Clearwater river (river type)

File:Amazonriverbasin basemap.png. Tributaries of the Amazon River in the eastern part of the basin are typically clearwater]]

The main clearwater rivers are South American and have their source in the Brazilian Plateau or the Guiana Shield. Examples of clearwater rivers originating in the Brazilian Plateau include Tapajós, Xingu, Tocantins, several large right-bank tributaries of the Madeira (notably Guaporé, Ji-Paraná and Aripuanã) and Paraguay (although heavily influenced by its whitewater tributaries).{{cite journal|author1=Blettler, M.C.M |author2=M.L. Amsler |author3=I.E. de Drago |author4=L.A. Espinola |author5=E. Eberle |author6=A. Paira |author7=J.L. Best |author8=D.R. Parsons |author9=E.E. Drago | year=2007 | title=The impact of significant input of fine sediment on benthic fauna at tributary junctions: A case study of the Bermejo-Paraguay River confluence, Argentina | journal=Ecohydrology | volume=8 | issue=2 | pages=340–352 | doi=10.1002/eco.1511 |hdl=11336/31047 |s2cid=55234507 |url=https://hull-repository.worktribe.com/447504/1/Article.pdf }} The Tapajós and Xingu alone account for 6% and 5%, respectively, of the water in the Amazon basin.{{cite web | url=http://amazonwaters.org/waters/flows-and-floods/ | title=Waters | date=28 May 2014 | publisher=Amazon Waters | access-date=10 October 2017 }} Examples of clearwater rivers originating in the Guiana Shield include the upper Orinoco (above the inflow of the blackwater Atabapo and whitewater Inírida–Guaviare), Ventuari, Nhamundá, Trombetas, Paru, Araguari and Suriname.{{cite journal|author1=Mol, J.H. |author2=B. de Mérona |author3=P.E. Ouboter |author4=S. Sahdew | year=2007 | title=The fish fauna of Brokopondo Reservoir, Suriname, during 40 years of impoundment | journal=Neotrop. Ichthyol. | volume=5 | issue=3 | doi=10.1590/S1679-62252007000300015 | pages=351–368| doi-access=free }}

Outside South America the classification is not commonly used, but rivers with clearwater characteristics are found elsewhere, such as the upper Zambezi River, certain upland streams in major river basins of South and Southeast Asia, and many streams of northern Australia.

File:Confluence of the Madeira River and Aripuanã River.jpg of the Madeira River and Aripuanã River (dark). Despite its colour on this photo, Aripuanã is clearwater, while the Madeira is whitewater]]

In South America, clearwater rivers typically have their source and flow through regions with sandy soils and crystalline rocks. These are generally ancient, of Precambrian origin, and therefore heavily weathered, allowing relatively few sediments to be dissolved in the water. This results in the low conductivity, relatively low levels of dissolved solids and clear colour typical of clearwater rivers.

Sand and kaolinite are the typical sediments transported by clearwater rivers, similar to blackwater, but unlike whitewater that also transports high levels of illite and montmorillonite, resulting in a significantly higher fertility of places influenced by the latter river type. Nevertheless, although clearwater rivers can have extremely low nutrient levels similar to blackwater, some such as the Tapajós, Xingu and Tocantins have nutrient levels that are intermediate between black and whitewater. The exact chemistry of clearwater rivers varies, but it is often very similar to rainwater, low in major nutrients with sodium as the relatively dominating chemical.{{cite web | url=http://amazonwaters.org/waters/river-types/clearwater-rivers/ | title=Clearwater Rivers | date=28 May 2014 | publisher=Amazon Waters | access-date=1 October 2017 }}

The water is typically neutral to slightly acidic, but the pH can range between 4.5 and 8. In the Amazon basin, clearwater rivers flowing through regions with sediments of Tertiary age are typically highly acidic, while those flowing through sediments of Carboniferous age are closer to neutral or slightly basic.

As suggested by the name, clearwater rivers are highly transparent with a typical visibility of {{convert|1.5-4|m|ft|abbr=on|0}}.{{cite book|author=Goulding, M. | year=1981 | title=Man and Fisheries on an Amazon Frontier | page=10 | publisher=Springer | isbn=978-90-481-8520-7 }} There can be large variations, even within a single river, depending on season or heavy rains.

id="table1" class="wikitable" style="float:left; clear:left; margin:0 0 0.5em 1em;" |+ Average physico-chemical characteristics
! Juruá River (typical whitewater) ! Tapajós River (typical clearwater) ! Tefé River (typical blackwater)
pH | 7.27 | 6.56 | 5.03
Electric conductivity (μS/cm) | 191.14 | 14.33 | 7.36
Total suspended solids (mg/L) | 51.42 | 10.56 | 7.90
Ca (mg/L) | 32.55 | 0.52 | 0.71
Mg (mg/L) | 4.42 | 0.26 | 0.22
Na (mg/L) | 10.19 | 1.50 | 0.40
K (mg/L) | 1.98 | 0.93 | 1.41
Total P (mg/L) | 0.080 | 0.010 | 0.033
Carbonate (mg/L) | 106.14 | 8.80 | 6.86
Nitrate (mg/L) | 0.031 | 0.040 | 0.014
Ammonium (mg/L) | 0.062 | 0.19 | 0.13
Total N (mg/L) | 0.39 | 0.35 | 0.24
Sulfate (mg/L) | 2.56 | 0.30 | 4.20
Colour (mg/Pt/L) | 41.61 | 4.02 | 54.90
Si (mg/L) | 5.78 | 5.25 | 0.33
Cl (mg/L) | 4.75 | 0.53 | 0.85

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File:Hypancistrus zebra4305.jpg is one of the many species, including several other catfish, that are restricted to clearwater rivers and threatened by dams{{cite web| title=Habitat | url=https://www.zebrapleco.com/core/zebra_pleco_habitat.php | publisher=zebrapleco.com | access-date=10 October 2017 }}{{cite web|author1=Hyland, T. |author2=M.H.S. Perez | title=Race against time | url=http://exelmagazine.org/article/race-against-time/ | publisher=Exel Magazine | access-date=10 October 2017 }}]]

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.{{cite journal|author1=Saint-Paul, U. |author2=J. Zuanon |author3=M.A. Villacorta Correa |author4=M. García |author5=N.N. Fabré |author6=U. Berger |author7=W.J. Junk | year=2000 | title=Fish communities in Central Amazonian white and blackwater floodplains | journal=Environmental Biology of Fishes | volume=57 |issue=3 | pages=235–250 | doi=10.1023/A:1007699130333 |s2cid=25361090 }}{{cite book | author=Kullander, S.O. | year=1986 | title=Cichlid fishes of the Amazon River drainage of Peru | publisher=Swedish Museum of Natural History | isbn=91-86510-04-5 }}{{cite journal |author1=Henderson, P.A. |author2=W.G.R. Crampton | year=1997 | title=A comparison of fish diversity and abundance between nutrient-rich and nutrient-poor lakes in the Upper Amazon | journal=Journal of Tropical Ecology | volume=13 |issue=2 | pages=175–198 | doi=10.1017/s0266467400010403|s2cid=84919041 }} 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.

File:White-blotched River Stingray Potamotrygon leopoldi 2284px.jpg is part of a species complex of blackish river rays with contrasting pale spots from Brazilian clearwater rivers{{cite journal| author=de Carvalho, M.R. | year=2016 | title=Description of two extraordinary new species of freshwater stingrays of the genus Potamotrygon endemic to the rio Tapajós basin, Brazil (Chondrichthyes: Potamotrygonidae), with notes on other Tapajós stingrays | journal=Zootaxa | volume=4167 | issue=1 | pages=1–63 | doi=10.11646/zootaxa.4167.1.1 | pmid=27701358 | doi-access=free }}]]

Many species of fish, which often are threatened (especially by dams), are only known from clearwater rivers. Large sections with rapids are home to specialized, rheophilic fish, as well as aquatic plants such as Podostemaceae.{{cite book |author=Collinson, A.S. | year=1988 | title=Introduction to World Vegetation | page=[https://archive.org/details/introductiontowo0000coll_r4b0/page/222 222] | publisher=Unwin Hyman | edition=2 | isbn=0-04-581031-1 | url=https://archive.org/details/introductiontowo0000coll_r4b0/page/222 }} There are major differences in the amount of macrophytes and this is mainly related to light: heavily shaded clearwater rivers have few, while those flowing through more open regions often contain many. Clearwater rivers have relatively low productivity compared to whitewater rivers, resulting in a comparably low insect abundance.

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Category:Aquatic ecology

Category:Rivers