September 18, 2011
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Meandering river in the Amazon. Photo by Rhett A. Butler.
Rivers in lowland Amazonia tend to meander due to the flatness of the basin. For example, the Amazon itself falls only 345 feet (105 m) from the Peruvian river port of Iquitos, a full 2,300 miles from the ocean. Thus the river descends at a rate of only 1.8 inches per mile (2.8 cm/km).
The lazy nature of rivers tends to produce oxbow lakes. An oxbow lake is a crescent-shaped lake formed when a river changes course. In lowland rainforests like the parts of the Amazon where soft alluvial soils dominate, meandering rivers gradually shift due to erosion and sediment deposition. Oxbow lakes typically form when loops in the river become so extreme that the main channel erodes a new straighter route, leaving the river bend apart from the river. As time passes, the oxbow lake becomes increasingly distant from the main channel.