Ox-bow lakes are formed when a river is at a very low gradient. A low gradient means that the slope or incline of the river is not very steep. When the river meanders or curves, the water flows faster on the outer curve or bend due to centrifugal force, and slower on the inner curve. Over time, the force of the faster moving water erodes the land on the outer bend, creating a neck between the bend and the main channel. This neck eventually becomes narrower and narrower until the river cuts through it, forming a new channel and leaving behind a U-shaped lake known as an ox-bow lake. It is important to note that if the river is moving too fast or is at a steep slope, the formation of an ox-bow lake is less likely.