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Abstract:
Forty-six rivers and creeks intersected by the 2,300 km long Cuiaba — Porto Velho-Manaus road in north—central Brazil were sampled and analysed for pH, specific conductivity, and the contents of Ca, Mg, Ba,
Na, K, Mn, Fe, Al, Cu, Zn_Organic C, total P and nitrate-N. The occurrence of aquatic macrophytes was recorded. The results show the close relationship between the chemistry of the waters and the geology
of the respective catchments. The waters of creeks draining towards the Rio Paraguay are rich in electrolytes and alkaline in nature, while waters flowing towards the Amazon are generally acidic and poor in
electrolytes. Significant differences between the electrolyte—poor waters are seen.
Waters from the creeks within the Parecis Formation of Mato Grosso are chemically extremely poor (µS around 3). Chemically richer, but very heterogeneous are the waters from the Archaic rock regions, while waters from regions of the Tertiary Barreiras sediments (as around 10) are very low in electrolytes. The relatively electrolyte—rich waters of the Rio Madeira and Solimoes do not fit into the classification, since their
chemical composition is due to gee-chemical influence in the pre—Andean
area. The above—mentioned differences are again reflected by the data on the concentrations of the elements analyzed, except for Cu, Zn, Al and organic C.
The occurrence and distribution of aquatic macrophytes cannot be correlated with the electrolyte content of the waters. The extremely poor waters of the Parecis-Formation sustain a rather luxurious flora of
submerged species Which is absent in the chemically similar waters of the Barreiras sediments. The role of light as a limiting factor in this case is discussed.
