Into the Amazon river flows over 1100 rivers, of which 17 are longer than 1500 kilometers. Together they form the largest inland waterway system. Rivers in the Amazon are divided according to the color of their water, which is determined by geological and biological structures where the waters originate. We distinguish between:
- white-water rivers – originate in the Andean slopes and have the color of coffee with milk. They are very turbid and contain high amounts of nutrients and eroded material, mainly clay. Their water has a higher conductivity and almost neutral pH. (rivers Amazon, Napo, Marañón, Tigre, Yuruá, Purús, Madeira ).
- blackwater rivers – The chemical properties of these waters are set by the sandy soil and type of vegetation known as Campina and Campinarana, which is growing in these soils. Not completely decomposed organic matter, leaves and wood in the soil and soil porosity allows the amount of humic acids to seep into rivers, reducing the pH of the water and creating characteristic dark color of the rivers (rivers Negro, Urubu).
- clear-water rivers – are usually transparent (up to 5 meters) and originate in the Brazilian and the Guyanese shield. They contain low amount of nutrients and eroded particles in suspension, thus the waters are chemically pure, with low conductivity and almost neutral pH (rivers Tapajós, Xingu, Trombetas).
Fig. 1 Main Amazon rivers. Adapted from: UNEP, Barthem, R. B., Charvet-Almeida, P., Montag, L. F. A. and Lanna, A.E. Amazon Basin, GIWA Regional assessment 40b. (2004) University of Kalmar, Kalmar, Sweden.
MEANDERS AND OXBOW LAKES
Meander (Fig. 2) is a bend of the river, caused by erosion on one side of riverbank, and deposition on the other side. Meander is composed of upper branch (upstream), arch and lower branch (downstream). Between two branches is spur or penninsula, which is connected with surrounding land by neck of meander (Fig. 2a). The inner bank (convex) of the meander, where the stream is slower is called point bar deposits, and the outer bank (concave) where the water flows faster, is cut bank (river cliff). Usually, the meander belt is created. During its development usually two arcs get close enough to each other narrowing the neck, till the cutoff of the meander. The meander can be completely separated from the river flow, by the subsequent deposition and erosion of the riverbed, and oxbow, or meander lake is created (Fig. 2b and Fig. 3). Meander lake after some time usually dries up, overgrows by vegetation and forms a meander scar.
Fig. 2a Meander. Fig. 2b Formation and development of meander and oxbow lake. (Adapted from geocaching.com)
Fig. 3 Ucayali river changes its course in a matter of few years (1986-2013). (Source: hinderedsettling.com)