The force of the water hitting the river bed and banks. This is most effective when the water is moving fast and when there is a lot of it.
Solution
When the load carried by the river repeatedly hit the bed and banks dislodging particles into the flow of the river
When the stone in the river hit each other, making them smaller.
When the river flows over a material like limestone or chalk they dissolve. This is because these materials are soluble in mildly acidic river water.