The body of water, which is often protected naturally from storm by barrier island and also contain a mixture of fresh and salt water is ESTUARY.
An estuary refers to a partially closed body of water, that have one or more rivers flowing into it and which has a connection to the open sea. Because the fresh and the sea water meet at this point, the water is always a combination of salt and fresh water. A good example of an estuary in America is the Chesapeake Bay.
