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A clearwater river is classified based on its chemistry,
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand ...
s and water colour. Clearwater rivers have a low conductivity, relatively low levels of dissolved solids, typically have a neutral to slightly
acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a se ...
ic pH and are very clear with a greenish colour. Clearwater rivers often have fast-flowing sections. The main clearwater rivers are South American and have their source in the Brazilian Plateau or the
Guiana Shield The Guiana Shield (french: Plateau des Guyanes, Bouclier guyanais; nl, Hoogland van Guyana, Guianaschild; pt, Planalto das Guianas, Escudo das Guianas; es, Escudo guayanés) is one of the three cratons of the South American Plate. It is a ...
. Outside South America the classification is not commonly used, but rivers with clearwater characteristics are found elsewhere. Amazonian rivers fall into three main categories: clearwater, blackwater and
whitewater Whitewater forms in a rapid context, in particular, when a river's gradient changes enough to generate so much turbulence that air is trapped within the water. This forms an unstable current that froths, making the water appear opaque and ...
. This classification system was first proposed by
Alfred Russel Wallace Alfred Russel Wallace (8 January 1823 – 7 November 1913) was a British natural history, naturalist, explorer, geographer, anthropologist, biologist and illustrator. He is best known for independently conceiving the theory of evolution thro ...
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

The main clearwater rivers are South American and have their source in the Brazilian Plateau or the
Guiana Shield The Guiana Shield (french: Plateau des Guyanes, Bouclier guyanais; nl, Hoogland van Guyana, Guianaschild; pt, Planalto das Guianas, Escudo das Guianas; es, Escudo guayanés) is one of the three cratons of the South American Plate. It is a ...
. Examples of clearwater rivers originating in the Brazilian Plateau include Tapajós, Xingu,
Tocantins Tocantins () is one of the 26 states of Brazil. It is the newest state, formed in 1988 and encompassing what had formerly been the northern two-fifths of the state of Goiás. Tocantins covers and had an estimated population of 1,496,880 in 20 ...
, several large right-bank tributaries of the
Madeira ) , anthem = ( en, "Anthem of the Autonomous Region of Madeira") , song_type = Regional anthem , image_map=EU-Portugal_with_Madeira_circled.svg , map_alt=Location of Madeira , map_caption=Location of Madeira , subdivision_type=Sovereign st ...
(notably Guaporé, Ji-Paraná and
Aripuanã Aripuanã is a municipality in the state of Mato Grosso in the Central-West Region of Brazil. It is located on the banks of the Aripuanã River. The city is served by Aripuanã Airport. The municipality contains the Rio Flor do Prado Eco ...
) and
Paraguay Paraguay (; ), officially the Republic of Paraguay ( es, República del Paraguay, links=no; gn, Tavakuairetã Paraguái, links=si), is a landlocked country in South America. It is bordered by Argentina to the south and southwest, Brazil to t ...
(although heavily influenced by its
whitewater Whitewater forms in a rapid context, in particular, when a river's gradient changes enough to generate so much turbulence that air is trapped within the water. This forms an unstable current that froths, making the water appear opaque and ...
tributaries). The Tapajós and Xingu alone account for 6% and 5%, respectively, of the water in the
Amazon basin The Amazon basin is the part of South America drained by the Amazon River and its tributaries. The Amazon drainage basin covers an area of about , or about 35.5 percent of the South American continent. It is located in the countries of Boli ...
. Examples of clearwater rivers originating in the Guiana Shield include the upper
Orinoco The Orinoco () is one of the longest rivers in South America at . Its drainage basin, sometimes known as the Orinoquia, covers , with 76.3 percent of it in Venezuela and the remainder in Colombia. It is the fourth largest river in the wor ...
(above the inflow of the blackwater Atabapo and whitewater IníridaGuaviare), Ventuari, Nhamundá,
Trombetas The Trombetas is a large river on the northern side of the Amazon River. Course The Trombetas is long, and is navigable by 500 ton vessels for a stretch of . The Trombetas river gives birth to very many rivers, including the Anamu river. It is ...
, Paru, Araguari and
Suriname Suriname (; srn, Sranankondre or ), officially the Republic of Suriname ( nl, Republiek Suriname , srn, Ripolik fu Sranan), is a country on the northeastern Atlantic coast of South America. It is bordered by the Atlantic Ocean to the nor ...
. Outside South America the classification is not commonly used, but rivers with clearwater characteristics are found elsewhere, such as the upper
Zambezi River The Zambezi River (also spelled Zambeze and Zambesi) is the fourth-longest river in Africa, the longest east-flowing river in Africa and the largest flowing into the Indian Ocean from Africa. Its drainage basin covers , slightly less than hal ...
, certain upland streams in major river basins of South and Southeast Asia, and many streams of northern Australia.


Chemistry and sediments

In South America, clearwater rivers typically have their source and flow through regions with sandy soils and
crystalline rock A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
s. These are generally ancient, of
Precambrian The Precambrian (or Pre-Cambrian, sometimes abbreviated pꞒ, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of th ...
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 Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahed ...
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 Fertility is the capability to produce offspring through reproduction following the onset of sexual maturity. The fertility rate is the average number of children born by a female during her lifetime and is quantified demographically. Ferti ...
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 Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
as the relatively dominating chemical. The water is typically neutral to slightly
acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a se ...
ic, but the pH can range between 4.5 and 8. In the Amazon basin, clearwater rivers flowing through regions with sediments of
Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...
age are typically highly acidic, while those flowing through sediments of
Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...
age are closer to neutral or slightly
basic BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
. As suggested by the name, clearwater rivers are highly transparent with a typical visibility of . There can be large variations, even within a single river, depending on season or heavy rains.


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 Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
in the Amazon basin. Many species of fish, which often are
threatened Threatened species are any species (including animals, plants and fungi) which are vulnerable to endangerment in the near future. Species that are threatened are sometimes characterised by the population dynamics measure of '' critical depe ...
(especially by dams), are only known from clearwater rivers. Large sections with
rapids Rapids are sections of a river where the river bed has a relatively steep gradient, causing an increase in water velocity and turbulence. Rapids are hydrological features between a ''run'' (a smoothly flowing part of a stream) and a ''cascade' ...
are home to specialized,
rheophilic A rheophile is an animal that prefers to live in fast-moving water. Examples of rheophilic animals Insects *Many aquatic insects living in riffles require current to survive. *'' Epeorus sylvicola'', a rheophilic mayfly species ( Ephemeroptera ...
fish, as well as
aquatic plant Aquatic plants are plants that have adapted to living in aquatic environments ( saltwater or freshwater). They are also referred to as hydrophytes or macrophytes to distinguish them from algae and other microphytes. A macrophyte is a plant that ...
s such as Podostemaceae. 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 Productivity is the efficiency of production of goods or services expressed by some measure. Measurements of productivity are often expressed as a ratio of an aggregate output to a single input or an aggregate input used in a production proces ...
compared to whitewater rivers, resulting in a comparably low insect abundance.


References

{{Wetlands Aquatic ecology Rivers