Explain the formation of a flood plain.

Flood plains form when the discharge of the river channel exceeds it's capacity. As the river transports sediment via traction, suspension, saltation and solution the sediment is held within the river channel however, when the river discharge is higher than the capacity of the channel, the river bursts its banks and so the water and the sediment spill onto the surrounding flat land. This material is then deposited on this flat land and builds up over many years/flood events thus forming a flood plain.