Levees are natural embankments which are formed when a river floods.  When a river floods friction with the floodplain leads to a rapid decrease in the velocity of the river and therefore its capacity to transport material.  Larger material is deposited closest to the river bank. This often leads to large, raised mounds being formed. Smaller material is deposited further away and leads to the formation of gently sloping sides of the levees. High pressure, caused by a river meeting its bank-full capacity, can cause a levee to burst.