What is the flat land adjacent to a river called?

The flat land adjacent to a river is called a floodplain, or floodplains when referring to multiple areas. A floodplain is the low-lying land beside a river or stream that can be covered by water when the channel overflows.

Floodplains form over long periods as rivers spread beyond their banks and deposit layers of sediment such as sand, silt, and clay. Repeated flooding can create very fertile soils, which is why many farming regions and historic settlements developed beside rivers. The same process, however, makes floodplain communities vulnerable to damaging floods.

A floodplain is not identical to a river channel. The channel normally carries the river, while the floodplain is the surrounding area used by excess water during high flow. Its width and shape depend on the river’s slope, sediment load, valley, and history of flooding.

People sometimes confuse floodplains with deltas. A delta forms where a river deposits sediment at its mouth, usually near a sea or lake; a floodplain can occur along much of a river’s course.