Why do alluvial soils build up on the Zambezi River floodplains?

Alluvial soils build up on the Zambezi River floodplains primarily because the river inundates these regions during its summer flood season.1

When the river overflows its banks, the velocity of the water is checked, causing it to deposit the materials it carries—such as silt, sand, clay, and gravel, along with organic matter. This process is most notable in broad, low-lying areas like the Barotse (or Zambezi) Plain, where these deposits accumulate to form fertile alluvial soils.231