Final answer:
The factor that causes the most erosion on river banks and beaches is waves coming onto shore.
Step-by-step explanation:
The factor that causes the most erosion on the edges of river banks and beaches is the waves coming onto shore. Waves carry a significant amount of energy, and when they crash onto the shore, they can erode the land by wearing away the Earth's surface. This erosion is a result of the movement of water, which is one of the key processes of erosion.