, accounts for 6%.^[22] 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 reachesSantarém (after the inflow of Rio Negro, Tapajós, and some smaller black- and clearwater tributaries), it has fallen to 40-70 μS/cm.^[8] At high elevations in the Andes near the headwater, the pH of whitewater rivers can be above 8.^[23]

In some parts of the Amazon where the rivers are not naturally whitewater, "pseudo-whitewater" exists because ofsoil erosion from human activities.^[3]

The difference in chemistry and visibility between the various black, white and clearwater rivers result in distinct differences in flora and fauna.^[7] 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.^[24][25][26] 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.^[7][25] These "barriers" are considered a main force inallopatric speciation in the Amazon basin.^[7]

As in South America, distinct differences between species in black- and whitewater can be seen in Asia and Africa. For example, the fish fauna in African whitewater rivers tend to be dominated bycyprinids,catfish, andelephantfish, whereas blackwater rivers usually have morecharaciforms andcichlids.^[6]

The high nutrient levels in whitewater rivers allow high levels ofperiphyton (in contrast to the nutrient-poor blackwater rivers), but the water turbidity restricts light, thereby limitingphotosynthetic processes, which are necessary toalgae and submergedmacrophytes, to the uppermost part of the water column. The periphyton roughly equals theproduction level in temperateeutrophic lakes.^[27]Bacterial abundance and production rates are roughly equal in whitewater and blackwater rivers, but both vary with water level and productions are higher during the high-water season.^[28]

The major seasonal Amazonian floodplains known asvárzea receive their water from whitewater rivers and are home to many animals and plants.^[2] In theBrazilian Amazon,várzea covers roughly 200,000 km² (77,000 sq mi), equalling 4% of the entire area (twice the area covered byigapó).^[29] In addition to forests and woodlands with trees and other plants that are seasonally covered by water, about one-third of this floodplain's area is covered by large floating meadows.^[30] These floating meadows are home to the richest Amazonian community of aquatic invertebrates^[31] and important to fish,^[32] especially species that visit during the flood season for feeding or breeding (a lower number of fish species live in the habitat year-round).^[30] The floodplains are also very important for fisheries. For example, in the Brazilian Amazon, 61% of the yields fromsubsistence and local market fisheries is from districts withvárzea.^[3] Several of the most important species in Amazonian fisheries rely on whitewater for breeding: Thetambaqui (Colossoma macropomum),black prochilodus (Prochilodus nigricans) andSemaprochilodus spp. move into whitewater rivers to spawn, and many largecatfish species (especiallypimelodids such asBrachyplatystoma) perform longmigrations up whitewater rivers to spawn.^[10][21][33] Most of the large cities in the Amazon region, such asIquitos,Manaus,Santarém andBelém, are located on clear- or blackwater rivers (which have fewer insects), but at the junction of whitewater rivers (which have better fishing).^[30] Due to the high level of prey fish, the largest group-size ofInia river dolphins are in sections of the Amazon and Orinoco basins that are directly influenced by whitewater.^[34]

• Aquatic ecology
• Rivers

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