The Pressure of the water breaks rock particles away from the bed and banks. It is strongest in rapids and Waterfalls
Eroded pieces of rock in the water scrape and rub against the bed and banks, removing material. Most Erosion of river beds and banks happens by Abrasion
Attrition- Eroded rocks Smash into each other and break into smaller fragments. Their edges get rounded off as the rub together. Attrition doesn't erode the bed and banks- it just makes the particles of rock in the river smaller and more rounded
Air bubbles in turbulent stretches of water implode causing shock waves the break pieces of rock off the banks and bed
The dissolving of rock by chemical processes. Carbon Dioxide dissolves in water to form a weak acid, which reacts with rocks like limestone and chalk, breaking them down.