In the middle course the river has more energy and a high volume of water. The gradient here is gentle and lateral (sideways) erosion has widened the river channel.
The river channel has also deepened. A larger river channel means there is less friction, so the water flows faster.
As the river erodes laterally, to the right side then the left side, it forms large bends, and then horseshoe-like loops called meanders.
The formation of meanders is due to both deposition and erosion and meanders gradually migrate downstream.
The force of the water erodes and undercuts the river bank on the outside of the bend where water flow has most energy due to decreased friction. This will form a river cliff.
On the inside of the bend, where the river flow is slower, material is deposited, as there is more friction. This will form a slip-off slope.
Over time the horseshoe become tighter, until the ends become very close together. As the river breaks through, e.g. during a flood when the river has a higher discharge and more energy, and the ends join, the loop is cut-off from the main channel. The cut-off loop is called an oxbow lake.