Geography – 2. The natural environment | e-Consult

Hydraulic action is the force of the water striking the banks and bed of the river. This force can compress air in cracks and fissures, causing them to widen. The sheer power of the water can also dislodge loose material. The effectiveness of hydraulic action is increased by the velocity of the water and the presence of air bubbles. For example, hydraulic action is particularly effective on vertical cliffs and rocky shorelines.

Abrasion occurs when sediment carried by the river is hurled against the riverbed and banks. This acts like sandpaper, wearing away the rock. The size and hardness of the sediment, as well as the velocity of the river, influence the rate of abrasion. Larger, harder sediment causes more rapid erosion. Abrasion is most effective when the river is carrying a significant load of sediment.

Landforms resulting from hydraulic action and abrasion include:

• Cliffs: Formed where hydraulic action attacks a coastline, undercutting the base and causing the overlying rock to collapse.
• Sea caves: Created when hydraulic action and abrasion exploit weaknesses in headlands.
• Arches: Formed when caves on either side of a headland meet, creating a natural arch.
• Stacks: Formed when an arch collapses, leaving a isolated pillar of rock.