A crescent-shaped lake formed when a meander bend of a river is cut off from the main channel. Oxbow lakes are important wetland habitats and indicators of past channel positions.
An oxbow lake is a U-shaped or crescent-shaped body of standing water that forms when a river meander is cut off from the main channel through the process of neck cutoff. As a meandering river progressively extends its bends through outer bank erosion and inner bank deposition, adjacent meander loops may approach each other until the narrow neck of land between them is breached during a flood event. The river then follows the shorter, steeper path through the cutoff, and the abandoned meander loop becomes isolated as sediment plugs seal both ends. Initially, oxbow lakes contain flowing water connected to the main channel during high flows, but they gradually become hydrologically isolated and fill with fine sediment and organic material through a process called terrestrialization. Oxbow lakes are ecologically valuable as they provide diverse wetland habitats including open water, emergent vegetation, and backwater environments that support fish spawning, waterfowl, amphibians, and aquatic invertebrates. The sediments accumulating in oxbow lakes preserve a detailed paleoenvironmental record that can be used to reconstruct past flood frequency, vegetation changes, and land use history. The presence and distribution of oxbow lakes across a floodplain are diagnostic indicators of channel migration history and rates.