151—Gulf Coast Marsh

This MLRA (shown in red in the figure above) is in Louisiana (95 percent), Texas (4 percent), and Mississippi (1 percent). It makes up about 8,495 square miles (22,015 square kilometers). The towns of Gretna, Chalmette, and Marrero, Louisiana, and the city of New Orleans, Louisiana, are in the eastern part of this MLRA. The town of Port Arthur, Texas, is in the western part. Interstate 10 and U.S. Highway 90 cross the area. The New Orleans Naval Air Station is in this MLRA. Fort Jackson, overlooking the mouth of the Mississippi River, and the Jean Lafitte National Historic Park and Preserve are in the MLRA. A number of national wildlife refuges and State parks occur throughout this area.

Physiography

Vermilion Bay splits this area into an eastern half and a western half. The eastern half is in the Mississippi Alluvial Plain Section of the Coastal Plain Province of the Atlantic Plain. The western half is in the West Gulf Coastal Plain Section of the same province and division. The land east of Vermilion Bay, part of the Mississippi River Delta, has a ragged shoreline. The land west of Vermilion Bay has a smoother shoreline. Low, narrow sandy ridges characterize much of the area. There are many rivers, lakes, bayous, tidal channels, and manmade canals. Elevation generally ranges from sea level to about 7 feet (2 meters). It is as much as 10 feet (3 meters) on beach ridges, canal spoil banks, and natural levees and as much as 165 feet (50 meters) on salt dome islands. Some areas that are protected by levees have subsided below sea level.

The extent of the major Hydrologic Unit Areas (identified by four-digit numbers) that make up this MLRA is as follows: Lower Mississippi (0809), 60 percent; Louisiana Coastal (0808), 31 percent; Galveston Bay-San Jacinto (1204), 5 percent; Lower Mississippi-Lake Maurepas (0807), 2 percent; Sabine (1201), 1 percent; and Pearl (0318), 1 percent. The Sabine River forms the boundary between Texas and Louisiana in this area, and the Beckwith and Calcasieu Rivers are at the western end of the area, in Louisiana. Other rivers in the part of the area in Louisiana include the Bayou Nezpique, Mermentau, Vermilion, Bayou Teche, Atchafalaya, and Mississippi Rivers.

Geology

The surface of this area is primarily Mississippi River clay, silt, and fine sand deposited over the past 2 million years. The eastern half of the area, part of the Mississippi River Delta, is underlain by a mixture of Recent alluvial material and Pleistocene-age marine sediments. The area west of Vermilion Bay is underlain by older alluvial and marine sediments. Salt domes, natural gas, and petroleum deposits are below the surface in this area.

Climate

The average annual precipitation in most of the eastern half of this area is 60 to 65 inches (1,525 to 1,650 millimeters). It is 49 to 60 inches (1,245 to 1,525 millimeters) in most of the western half. About 70 percent of the precipitation occurs during the growing season. Rainfall typically occurs as post-frontal precipitation in the winter and heat-convection showers and thundershowers in the spring and summer. In addition, tropical storms can bring large amounts of rainfall. The average annual temperature is 67 to 69 degrees F (19 to 21 degrees C). The freeze-free period averages 325 days and ranges from 290 to 365 days, increasing in length from north to south.

Water

Following are the estimated withdrawals of freshwater by use in this MLRA:

Public supply—surface water, 6.5%; ground water, 0.8%
Livestock—surface water, 3.9%; ground water, 0.8%
Irrigation—surface water, 7.6%; ground water, 0.8%
Other—surface water, 75.3%; ground water, 4.4%

The total withdrawals average 1,310 million gallons per day (4,958 million liters per day). About 7 percent is from ground water sources, and 93 percent is from surface water sources. Most of the water used in this area is for public supply, thermoelectric power plants, and industry in the northeast corner. This area has many rivers, lakes, bayous, tidal channels, and manmade canals. About one-half of the marsh is fresh, and one-half is salty. Tidal channels allow free movement of salty water from the Gulf of Mexico into the parts of this area adjacent to the Gulf. Most of the area is susceptible to flooding either by freshwater drained from lands adjacent to the marsh or by saltwater from the Gulf of Mexico. Daily tides flood some areas. High tides and storm surges resulting from hurricanes or tropical storms can be as much as 10 feet (3 meters) above sea level and can flood most of the area.

River water in this area is generally of poor quality and requires treatment prior to human consumption. Contamination by fecal coliform bacteria is high in some rivers, and sediment problems from nonpoint sources and contamination by industrial wastes can occur in all of the rivers. Most soils of this area are very poorly drained, having a water table at or above the surface most of the time.

Ground water is scarce east of Vermilion Bay, in the Mississippi Delta. Fresh ground water is available in moderate to large quantities from the Gulf Coast (Texas) and Chicot (Louisiana) aquifer systems west of Vermilion Bay. The water from these Pleistocene-age river deposits is hard, and its median level of total dissolved solids is 300 to 450 parts per million (milligrams per liter). Iron concentrations exceed the national secondary standard for drinking water and approach 1,000 parts per billion (micrograms per liter) in some wells in Louisiana.

Soils

The dominant soil orders in this MLRA are Entisols and Histosols. The soils in the area dominantly have a hyperthermic soil temperature regime, an aquic soil moisture regime, and smectitic mineralogy. They generally are very deep, very poorly drained, and clayey. Hydraquents (Bancker, Creole, Larose, and Scatlake series) formed in clayey sediments in coastal marshes. Haplosaprists formed in organic deposits over alluvium (Allemands, Clovelly, and Lafitte series) or entirely in organic deposits (Kenner and Timbalier series).

Biological Resources

This area supports freshwater and saltwater marsh vegetation consisting of grasses, sedges, rushes, and other plants. Alligatorweed, spikerush, maidencane, cutgrass, and bulltongue characterize the freshwater vegetation. Roseau, common reed, bulltongue, and marshhay cordgrass characterize the intermediate water vegetation. Marshhay cordgrass, saltgrass, and Olney bulrush characterize the brackish water vegetation. Saltgrass, marshhay cordgrass, smooth cordgrass, and black needlerush are included in the saltwater vegetation.

Some of the major wildlife species in this area are white-tailed deer, alligator, nutria, raccoon, otter, muskrat, swamp rabbit, cottontail rabbit, mink, mottled duck, bobwhite quail, mourning dove, meadowlark, lark bunting, and crawfish.

Land Use

Following are the various kinds of land use in this MLRA:

Cropland—private, 16%
Grassland—private, 6%
Forest—private, 8%
Urban development—private, 3%
Water—private, 33%
Other—private, 30%; Federal, 4%

Most of this area supports marsh vegetation and is used for wildlife habitat. The area is almost treeless. Much of the area is uninhabited. The area is in the fertile and productive estuarine complex that supports the marine life of the Gulf of Mexico. The area provides wintering ground for millions of migratory ducks and geese and habitat for many fur-bearing animals and for alligators. A significant acreage west of Vermilion Bay is firm enough to support livestock and is grazed by cattle in winter. A small acreage of freshwater marsh is drained by pumping systems and is used for pasture or for rice.

The major resource concerns are determined by land use and marsh type. Flooding is a major concern in New Orleans. The concerns in areas of native marsh include maintenance of the salinity level in the soils, ingress and egress of freshwater or saltwater, and the content of organic matter in the soils. The concerns on pasture and cropland include maintenance of the content of organic matter and control of the salinity level in the soils. Erosion caused by overland water from high rainfall or a storm surge in the Gulf is a concern in areas where the native vegetation has been altered.

Conservation practices on cropland include systems of crop residue management, which help to control erosion and maintain the content of organic matter in the soils. Timely tillage and planting can help to maintain tilth and the supply of soil moisture and control salinity. The practices on pasture include prescribed grazing, brush and pest management, prescribed burning, and watering facilities. Management of upland and wetland wildlife habitat is needed.

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