Natural Levee Formation

The buildup of low ridges paralleling a river channel as floodwater spilling over the banks loses velocity and drops its coarsest sediment closest to the channel, leaving progressively finer material further out across the floodplain. Repeated flooding raises natural levees higher over time, and where a river's bed also aggrades within them, the channel can come to sit above the level of its surrounding floodplain, as the lower Mississippi and Yellow rivers both do.