Water flows at different speeds at different parts of the river. In the middle, farthest away from the sides (or banks), the water flows fastest. At the banks, the water flows slowest. When the river bends, water flows faster on the outside of the bend than the inside. The fast water on the outside 'erodes' the outside of the bend. This means it breaks bits of the bank off. At the same time, the slow water on the inside of the bend leaves behind mud, sand and parts of plants (deposition). Together, these make the bend move in the direction of the outside of the bend. 