When a river carrying sediment overflows it's banks the coarsest sediment is dropped first as the river no longer has the energy to carry it. The coarse material forms a small natural embankment...
Banks of coarse river load and other material that has been transported down river, that are made when the river floods and bedload is carried out wider then before and desposited. It is left with...
well it's a bit obvious, because artificial levees are man-made so they can make the levees as big as they want, but natural levees are natural (obviously), but they are just sloww process sort of...