Answer:
Floodplains and levees are formed by deposition in times of river flood. The river's 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. The sands, silts and clays, are similarly sorted with the sands being deposited next, then the silts and finally the lightest clays. Every time the river floods, deposition builds up the floodplain.
