Mississippi River Delta: Land Subsidence and Coastal Erosion

The Mississippi River Delta is a major center for transportation, industry, human population and ecosystem services. Critical areas included energy production, navigation, fisheries, flood protection of coastal communities, and restoration of damaged habitats. Complex environmental management in a great river system requires broad-base complex science, engineering and monitoring. A major national and state objective has become the restoration of the Mississippi River Delta that is threatened by subsidence, flooding, storm surges, compaction, oil extraction and gas extraction. The primary objectives of the paper are to document the landscape and geological properties of the Mississippi River Delta which have contributed to the successful resource and economic development of a historically-rich region of North America and to document the natural resource and environmental risks to the Mississippi River Delta. Economic and urban development of the Mississippi River Delta by the oil and gas industry and creation of levees by the USACE has contributed to land subsidence problems. Environmental challenges include land subsidence as a result of the pumping of vast amounts of oil and gas, the lack of sediment deposition in the Mississippi River Delta as a result of a system of levees, coastal erosion impacts of hurricanes, disposal of untreated and treated wastewater, periodic flooding and water pollution.


Introduction
The Mississippi River's major deltaic cycle began approximately 7000 years ago.
The modern Mississippi River Delta was formed as the Mississippi River depo-

Historical Geology of the Mississippi River Delta
Holocene Mississippi Delta began to form approximately 5000 BCE as the rate of sea-level rise slowed and was outpaced by the rate of sediment delivery to the basin [2]. The delta was comprised of smaller sub-deltas that overlapped over time [3]. Eventually, the Mississippi River Delta was formed by at least six delta complexes ( Figure 1). The Mississippi River Delta complexes consist of smaller areas known as delta lobes, which include basins and other natural landscapes of the coastline. These are identified as Mariggouin, Teche, St. Bernard, La Fourche, Plaquemine-Balize and Atchafalalya and Wax Lake Outlet sub-deltas.
After each deltaic cycle, which offered a shorter route for the Mississippi River to the Gulf of Mexico, the older delta lobe was abandoned. This cut off the primary supply of fresh water and sediments to an older lobe, where these sediments underwent compaction, subsidence and erosion.
The sub-deltas delivered their sediment load to the Mississippi River basin and built land through processes of pro-gradation into bay environments, aggradation by crevasse and overbank deposition. Sub-deltas were initiated with stream capture, which delivered both freshwater and sediment to new depo-centers and were ultimately abandoned as the depo-center changed positions in response to river changes in alignment [3]. The historic and prehistoric delta lobes of the Mississippi River Delta have influenced the formation of the Louisiana coastline and created over 1.6 m ha of coastal wetlands [4]. Periods of land In the natural deltaic cycle, the land (delta lobes) that form where the river meets the sea grows outward extending from the river as it overflows its banks and deposits sediment into the basins. This process is known as pro-gradation.
Further from the river's mouth, the processes of subsidence, erosion and sea-level rise dominate and can result in coastal retreat. The creation of the Mississippi River Delta with integrated natural pro-gradation and degradation processes resulted in a mix of unique habitats, ecosystems and landforms.
The Atchafalaya River is the largest distributary of the Mississippi River and is also considered to be a significant part of the continual land-building process within the Mississippi River Delta. Around the middle of the twentieth century, the Atchafalaya River's tributary channel was formed as part of the Atchafalaya and Wax Lake lobes [5].

Cultural History
Native Americans called Paleoindiano settled on prairies west of the Teche re-  [1]. The $15 million land acquisition became known as the Louisiana Purchase. The delta became a significant part of farming industries (Figure 8). Navigation and trade on the Mississippi River increased after the Civil War, and this economic development eventually moved into the Delta. During the 1870s, former Delta swamplands were converted to fertile farmlands via levee construction. Timber companies harvest forests, and crop planters followed, to capitalize on new agricultural opportunities. Railroads entered the Delta replacing steam boats as the primary means of transporting crops and produce. Vast gas and oil deposits brought further environmental and economic changes in the Delta. The openings of the Panama Canal in 1914 increases the Mississippi River Delta traffic. Shipping and commercial and local fisheries continued to expand.
The Mississippi River Delta has become home to 2 million people. The location of the Delta at the mouth of the Mississippi River allowed the area to be the Gateway to the United States and resulted in a mix of nationalities creating the diversity of the region. The French were Louisiana's first 18 th century colonists who were later joined by the Spanish and the Acadian settlers. By the 19 th century, European immigrant groups included the German, Sicilian and Irish. Later the African, West Indians and Native Americans were added to the mix. The combination of these groups over time created the unique culture found in the Mississippi River Delta. Two unique groups were Creoles and Cajuns. Cajuns refer to a white, black or mixed-race native of Louisiana [6]. They were the offspring from the union of various ethnic groups. Creole populations before 1803 were typically born of French and/or Spanish parents who kept their European characteristics and cultures.

Bayou Teche
The Bayou Teche begins in Port Barre and flows south to meet the Lower Atc- to headwaters of the Teche, then south between the bayou and Atchafalaya swamp, and for a few miles past the bayou's mouth at Patterson ( Figure 10).
The huge West Atchafalaya basin berm was large enough to support a gravel road and changed the Teche by separating the bayou from its important fresh water sources including the Atchafalaya River, the Red River and the Mississippi River. It also blocked Lake Daulerive and Lake Fausse Point from discharging seasonal rainwater into the Atchafalaya Basin.

Delta Today
Louisiana's wetlands are one of the US's most important and productive natural assets consisting of natural levees, barrier islands, swamps, forests ( Figure 11)

Fisheries in the Mississippi River Delta
The  into the Gulf of Mexico [9]. Despite these changes, many commercial fisheries in Louisiana have remained stable over the past few decades, which is puzzling with the magnitude of degradation is considered [9] [12]. This trend may be occurring if changes in the population first occurred prior to the period of record for fisheries, if the edge of marsh habitat is the key factor influencing fish populations rather than total area, or if nutrient inputs that stimulate productivity to compensate for habitat degradation [9]. At present, links between the Mississippi River plume ecosystem and fisheries productivity need additional research to clearly identify trends and patterns and better inform predictions of future changes.
A challenge of restoration activities to restore coastal lands is the tradeoff between the long-term benefits of land building and potential short-term impacts to marine fisheries and populations of commercially important species. Freshwater diversions intended to deliver sediments and nutrients to help build wetlands can result in enhanced productivity for fisheries, but can also lead to eutrophication, algal blooms or hypoxia that can negatively impact marine fisheries [13]. For example, models suggest that the diversion of Mississippi River water into estuaries can alter salinity or chlorophyll concentrations that can cause the movement of fish to other habitats, often further offshore [11] [14], although this pattern is not universal [9]. Similarly, measures to reduce nutrient inputs into the Gulf of Mexico can result in reduced areas of hypoxia, but may negatively impact the productivity of marine ecosystems [9]. Research is therefore needed to define the optimum balance between the amount and timing of freshwater diversions, resulting in the maximum delivery of sediment while transporting a small amount of water [14].

Natural and Anthropogenic Threats to the Mississippi River Delta
Sediment is considered the key to the management of river-dominated coastal zones. Previously, most sediments from the Lower Mississippi River were transported into the deep water of the Gulf of Mexico [15]. However, 50% of sediments load in the Lower Mississippi River is shunted through the South and Southwest passes, the major discharging outlets, after these sediments settle out of the river's water column and aggrade on the channel floor [16].
In the 19 th century, the Mississippi River channel dynamics shifted from a plentiful sediment supply to a supply-limited system. This shift occurred between Cairo (Illinois) and the confluence of the Mississippi and Ohio rivers, adding to the levees that increased velocities and limited overbanking which enhanced sediment export. During the middle part of the 20 th century the river was in an erosive site [17] but now is a dynamic equilibrium or aggrading (adding sediment). Shoaling, where a river channel becomes shallower, is occurring. Shoaling increases when sea levels rise and where the Mississippi River slope is reduced, or if changes in the volume of water carried by the river's distributaries occur. Both could affect navigation and commerce.
Coastal restoration plans include diverting part of the river flow to bring freshwater and new sediments to the subsiding basin [18] [19]. Flow through gated, controlled diversions can optimize land building and reduce adverse impacts such as shoaling and/or eutrophication. River diversions are proposed in Louisiana's coastal plan [20].
Subsidence is a significant threat to the Mississippi River Delta. In the absence of sediment inputs, the Mississippi River Delta subsidence is occurring at a greater rate than for other deltas in the United States. The subsidence rate may be further exacerbated through fluid extraction by the oil and gas industry ( Figure 17). Storms and hurricanes are natural threats to the Delta. The first line of protection for communities and cities from hurricanes and storm surges are the coastal wetlands and barrier islands. As these landscapes weaken, the coast is more vulnerable to flooding and strong winds. Permanent loss of 518 km 2 of wetlands occurred during the Katrina and Rita hurricanes.
Sea level rise in combination with saltwater intrusion, subsidence, hurricanes and storms has resulted in coastal erosion. Lack of sediments into these flooded wetland areas also accelerates the rate at which sea-level rise rate which affects the region [8]. The same combination of events is occurring in the Mekong Delta [21]. As global sea levels rise, the Mississippi River Delta areas that experience subsidence may flood and become open water ( Figure 18). The levees, floodways, and basin and channel improvements were created to improve flood Open Journal of Soil Science  protection for residents (Figure 19), agriculture (Figures 20-23) and communities, and has been effective. However, this protection has come at a high environmental cost for the delta's natural landscape and ecosystems ( Figure 24) as the levees serve as the link between the river and surrounding wetlands. The river carried freshwater and sediment that is needed for land growth and in the delta, but the levees block the process and cut off the deposition of sediment in most of the delta. Since the 1950s, ten major navigation canals ( Figure 13

Mississippi River Delta Restoration
During the past 10 to 20 years a number of steps have been taken to increase the resiliency of coastal Louisiana. It took thousands of years to build the Mississippi River Delta, but the land loss is now happening at a much faster pace. Research is occurring to find feasible and effective projects to mitigate additional land loss and accelerate the delta rebuilding processes. By 2100, as much as 10,000 to 13,000 km 2 of the Mississippi River Delta may be lost or submerged.
After Hurricane Katrina, the Mississippi River Gulf Outlet Canal caused the USACE to develop an ecosystem restoration plan that included closing the    The Mississippi River suspended sediment concentration decreased between 1950 and 1966 (before the construction of 6 dams on the "Big Muddy" Missouri River) [21]. Since then, observed decreases in the suspended sediment load has been minimal, even with the use of reduced tillage and no till conservation prac-

Case Study: Sediment Needed in Coastal Zones
Sediment is considered the key to managing river-dominated coastal zones. Previously, the sediment from the Lower Mississippi River was transported into the deep waters of the Gulf of Mexico [15]. However, now 50% of the sediment load passes the major discharging outlets after these sediments settle out of the river's water column and aggrade (add sediment) to the channel floor [16].
In the 19 th century, the Mississippi River channel dynamics shifted from a plentiful sediment supply to a supply-limited system. This shift resulted in the Mississippi River channel south of the confluence of the Ohio and Mississippi rivers becoming a natural, self-scouring system [17]. This shift resulted in a shortening of the river, increased velocities and adding levees that limited overbanking and enhanced sediment export. Shoaling where a river channel becomes shallower is occurring. Shoaling reduces the slope of the Mississippi River or if changes in the volume of water carried by the river's distributaries occur and increases when sea levels rise. Both scenarios could affect navigation and commerce ( Figure 26 and Figure 27).
An important policy objective is restoring the Mississippi River Delta and adjacent environments. The Mississippi River Delta is a major center for human   [24]. River diversions are proposed in Louisiana's Coastal Master plan [20]. Critical aspects of this plan include navigation [25], energy production, fisheries, salinity, water quality, restoration of damaged habitats and flood protection of coastal communities. Coastal restoration plans rely in part on diverting a portion of the river flow to bring freshwater and new sediments into the subsiding basin [18] [19]. Flow through gated and controlled diversions can reduce adverse impacts such as shoaling and eutrophication and maximize land building. Complex environmental management in great river systems requires broad-based complex science, monitoring and engineering [25].

Summary and Conclusions
The lack of sediments can be attributed to conservation practices on the uplands in the Mississippi River basin, levee systems including dikes and weirs, earthen levees and navigation channels. Many of the structures slow or eliminate the Mississippi River flow to bottomland and marsh areas and increasing saltwater intrusion from the Gulf of Mexico into freshwater wetlands.
Measures to accommodate deep-draft navigation, such as dredging, are needed between the Mississippi River and the Gulf of Mexico. Any reduction in the flow of the Mississippi River will result in an increase in dredging requirements and a reduction of the depth of naturally occurring channels. The need for oil and gas for national security is also a national priority but appears to be contributing to the subsidence. Sediment collected in the 6 huge Missouri river reservoirs reduces the Lower Mississippi sediment load. In addition, there is a need to cut soil loss from erosion on the North Central uplands and reduce transport to the streams in the Mississippi River Valley. This has resulted in the reduction of the sediment load of the major tributaries to the Lower Mississippi Open Journal of Soil Science River, including the Missouri, Upper Mississippi and Illinois rivers and their tributaries. The jury is still out on the use of the thousands of dikes and weirs on the Mississippi and Missouri rivers and their tributaries, which appear to sustain the navigation river channels but contribute to moving the sediment load to the Gulf of Mexico via suspension and dispersion.
The Mississippi River Delta shoreline is eroding and significant land areas and wetlands are being converted to open water. The wetlands and landmasses are also subsiding as a result of oil and gas extraction, consolidation, and reduced sediment loads in the lower Mississippi and Missouri rivers as a result of the use of 13,000 weirs and dikes that create turbulence and keep sediment in suspension into the Gulf of Mexico. These activities should help restore critical habitat for many commercially important marine fisheries, and ensure sustainability into the future.
The USACE is trying to mitigate by adding sediment to existing Lower Mississippi River wetlands by creating conditions similar to the Atchafalaya River Delta. A USACE plan is needed if any attempt is made to preserve the existing shoreline and wetlands that buffer future storm surges. The object of stabilizing the existing delta and marshes is a secondary concern. Attempts to stabilize the current shoreline will require the partnering with two major land owners the U.S. Fish and Wildlife Service (USFWS) and the Louisiana Department of Wildlife and Fisheries (LDWF). Additional mitigation attempts are needed to address the loss of wetlands and shorelines.