Floodplains form due to erosion and deposition. Erosion removes any interlocking spurs, creating a wide, flat area on either side of the river. During a flood, the material being carried by the river is deposited (as the river loses its speed and energy to transport material). Over time, the height of the floodplain increases as the material is deposited on either side of the river. The floodplain is often a wide, flat area caused by meanders shifting along the valley.
Find out more about landforms in the lower course of the river.