On the frozen plains of central South Dakota, the Missouri River takes a dramatic turn known as a meander bend, creating the 130-kilometer-long Lake Sharpe. Meander bends form as fast-moving water erodes the outer bank and deposits sediment on the more placid inner bank. Eventually the Missouri River will cut through the skinny peninsula in the lower right of this image, creating a shorter path to the sea. In time, sediment deposition will sever Lake Sharpe from the river, forming what is called an oxbow lake—a freestanding, horseshoe-shaped body of water.
This article was originally published with the title "What Is It?" in Scientific American 310, 4, 21 (April 2014)