* Erosion: Erosion is the process of wearing away and transporting material. While erosion plays a role in shaping the landscape around a river, it's not the primary force behind floodplain formation.

* Deposition: Deposition is the process of dropping sediment that has been transported by water, wind, or ice. Floodplains form when a river overflows its banks during floods, causing the water to slow down and deposit sediment. Over time, these deposited layers build up, creating a flat, fertile area alongside the river.

In short: Erosion helps carve out the river channel, but deposition builds up the flat, fertile area we call a floodplain.