EM 1110-2-1100 (Part I)
30 Apr 02
Figure I-2-5. Length (in km) of barrier islands and spits in the United States. Data measured from
U.S. Geological Survey topographic maps (see Table IV-2-3 for details)
d. Gulf of Mexico East: Wetland mangrove, and barrier coasts (Figures I-2-8, I-2-9). On Florida's Gulf
of Mexico coast, 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 doted with small vegetated
islands. The rock strata in this area are limestone, which, along with the low river gradients and numerous
ponds or sinkholes, accounts for the absence of sand in the region. Due to its location and the large shallow
water area offshore, little wave energy is present except during rare hurricanes. Some 130 km to the
northwest, the swamp coast ends. Here the coastal trend changes direction from north-south to east-west, and
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 (Figures I-2-10 and I-2-11).
Studies of the Mississippi delta indicate that the river has built a series of deltas into the Gulf of Mexico
during postglacial times and that the Balize Delta (bird foot) is the latest, with an age of about 1500 years.
The Bird Foot delta is southeast of New Orleans, lying among a series of old passes that extend for 300 km
(186 miles) along the coast. Most of the greater Mississippi delta is marshland and mud flats, with numerous
shallow lakes and intertwining channels (Figures I-2-12 and IV-3-9). The principal rivers have built natural
levees 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. In the areas of old delta
lobes, subsidence has left only the natural levees above water in some instances.
e. Gulf of Mexico West: Barrier coast. From western Louisiana, west of the Mississippi Delta marsh
coast, toward the southwest, barrier islands become the dominant coastal features. Some of the longest