What is the process by which a floodplain forms?

Answer: Floodplains are formed in two ways : erosion and aggradation

Explanation:

The floodplain caused by erosion is created as stream cuts deeper into the channel and laterally onto its banks. A stream with a steep gradient will tend to down cut faster than it causes lateral erosion, resulting in a deep, narrow channel with little or no floodplain at all.