What are river cliffs and how are they formed?

River cliffs are features of meanders. In a meander the outside bank is where the Thalweg flows(fastest flowing part of the river), this means the velocity is greatest here so there is more available energy for erosion. The Helicodal flow (cock screw like flow of water that goes through rivers) causes surface water to flow towards the outside bank which causes lateral erosion through hydraulic action. Erosion of the outside bank also takes place through abrasion; as the velocity is greater here the river is able to carry larger sediment which increases the force of abrasion. The combined effect of this erosion causes the outside bank to be undercut and eventually causes it to collapse thereby forming a river cliff.