New insights on the classification of major Amazonian river water types

Traditional water classifications for Amazonian rivers are based mainly on optical characteristics that do not fully capture their complexity. Today, an increasing amount of hydrochemical data indicates that the chemical composition of Amazonian rivers varies much more than this simple classification indicates. Revising river classification based on hydrochemical properties is proposed here. In this first comprehensive review of Amazonian river chemistry, we synthesized critical information from 168 scientific publications and distinguish unusual white, black, and clearwater rivers. It is shown the distribution of such rivers across the basin and the limitations of using generalized designations for river typologies. For example, some optically clearwater rivers draining carboniferous stripes have chemical properties similar to whitewater rivers, regardless of their high transparency. Furthermore, a clear or blackwater stream that becomes turbid because of soil erosion does not become a whitewater stream. Its hydrochemical parameters of acidity and nutrient poverty depend on the geology of the catchment area and remain relatively unaffected. These insights into Amazonian river classification provide a new understanding of their baseline limnological conditions. They have implications for sustainable management of freshwater systems and for monitoring potential impacts of large development projects and climate change on the Amazonian aquatic systems.