The coasts along the Gulf of Mexico are a collection of mangrove and marsh coasts, coral reefs, river deltas, and barrier islands. On Florida's Gulf of Mexico coast, carbonate coal reefs are found around the Florida Keys and scattered to the north and west. Barrier islands begin at Cape Romano and extend north as far as Cedar Keys. Enclosed bays usually have an abundance of mangrove islands and the topography is low with many lakes and marshes. North of Cedar Keys, the barrier islands end. They are replaced by a vast marsh dotted with small vegetated islands. Some 130 km to the northwest, the swamp coast ends.
The coastal trend changes direction from north-south to east-west. Ochlockonee Bay, with drainage from the southern Appalachian Mountains, provides quartz sand for redevelopment of barrier islands. These sandy islands, with their various openings for access to the lowland port cities, continue westward as far as the Mississippi River delta.
Most of the greater Mississippi delta is marshland and mud flats, with numerous shallow lakes and intertwining channels. The principal rivers have built channels along their course. These natural levees are about a meter above the normal water level, but many of them have been artificially raised to provide protection to towns and cities from floods. Aquatic plants cover the marshland, which is remarkable for the huge population of waterfowl it supports.
West of the Mississippi delta marsh coast, toward the southwest, barrier islands again become the dominant coastal features. Some of the longest barrier islands in the world are located along the Texas coast.