Boston Harbor is located in Massachusetts Bay, itself part of the Gulf of Maine which is a projection of the Atlantic Ocean. Massachusetts and Cape Cod bays stretch between Cape Ann in the north and Plymouth in the south and eastward to a boundary marked by Gary E. Studds Stellwagen Bank National Marine Sanctuary. Together they encompass approximately 1,400 square miles; Massachusetts Bay itself embraces 800 square miles. East of the Boston Harbor Islands, Stellwagen Bank is a glacially deposited, submerged sand bank that forms critical feeding and breeding grounds for endangered marine species. The National Marine Sanctuary was designated by Congress in 1992 to protect and conserve the fragile ecosystem that the bank supports. It protects the feeding areas of migrating cetaceans (including Atlantic white-sided dolphins, harbor porpoises, and orca, pilot, and minke whales) and the nursery and feeding grounds of humpback, fin, and Northern right whales. Stellwagen is one of only 14 such sanctuaries in the United States.
Boston Harbor is an estuary system where the salt water of Massachusetts Bay mixes with fresh water from three rivers: the Charles, the Mystic, and the Neponset. The harbor shores include six of Boston’s neighborhoods (East Boston, Charlestown, North End, Fort Point, South Boston, and Dorchester) and seven other municipalities: Hull, Hingham, Weymouth, Quincy, Chelsea, Revere, and Winthrop. Although it has extensive development on its edge, the estuary provides valuable habitat for wildlife, a nursery for marine organisms, water filtration, and flood control.
The Inner Harbor includes the mouths of the Charles and Mystic rivers, Chelsea Creek, and the port of Boston; the Outer Harbor includes the three bays of Dorchester, Quincy, and Hingham and the mouth of the Neponset River. Combined, the Inner and Outer harbors comprise approximately 50 square miles and are bounded by 180 miles of shoreline.