As a river flows through countryside, it rarely follows a straight line, but bends and twists following the natural contours of the ground and washing away the softest soil. Water flows fastest on the outer side of the bends, causing that bank to wash away further. In the meantime, soil being carried along in the river water, called silt, is deposited on the opposite bank. Over time, especially if there is flooding, the river may cut across the neck of the bend, creating an oxbow lake beside the river.
Oxbow lakes, sometimes called horseshoe lakes, loop lakes, or cutoff lakes, get their name from their resemblance to the U-shaped collar put around the neck of an ox for plowing. Their unique shape stems from the way in which they’re formed. An oxbow lake starts out as a curve, also known as a meander, in a river. As water travels around the curve, it flows faster on the outside of the curve and slower on the inside of the curve.
This has two effects. The water on the outside of the curve eats away at the river bank in a process called erosion. The water on the inside of the curve, on the other hand, moves more slowly and leaves behind dirt, sand, silt, and other types of sediments in a process called deposition. Over time, these processes create a curve with a distinct U or crescent shape. As the processes of erosion and deposition continue, the piece of land at the narrow ends of the curve closest to the straight path of the river gets smaller and smaller.
Eventually, the river cuts a new, straight path through that small piece of land, creating a shortcut that straightens the path of the river and leaves an oxbow lake behind. When you look at a photograph of an oxbow lake from above, you can often see how it used to be simply a bend in the river. Cut off from the main river channel, oxbow lakes don’t have any water flowing into or out of them. Without sufficient rain, they may dry up completely. Many oxbow lakes that do receive some rain turn into swamps that become thriving wildlife habitats.