The correct answer is floodplain.
Floodplains are formed by a meander eroding sideways as they move downstream. When a river breaks its banks and floods, it leaves behind the layers of silt (alluvium). These slowly accumulate to form the floor of the floodplain.
The floodplains usually comprise unconsolidated sediments, generally stretching below the bed of the stream. These are gatherings of gravel, sand, silt, loam, and/or clay, and are usually important aquifers.
