Whitewater river (river type)

A whitewater river is a type of South American river that 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, important to local fisheries and the major seasonal Amazonian floodplains known as várzea receive their water from them.

Most –but not all– whitewater rivers are Amazonian and have their source in the Andes.

Amazonian rivers fall into three main categories: Whitewater, blackwater and clearwater. This was first proposed by Alfred Russel Wallace in 1853 based on water colour, but the types were more clearly defined by 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.

Most –but not all– whitewater rivers originate in the Andes where they collect high levels of nutrient-rich sediments. This results in 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 usually is between 20 and 60 cm (0.7–2.0 ft). 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 at 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.

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