Mississippi Delta

James S. Aber

Wetland Environments
Emporia State University

Table of Contents
Introduction Barrier islands
Hurricanes Related sites

Introduction

The Mississippi Delta region comprises much of coastal Louisiana and adjacent Mississippi, stretching from the Atchafalaya Bay on the west to the Chandeleur Islands on the east, and includes the metropolitan area of New Orleans. The delta complex contains major river channels and levees, numerous bayous, swamps and marshes, lakes, tidal flats and channels, barrier islands, and shallow sea enviroments. Water chemistry grades from fresh to brackish to marine. The climate is subtropical; freezing conditions occur rarely, almost never near the coast. The major climatic events are hurricanes that strike the region every few decades, and floods derived from upstream runoff. Economic activities include shipping, traditional fishing and farming enterprises, as well as oil production and petrochemical processing. Recreation and tourism are likewise quite siginificant for the local economy.

Space-shuttle photograph of the Mississippi Delta region, southeastern Louisiana and southwestern Mississippi. A - Grand Isle, B - Head of Passes, Mississippi River, C - Chandeleur Islands, D - Mississippi Sound. Adapted from NASA JSC, STS-51C-143-027, 1/85.

See pdf handout for additional maps and illustrations.

Mississippi Sound--prior to Hurricane Katrina
View inland near Buccaneer State Park, Mississippi showing a tidal-flat meadow with a meandering stream in foreground and artificial reservoir in background. Forests occupy dry, sandy soils of higher ground. Kite aerial photograph, 3/04; © J.S. Aber.
Closeup view of tidal-flat meadow and upland forest. Deciduous trees are just beginning to leaf out in this early spring scene. Kite aerial photograph, 3/04; © J.S. Aber.

The foundation of the modern Mississippi Delta was constructed during the Pleistocene (ice age), when melt water from ice sheets poured down the Ohio, Missouri and Mississippi valleys. These melt-water floods transported huge volumes of sediment that accumulated in the delta. Since the end of glaciation, the delta has undergone continual change during the past several millennia. As distributary channels (passes) build farther and farther into the sea, river gradient is reduced. Eventually the channel gradient becomes too low for water to flow readily. When this happens a new channel, with a shorter route and steeper gradient, may be carved during flooding, and then a new delta lobe begins to build. In this way, the water flow within and across the delta shifts back and forth, and many delta lobes have been constructed during past centuries.

Landsat TM false-color image of the modern Mississippi Delta. Pink and red depict active, emergent vegetation; cyan shows water rich in suspended sediment (mud); dark blue to black is relatively clear water. Image obtained from NASA GSFC.

Ecosystems of coastal Louisiana.

Coastal Louisiana has experienced rapid loss of wetland habitats, in which wetlands become open-water environments. Rates of loss exceeded 100 km² per year in the early 1980s. These rates have declined to 60-70 km² yearly loss more recently, which remains the greatest wetland conversion for any state in the nation. The losses are due to a combination of natural and human factors, including land subsidence, rising sea level, and human drainage modifications. For more information, see Guide to wetlands (Dugan 2005, p. 84-86).

Barrier islands

When a lobe of the delta is abandoned by a shift in drainage, that portion begins slowly to subside into the sea and is further reduced by erosion. Some of the sediment may be reworked by wind and waves into barrier islands; the Chandeleur Islands are an excellent example of this situation. Another example is Grand Isle; it is the only barrier island accessible via highway in Louisiana. Grand Isle has a long history of agricultural, recreational, and industrial development. First colonized by the Spanish in the 1700s, the island was shifting sand without any tree cover. French Creole settlers took over in the 1800s with agriculture and fishing. Gradually woodland vegetation became established on the dune sands--oaks and oleander. Salty meadows, marshes and lagoons occupy the lower terrain. The island remained relatively inaccessible until offshore oil production began in the late 1940s. Electricity and telephones arrived in the 1960s along with a recreational boom.

Grand Isle, Louisiana--prior to Hurricane Katrina
View toward the southwest; open sea to left, protected water of Barataria Bay to right. Notice the recreational housing and industrial development. Kite aerial photograph, 3/04; © J.S. Aber.
Recreational development in foreground includes raised houses, boat sheds and canals. An oil terminal can be seen in the background. Kite aerial photograph, 3/04; © J.S. Aber.
U.S. Coast Guard station to right, recreational houses and boat sheds to left, and Barataria Bay visible in background. Kite aerial photograph, 3/04; © J.S. Aber.
View toward northeast; open sea to right, Barataria Bay to left. Another barrier island is visible in the distance. The open channel provides passage for ships between the sea and bay. Kite aerial photograph, 3/04; © J.S. Aber.
Seaward view over vegetated sand dunes and beach. The elevated walkway provides access to the beach without disturbing the dunes at Grand Isle State Park. Photo date 3/04; © J.S. Aber.
View over vegetated dunes at Grand Isle State Park. The woodland consists of oaks and oleander. A salty meadow and lagoon appear in the left background. Photo date 3/04; © J.S. Aber.

Hurricanes

Hurricanes are a way of life in the Mississippi delta and adjacent Gulf coast region. For example, major hurricanes impacted Grand Isle in 1893 and 1965. The hurricane season of 2005 was the most devastating in recent times, when Hurricane Katrina struck New Orleans and Hurricane Rita came onshore in western Louisiana. Some 100 square miles (260 kmē) of marshes were converted into open water, as a consequence of erosion during storm surges. Most of this wetland loss was attributed to Hurricane Katrina and took place east of the Mississippi River in St. Bernard and Plaquemines parishes and in Breton Sound. Hurricane Rita caused marsh loss west of the Mississippi River to the Texas border.

The eye of Hurricane Katrina passed directly over the Chandeleur Islands, a chain of delicate barrier islands east of the modern delta (see above). These islands were heavily damages by the storm; initial estimates indicate the land area of the islands was reduced by about one half. Hurricane Katrina was the latest of five hurricanes to impact the Chandeleur Islands during the brief period 1998 to 2005. Grand Isle also was heavily affected by Katrina's storm surge and high winds that caused widespread structural damage to most buildings on the island.

The long-term wetland retreat in the Mississippi delta region (see above) may have contributed to severity of damage during the hurricane season of 2005. Loss of coastal wetlands has reduced the capacity to absorb storm surges. Flooding has become more frequent and deeper in some delta areas. The exact contribution of wetland loss to storm damage is difficult to quantify, but it is undoubtedly significant. Efforts to mitigate this situation will be quite expensive ($2 billion) and require decades to put into place, as detailed in the Coast 2050 plan.

Related sites


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© Notice: Wetland Environments is presented for the use and benefit of students enrolled at Emporia State University. Any other use of text, imagery or curriculum materials is prohibited without permission of the instructor, J.S. Aber (2007).