An oxbow lake is a crescent-shaped (often temporary) lake that is formed when a bend in a river is cut off from the main channel by the forces of erosion. An oxbow lake will slowly be created as soil erodes and re-deposits, changing the river's original course.

You can see how an oxbow lake forms below:

On the inside of the loop, the river travels more slowly leading to deposition of silt, while water on the outside edges tends to flow faster, eroding the outside banks.

Over time the loop of the meander widens until the neck vanishes altogether.

Eventually the bend becomes isolated from the river's path and a horseshoe shaped oxbow lake is formed.

Once the water stops flowing in the former river bed, sediment begins to build up in the lake. Eventually the lake will become a wetland, then a meadow, then trees will take root. In a process known as "succession," what was once river will one day become a forest.