Alluvial plains are plain landforms formed by river sedimentation. The water flow in the lower reaches of the river is not as fast as that in the upper reaches. A large amount of silt is eroded from the upper reaches. When the flow rate is no longer sufficient to carry the silt to the lower reaches, the silt is deposited in the lower reaches. Especially when the river is flooded, the silt is deposited on both sides of the river, and the alluvial plain is gradually formed.

Famous alluvial plains include the Amazon Plain, the middle and lower reaches of the Yangtze River Plain, the Huanghuaihai Plain, the Ningxia Plain, etc.