A meander is a bend in a river. It is caused by helicoidal flow (the uneven movement of the river's discharge.
During the river's lowland course, lateral erosion becomes more important than vertical erosion since it's got too close to sea level. Therefore, as a result of lateral erosion, the sides of the river channel is eroded,. The force of the river water undercuts the bank on the outside of a bend to form a river cliff. An underwater current which carries the eroded materials, is deposited to form a gentle bank called a slip-off slope.
As lateral erosion continues, the bend of the meander becomes even more pronounced. Especially in times of flood, when the river's energy is much greater, the narrow neck of the meander may be breached so that the river flows straight again. The redundant meaner loop retains some water, forming a crescent-shaped ox-bow lake. Deposition during the flood helps to seal off edges and ends of the lake.