The Liquid Forest

TROPICAL LAKES

The vast majority of natural lakes in lowland rainforests are 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. Water conditions change as the water stagnates.

Profile of an Oxbow lake in the Peruvian Amazon

The characteristics of an oxbow lake generally depend on its age. Younger oxbow lakes may be "flushed" by river currents, especially during the flood season. These lakes tend to be slightly more turbid (less transparent) and less acidic than older lakes more distant from the river channel.

Oxbow lakes are key habitats for many species of fish and aquatic animals. A number of popular tropical aquarium fish species live exclusively in oxbow lakes, which also serve as the primary habitat for giant river otters in South America.

Free-standing lakes in the rainforest also exist, though they are not as abundant as tropical rivers. Like oxbow lakes, the tend to become more acidic with time as vegetation decays.

The Death of Lac Alaotra, Madagascar

Review questions:

• What is an oxbow lake?

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Continued / Next: Threats to Tropical Rivers and Lakes