Summary
The narrow, elongated morphology of the river islands called the Anavilhanas Archipelago, found in the lower Rio Negro, suggests sedimentation processes which differ from other Amazon tributaries, which have lens-shaped river islands. A study of the geochemistry of the water and sediment of the Rio Negro, Rio Branco, and Rio Solimoes suggested that kaolinitic suspended sediments from the Rio Branco were being flocculated by the acid black water of the Rio Negro, due to pH reduction to the zero point of charge of the sediment, and because of the formation of humic-kaolin clay complexes. No sediment flocculation was observed where the Rio Negro combined with the Rio Solimões; as the pH change at this convergence was slight. Sedimentation of flocculates in the lower Rio Negro produces unstratified deposits, which are manifested by elongated islands consisting mainly of silt and clay, and bed sediment of very-high clay content.