* Erosion: Erosion is the process of wearing away and transporting material. While erosion plays a role in the initial formation of a river valley, it's not the primary force behind the flood plain itself.

* Deposition: Deposition is the process of dropping off sediment. During floods, rivers overflow their banks, carrying sediment and depositing it on the surrounding land. Over time, these deposits build up, forming the flat, fertile areas we call flood plains.

Think of it this way:

* Erosion: The river carves out the valley, making room for the flood plain.

* Deposition: The floodwaters spread out and drop off sediment, filling in the valley and creating the flat, fertile plain.

So, the flood plain is a result of the river depositing material during its periodic overflows.