Oxbow lakes begin life as the curves or meanders formed by slow-moving rivers as they wind their way through the lowland forests of the Amazon basin, and other tropical forest systems throughout the world. A lake is formed as the river gradually alters its course in its never-ending evolution towards the path of least resistance. Rivers alter their course in this way as sediment builds up on the convex curve of a meander, while the bank is eroded on the concave curve. Eventually, a horse-shoe shaped body of water is left behind by the diverted river.
The still waters of these vestigial lakes provide the ideal habitat for many species of fauna and flora. In Tambopata National Reserve, oxbow lakes are known by their Quechua name, “cocha”, and they offer some of the best opportunities in the entire reserved area for the observation of wildlife.
Among the reed beds that tend to thrive in oxbow lakes as they mature into marshy wetlands, it is often possible to spot animals it would be more difficult to see in the forest itself.
Oxbow lakes are one of the best places to spot black caiman. As the largest reptile in the Neotropical Zone, these prehistoric creatures can grow up to five meters in length, and they prey on a variety of fish, reptiles, birds and mammals.
The increasingly rare giant river otter has made a home for itself in the oxbow lakes of Tambopata National Reserve. Charming as it may seem to observers, this is an efficient carnivore. It can grown up to 1.7 meters in length and consume as many as 5 kilos of fish in a single day. Poaching in the 1950s and 1960s throughout South America by hunters in search of its pelt severely reduced its numbers, and the giant river otter was listed as endangered in 1999. Today, protected areas like Tambopata offer this fascinating species a refuge from habitat loss and other pressures.
These crescent-shaped lakes are also home to many aquatic bird species, and bright macaws can sometimes be seen as they overfly the water. Perhaps the most unusual bird found in this type of habitat is the hoatzin. About the size of a turkey, crowned with a distinctive crest and bearing blue markings around their eyes, hoatzins nest in the trees around oxbow lakes and rarely show signs of discomfort when observed from relatively close quarters by human visitors. Amid considerable debate, some ornithologists have classified the hoatzin as an ancient species with no extant close relatives. Local people have their own name for them: they call them “stink birds”, because of the unpleasant odor produced in their crops by their fermenting herbivorous diet.