Hydraulic action is the erosive effect of water. The force of moving water can be an effective agent of erosion. It is important both in river a coastlines. In rivers the movement of currents causes hydraulic action, though is a load of silt or sand were added the process would be called corrasion. Along the coast, breaking waves batter cliffs and may open out joints in the rock, forming caves or undermining the cliff and leading to collapse.