An oxbow lake is a crescent-shaped lake formed when a river meander is cut off from the main channel.
Meandering rivers erode their outer banks while depositing sediment along their inner bends. As a bend becomes more pronounced, the river may eventually break through the narrow neck, often during a flood. The river then follows the shorter route, leaving the old loop behind.
Sediment gradually seals the loop’s entrances, turning the abandoned bend into a still-water lake. Oxbow lakes are important freshwater habitats because they provide sheltered conditions for aquatic plants, fish, amphibians and wetland birds.
The name comes from an oxbow, the U-shaped wooden collar used with draft oxen. Oxbow lakes may eventually fill with fine sediment and organic matter, becoming marshes, swamps or dry depressions. In some regions, they have local names: Rio Grande oxbows are called resacas, while Australian examples are often called billabongs.