Whitewater river

The classification goes back to Harald Sioli's mid-century limnology of the basin: rivers born in the young, eroding Andes carry heavy loads of suspended silt that turn them opaque café-au-lait. Of the three Amazonian water types they are the richest — near-neutral pH and the highest dissolved-mineral and nutrient loads, which is why the atlas's Amazon page shows the mainstem running clearly above its black and clear tributaries in conductivity.

The engine of a whitewater river is its flood pulse. Each year the river rises many metres and spills into the várzea, the whitewater floodplain forest, and that annual drowning — not the channel itself — drives the system's productivity. In the channel, visibility is measured in centimetres; whitewater fish live by senses other than sight, over shifting sandbanks and under floating meadows.

Much of the familiar Amazonian aquarium fauna comes from whitewater floodplains rather than the famous blackwaters, and the distinction matters: these are fish of near-neutral, mineral-carrying, seasonally moving water. 'Amazon' was never one kind of water, and the whitewater rivers are its generous half.

The water of this character in the atlas

Sources

1. The Amazon: Limnology and Landscape Ecology of a Mighty Tropical River and its Basin (Sioli, ed., 1984)
2. Physico-chemical Features of Major Amazonian Water Typologies (Ríos-Villamizar et al., Water Resources & Wetlands, 2016)
3. The Flood Pulse Concept in River-Floodplain Systems (Junk, Bayley & Sparks, 1989)