They are formed by the effects of water erosion and river sedimentation respectively.

River Valley:

Water flow carries silt to erode the river valley; the lateral erosion of water flow causes the valley slope to erode and retreat, including sheet erosion, gully erosion and block collapse on the valley slope; upstream erosion causes the river valley to extend upward and lengthen the valley.

Alluvial Plain:

In the lower reaches of the river, the water flow is not as fast as in the upper reaches. A large amount of silt is eroded from the upper reaches. When it reaches the lower reaches, the flow rate is no longer sufficient to carry the silt, and as a result, 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.