The four types, or processes, by which a river erodes are hydraulic action, abrasion or corrasion, attrition and corrosion. Each process affects the river in different ways.
Hydraulic action is where the force of the river water pushes into holes in the riverbank and causes the riverbank to collapse. Hydraulic action also breaks rocks away from the bed and bank. Abrasion, or carrasion, occurs when the water picks up sediment and small pebbles, and they rub against the riverbed, causing holes to form.
Attrition smooths the edges of broken rock, and sediment becomes smaller. Corrosion happens when carbon dioxide dissolves in the water and forms an acid. This dissolves rocks like limestone and chalk.