Lagoon

A lagoon is a shallow body of water separated from a larger body of water by a narrow landform such as a reef, a barrier island, a barrier peninsula or an isthmus. Atoll lagoons form when a tropical coral reef grows upward as the volcanic island it surrounds slowly subsides, eventually leaving only the reef standing above the water and often enclosing a lagoon more than 20 meters deep. Coastal lagoons instead develop along gently sloping shorelines where rising sea level or land subsidence allows a barrier island or reef to build offshore, producing a shallower lagoon behind it; they line roughly 15 percent of the world's coastlines and more than three-quarters of the coastline of the eastern and Gulf coasts of the United States. Coastal lagoons are further grouped as leaky, restricted or choked depending on how freely they exchange water with the sea, and their salinity, sediment and ecology vary with inlet size, rainfall, evaporation and freshwater inflow.