Floodplains and levees are formed in the middle and lower course of a river. They are formed by deposition in times of river flood. The rivers load is composed of different sized particles. When a river floods it deposits the heaviest of these particles first. The larger particles, often pebble sized, form the levees.
Levees are raised banks right next to the river banks. As water continues to flood further away from the river banks, the sands are deposited next, then the silts and finally the lightest clays. This build up of sand, silt and clays leads to the formation of a flat piece of land ( floodplain) either side of the river, just after the levees. Every time the river floods deposition builds up the floodplain.