When a river overflows its banks, it deposits material called alluvium.

Here's why:

* Alluvium is a general term for sediment deposited by a river.

* When a river floods, it carries sediment (sand, silt, clay, and sometimes larger debris) downstream.

* As the water spreads out over the floodplain, it loses energy and drops the sediment it was carrying.

* This deposited sediment is called alluvium, and it can create fertile floodplains that are ideal for agriculture.