The energy of a river flowing downhill causes erosion. There are five main ways in which river erosion happens.
Hydraulic action - the pressure of the water breaks rocks particles away from the bed and banks. Its strongest in rapids and waterfalls and during floods.
Abrasion (corrasion) - 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 also get rounded off as they rub together. Attrition doesn't erode the bed and banks - it just makes the particles of rock in the river smaller and more rounded.
Cavitation - air bubbles in turbulent stretches of water implode causing shockwaves that break pieces of rock off the banks and bed.
Corrosion (solution) - the dissolving of rock by chemical processes. Carbon dioxide dissolves in water form a weak acid, which reacts with rocks…