Why do rivers sometimes cut off meanders and create oxbow lakes?

As a meander grows, the river neck between two bends can become very narrow. During a flood or high-water event, the river may burst through this neck and follow the shorter route. Once the new channel captures most of the flow, sediment seals off the old loop from the main river. The result is an oxbow lake, named for its horseshoe shape. Many floodplains, including parts of the Mississippi system, contain dozens of these abandoned meanders.