* River Erosion: Rivers constantly erode their channels, carrying sediment downstream.

* Flooding: During floods, the river overflows its banks, spreading out over a wider area.

* Deposition: As the floodwaters slow down, they lose energy and deposit the sediment they were carrying. This sediment builds up over time, creating the flat, fertile land of a floodplain.

In summary: Floodplains are the result of a river's erosional and depositional processes, with deposition being the key factor in their formation.