The Amazon waters are composed of several miles of tributaries feeding the major Amazon river, which is the most voluminous river in the world. Amazon tributaries are generally classified according to their color: blackwater, whitewater, and clearwater rivers. Blackwater rivers such as the Rio Negro drain highly weathered sandy soils of the central Amazon basin. They are characterized by low sediment and nutrient concentrations, and brown-colored acid waters, rich in dissolved humic substances. Whitewater rivers such as the Rio Solimoes (the Amazon River upstream of Manaus) and the Rio Madeira originate in the Andes. They are characterized by a high suspended sediment load and are rich in dissolved nutrients due to rapid weathering in piedmont regions. Clearwater rivers are common everywhere in Amazonia due to several reasons: geology, lithology, climate, soils and vegetation cover. The clearwater observed in Rio Trombetas seems to be due to the morphology of dissected terraces by a stepped sequence of flood plains. The lowest plain is several meters below the datum and almost permanently submerged. The Rio Tapajos bed widens disproportionately when the river leaves the regions of hard rocky subsoil. The current slows down and the suspended sediment settles. Consequently the waters become very transparent.The chemical contents of the water vary slightly along the tributaries and the main river.
Many minerals in the waters of the Amazon are either dissolved in the rivers of the Amazon or suspended colloidally.
The amounts of some minerals dissolved in the rivers appear to be determined by soluble rocks in and around the rivers. Metals like: Strontium, Barium, Rubidium, Molybdenum and Uranium are passed to the rivers via carbonates, evaporates, sulphides and nitrates which are contained within shales(breakable rocks with layers of sediment). Other minerals (like Chlorine, Sodium and Magnesium) enter the water systems via erosion, which is escalated by the lack of trees and consequent open soils. The concentrations of all these metals, with the exception of Cadmium and Zinc, are found to be within accepted limits. Cadmium and Zinc concentrations are debatable since different studies have found different amounts in the rivers. Cadmium, however is quite a toxic metal, hopefully, its concentration in the rivers is low. Unfortunately the concentrations of Cadmium, Arsenic and Zinc are strongly influenced by mining which is a popular money making venture done in the Amazon. This then shows directly the dangers of the mining activities performed in the Amazon.
Different types of clays are mixed in with the water in the Amazon rivers, along with quartz giving rise to the colloidally suspended metals. 53% of the clay is of the type illite, but included are chlorite, kaolinite and smectite which, all together have the main elements: Silicon, Potassium and Aluminium.
The pH values of the waters also vary, with the Lake Negro(blackwater) having a pH value of 4.43 and the Solimoes(whitewater) river having a fairly neutral, value of 7.10. While the blackwater rivers have low pH, the clearwaters tend to have slightly higher(but still in the acidic region) pH values. The whitewaters have slightly acidic to slightly alkaline pH.
---Source: L Maurice-Bourgin et al., (IRD [ex-ORSTOM]), 1999, 'Hydrological and Geochemical Processes in Large Scale River Basins'
---Source: A Saliot et al., 1989 "Particulate organic carbon, sterols, fatty acids and pigments in the Amazon River system"
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Ryan F Allard(Class of 2006)
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