Rivers which flow with water derived from catchments within the Amazon forest are classified as either black or clear waters. The soil within the catchment, the vegetation and the ground water level are the three main factors determining if a river will hold clear or black water. Clear water tributaries such as the Xingú and Tapajós have higher mineral content and lower levels of humic and fulvic acids than the Rio Negro and other blackwater rivers.
Because of their higher mineral content and increased clarity clear water rivers can support phytoplankton and large blooms are sometimes seen in the vicinity of the union with the river Amazon. Some forest streams also flow with clear water. These normally receive water from catchments which do not contain much regularly inundated low land and swamps. Some streams flow with clear water during the rainy season and black water during the dry season. During the dry season they are fed with water that has laid in contact with dead leaves and other rotting vegetation for sufficient time to take up large amounts of humic acid.