When a river picks up material and then rubs it against its bed and banks. Erosion occurs by the process of abrasion, and is most effective during times of flood. It is the main method of both vertical and horizontal erosion.
Boulders collide with one another as they move down the river, and can break into smaller pieces. Over time rocks become more rounded in appearance.
The force of water that hits river banks, and then pushes water into cracks. Air becomes compressed, pressure increased and the riverbank may, in time collapse.
A continual process, which is as a result of the chemical composition of the water.