barrier island

A long, narrow island of sand running parallel to the mainland coast, separated from it by a lagoon, bay, or marsh. Barrier islands are not permanent; they migrate, erode, overwash, and reform in response to storms, sea level, and sediment supply. They are the coast's expendable outer wall — absorbing wave energy, sheltering the mainland, and slowly rolling landward as the sea rises. Most of the Atlantic and Gulf coasts of the United States are fringed by barrier islands.