Clearwater river (river type)

A clearwater river is classified based on its chemistry, sediments and water colour. Clearwater rivers have a low conductivity, relatively low levels of dissolved solids, and are very clear with a greenish colour. Their chemistry is often very similar to rain and they are typically neutral to slightly acidic, but the pH can range between 5 and 8. Clearwater rivers are often fast-flowing.

Amazonian rivers fall into three main categories: clearwater, blackwater and whitewater. 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 Harald Sioli in the 1950s and 1960s. 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 main clearwater rivers are South American and have their source in the Brazilian Plateau (notably Tapajós, , Xingu and some right tributaries of the Madeira) or the Guiana Shield (such as the upper Orinoco, Nhamundá, Paru and Araguari). The sandy soils and crystalline rocks of these regions are generally ancient, of Precambrian origin, and therefore heavily weathered, allowing relatively few sediments to be dissolved in the water. 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.

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