Both of these features move over time, migrating down stream. Material on the gently sloping stoss-side of these features rolls and jumps up the slope under the influence of water flow. Particles move up the slope until they reach the crest of the feature and then avalanche down the steeper lee-side to collect at the base of the next dune or ripple. This process is then repeated over and over again until the material reaches a location down stream where it is more permanently deposited. Floodplains develop when streams over-top their levees spreading discharge and suspended sediments over the land surface during floods .
Levees are ridges found along the sides of the stream channel composed of sand or gravel.
Levees are approximately one half to four times the channel width in diameter. Upon retreat of the flood waters, stream velocities are reduced causing the deposition of alluvium . Repeated flood cycles over time can result in the deposition of many successive layers of alluvial material. Floodplain deposits can raise the elevation of the stream bed . This process is called aggradation .