How does an oxbow lake form?
An oxbow lake is an arcuate or horseshoe-shaped lake created when a river bend becomes separated from the main channel. Its outline preserves the river's former course.
Cutting off a bend
Outer-bank erosion and inner-bank deposition can narrow the neck between adjacent limbs of a meander. High flows may open a shorter route, leaving the old bend outside the main current. Sediment deposited at its entrances reduces connectivity. A cutoff does not always immediately create a permanent lake; water levels, groundwater connections and later floods influence the outcome.
How the lake changes over time
Fine sediment and organic matter can gradually fill the lake, turning it into a wetland or damp depression. The former channel may remain visible in aerial imagery. These lakes diversify floodplain habitats and record past channel movements. Separation from the main channel does not mean complete isolation from flooding: major floods can reconnect them. Nor does every rounded lake originate from a river meander.