Hydraulic action is the movement or wearing down of material by flowing water. In geographic processes, hydraulic action is also known as erosion.
In hydraulic action, the flow of water creates a force that knocks loose rocks and other materials and then sweeps these sediments away, creating erosion. This process is often seen in streams flowing through rocky or gravel banks, which might collapse over time.
Another type of stream erosion is abrasion, in which sediment scrapes along riverbeds and banks to wear them down and possibly form potholes. The third type of river erosion is corrosion or solution, in which carbon dioxide from carbonate rocks like limestone turns into acid and dissolves rock.