Accretion is the gradual buildup of sand and silt along the shores or banks of a waterway. For an easy way to picture accretion, the Gulf of Mexico and the Mississippi River provide a clear picture. The Mississippi River is one of the largest in the country and carries a variety of dirt, sediment and debris in its waters. When the Mississippi reaches the Gulf of Mexico, much of the sand and dirt that is carried is deposited. The effects of accretion have caused all of the deltas that exist in the area. Barrier islands are another example of accretion. The ocean routinely takes sand with it when it laps at the beach on a barrier island. The sand is then carried in the ocean's waters and deposited at another location. The deposition of any silt, sand or debris along the banks of a river, stream or ocean is accretion.