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Major Ecosystems and Regions of the Acadian-Pontchartrain NAWQA
The study area can be divided into five "ecoregions", or areas of similar physical and biological characteristics as defined by Omernik in 1986.
The South Central Plains and the Southeastern Plains, in the northern part of the study unit, are hilly and forested with longleaf and slash pine, with dendritic and trellis drainage. Longleaf pine savannah and piney hills dominate this area.
In the southwest, the Western Gulf Coastal Plain consists of clayey flat plains with slow-moving bayous. Vegetation ranges from mixed pine forest in the north to wet prairie grassland in the south with interspersed oak chenier uplands.
The Mississippi River Valley Loess Plain, east of the Mississippi River, consists of bluff uplands, loessal (wind-blown) soils, small dentritic streams, and mixed hardwood forests.
The Mississippi River Alluvial Plain is characterized by low relief and slope, with cutoff lakes and distributary bayous, cypress-tupelo gum swamps, and fresh-to-saline marshes.
The red line shows the "Pleistocene Ridge", partly a political boundary, partly a geophysical boundary based on the high tide line and ancient coastline ridges, demarcates the uplands from the coastal lowlands.
Formation of the Mississippi Delta:
Over the last 8,000 years, the Mississippi River has created coastal land where there was once ocean, and reworked these floodplains through natural meandering and flooding. In geologic times, the ancient Mississippi Embayment filled in with river deposits, forming the flat Delta region of northern Louisiana and Mississippi, and later the coastal wetlands of southern Louisiana. The ancient Mississippi River created large delta lobes where it reached the Gulf of Mexico, and changed the location of its main channel several times, forming overlapping deltas with remnant channels. In the present time, the Mississippi River Deltaic Plain consists of a series of interdistributary streams (interconnected streams with complex flows) separated by former and present natural levees. Old main-river channels remain as smaller distributary bayous, such as Bayous Lafourche, Des Allemands, and Teche.
The Coastal Zone:
The coastal zone is a broad area affected by ocean-water and its tides. Different wetland types are determined by the salinity of the water in them, which may infiltrate naturally through bayous or reach further inland through canals. The Mississippi and its distributaries spread fresh water into the Gulf. Note the newly forming freshwater deltas of the Atchafalaya River. In this picture, dry land is dark green, open water dark blue, with the gradient of fresh, brackish, and saline in between. You can view the eastern and western coastal areas of Louisiana in greater detail.
The Atchafalaya, Modern Delta Formation, and Wetlands Loss:
The coastal zone of Louisiana contains almost 40 percent of the lower 48 states' coastal wetlands and includes the Atchafalaya River Basin, a large area of bottomland forest. The Atchafalaya River is the largest distributary of the Mississippi River, and historically has flooded regularly, allowing the development of large cypress-tupelo forests. The Atchafalaya is actively building a delta in Atchafalaya Bay, and its fresh waters are distributed across the coastal zone by the Gulf Intracoastal Waterway and natural bayous. Wetlands more distant from the mouth of the Atchafalaya are degrading more rapidly. Theories of the cause of coastal wetlands loss include the decreased supply of freshwater or new sediment, effects of increased agrichemicals on marsh plants, increased saltwater intrusion from navigation canals, and subsidence (natural sinking and compaction).
Cypress-tupelo swamps are one of the coastal, tidally-influenced ecosystems characterized by low-gradient, muddy-bottom channels with silty, moderate ionic-strength waters. Regular inundation prevents the forest from being overtaken by upland tree species. Tides and gentle change in elevation keep the water flowing very slowly, if at all, so that aquatic habitats are intermediate between streams and lakes in characteristics. Swamps require dry periods to allow tree seedlings to establish themselves.
Freshwater marshes develop where there is sufficient fresh water to prevent saltwater intrusion. Marshes will replace swamps where the soil is always covered with water. Some marshes develop on the mineral sediments of flood deposits. Coastal peat marshes (locally called flotant) develop in the absence of mineral sediment input, and resemble peat bogs.
Brackish and saltwater marshes develop where tides mingle with freshwater outflows, yet velocities are lower than the pounding of the surf. The gentle water flow allows the growth of plants like Spartina.
Wet prairies were the original vegetation of southwestern Louisiana before cultivation. The clayey soil retains water, and is ideal for growing rice. Streams in this area are muddy, slow-moving, and have very steep banks. Ground-water systems in this area are highly susceptible to saltwater intrusion. Our Mermentau Basin Study samples water quality and ecology of streams in this area.
Remnant beach dunes covered with oaks and brush, locally called cheniers, are scattered across the most costal areas of the wet prairie. Cheniers are dryer than the surrounding prairie, and allow trees and cacti to grow. The lack of surrounding vegetation, and their elevation above the prairie, exposes them to more winds, resulting in windswept growth of the woody vegetation.
Bottomland hardwood forest:
Bottomland hardwood forests are found in stream valleys and poorly drained areas of the uplands. Tree species in these places are tolerant of occasional flooding.
The Piney Hills are gentle-relief uplands in the more northerly parts of the study unit. Soils here are sandy and easily eroded, streams are characterized by unidirectional flow, low-ionic strength, and sandy/gravelly bed material. Streams like Whiskey Chitto can run very fast at high water, which will shift streambeds and not prevent aquatic plants from taking root.
Longleaf pine savanna:
Longleaf pine communities, largely flat uplands composed of longleaf pine, wiregrass, and abundant rare herbs, historically have grown in the far eastern part of southern Louisiana. Soils are sandy and well-drained, creating dry conditions. In natural conditions, fire maintains the savannah by preventing the growth of hardwoods that would shade out trees and herbs of this community.