Alluvial plains are formed when the flow of rivers slows down due to tectonic movement (upward to downward), topography (from mountainous areas to low valleys) and human factors (embankment construction, reservoir building). The formation of alluvial plains is based on the continuous and abundant supply of sediment in the upper reaches of the river and the continuous subsidence (or relative subsidence) of the crust in the accumulation area.

There are three conditions for the formation of alluvial plains:

①In terms of geological structure, it must be a relatively subsided or relatively stable area. In relatively subsided areas, thick alluvial plains are formed, and in relatively stable areas, thin alluvial plains are formed;

②In terms of topography, there must be a fairly wide valley or flat land;

③There must be enough sediment sources.

Basically, any river will have sedimentation in its lower reaches, especially some longer rivers. The largest alluvial plain in the world is the Amazon Plain, which is formed by the sedimentation of the upper reaches of the Amazon. The Yellow River Delta and the middle and lower reaches of the Yangtze River in China also belong to this terrain, and the Ningxia Plain in my country is also a typical alluvial plain. The North China Plain was formed by sedimentation in the Yellow River, Huai River, Haihe River and other large rivers.