Water flows slowly over shallow areas (riffles) in the riverbed and faster through pools, deeper sections of the river. This leads to helicoidal flow that corkscrews from one bank to another. Fast flowing water on the outside bank causes lateral erosion through hydraulic action and abrasion which undercuts the bank, creating a river cliff. The eroded material is then deposited on the inside of the next meander where friction slows the flow, creating a slip-off slope.
Find out more about meanders and oxbow lakes.