The Historic and Existing Louisiana Coastal Systems
River deltas are coastal accumulations of sediment derived from the land and organic material that rivers have brought to the sea as well as organic matter, predominantly peat, developed on the terrestrial delta surface. The shape or morphology of a delta is determined by the delicate balance of sediment accumulation, compaction, subsidence of the seafloor upon which the sediment accumulates, and eustatic (worldwide) sea level rise. Understanding these processes, and how human activity can alter them, is an important step in understanding and anticipating the evolution of a given delta.
The prevailing shape of any given delta depends on the rates of sediment supplied by the rivers and the patterns and rates of sediment dispersal by coastal ocean processes and by gravity (Wright, 1985). The size
of a delta depends, most importantly, on the annual sediment discharge of the river, but the most extensive deltas also tend to be developed where wide, gently sloping continental shelves provide a platform for prolonged organic and inorganic sediment accumulation and morphological progradation. The processes that disperse, transport, and deposit the sediment discharged by a river determine where, and at what rate, sediments accumulate or erode and thereby control the configuration of the resulting delta. This is true for both the subaqueous (underwater) component and the subaerial (terrestrial) delta, which must surmount the subaqueous deposits in order to prograde. Wright and Nittrouer (1995) argue that the fate of sediment seaward of river mouths involves at least four stages: (1) supply via river plumes, (2) initial deposition, (3) resuspension and onward transport by marine forces (e.g., waves and currents), and (4) long-term net accumulation. Different suites of processes dominate each stage. Interruption or alteration of any of these stages can impact the essential continual sediment nourishment of the delta.
The Mississippi River Delta, one of the world’s most extensively studied deltas, is composed of sediments from a catchment that covers much of the continental United States (3.2 million square kilometers [km2] or 1.2 million square miles [mi2]) or 41 percent of the lower 48 land area of the United States (Environmental Protection Agency, 2005). Unlike some deltas, the Mississippi River Delta contains more subaerial material than subaqueous deposits. Mississippi River sediments generally prevail over the central portion of the wide, passive Gulf Coast continental margin (Uchupi, 1975). These sediments have accumulated on the shelf and at the base of the continental slope as the Mississippi River cone.
Past and present subsidence of the shelf and coastal plain reflects the large-scale response to the loading of these sediments. Isopach maps, which depict the thickness of sedimentary deposits based on syntheses of cores and seismic data (Coleman and Roberts, 1988a,b), show that late Quaternary deposits exceed 0.1 kilometers (km) (0.06 miles [mi]) in thickness over most of the shelf, and Holocene thicknesses are as great as 0.5 km (0.3 mi) in locations of maximum deposition.
In a recent analysis of deltaic systems of the world, Walsh et al. (in press) characterize the Mississippi River Delta as “proximal-deposition dominated,” meaning that the bulk of the sediments discharged by the river were deposited close to the river mouth. In recent geologic history, a series of lobate deltaic projections were followed by avulsions, or channel switches. According to Kolb and Van Lopik (1966), at least 16 such lobes were created and abandoned in late Quaternary time (the last 20,000 years). More recent data indicate that since sea level reached its present, postglacial maximum approximately 7,000 years ago, six major lobes, including an incipient new one at the mouth of the Atchafalaya River dis-
tributary, have existed (Draut et al., 2005). The fifth lobe, the Balize Delta, has existed for the past 800–1,000 years, during which time it has evolved into the modern Birdsfoot Delta (Roberts, 1997).
Figure 2.1 shows the chronological distribution of the six major lobes. Abandoned deltaic lobes make up most of Louisiana’s coastal plain, and over the long term, the large-scale east-to-west dispersal of sediments has been affected more by these episodic channel switching events than by oceanographic factors, such as waves and currents. Throughout the middle to late Holocene, land loss in areas recently abandoned was the norm; however, these losses were balanced approximately by accretion in areas occupied by new lobes. The alternation alongshore between areas of erosion and areas of accretion has produced a highly irregular shoreline and corresponding irregularities in the inner shelf contours. As pointed out in the programmatic environmental impact statement of the Louisiana Coastal Area (LCA), Louisiana—Ecosystem Restoration Study (LCA Study) (U.S. Army Corps of Engineers, 2004a), coastal erosion has been a chronic feature of coastal Louisiana throughout the late Holocene because of a combination of subsidence, delta lobe abandonment, and wave-induced erosion. However, prior to modern interventions in the form of control structures, the channel switching that allowed wetlands to be lost in aban-
doned areas also caused new wetlands to be created in coastal areas that were supplied by a new sediment source.
The coastal region, which is west of the deltaic plain region influenced by the delta lobe avulsions, has historically been nourished by Mississippi River sediments transported westward by coastal processes, such as waves and currents, rather than delivered directly by river mouths. This region, Region 4, is the Chenier Plain (Figure 1.1). Louisiana’s Chenier Plain is a coastal morphological province characterized by a series of low-lying sandy ridges, often supporting stands of oak trees (chêne in French), separated by wide prograded mudflats capped by marshes (Gould and McFarlan, 1959). Episodic winnowing and reworking (by storms) of sands from within the tidal flats have formed the Chenier ridges. Mud accretion during intervening low-energy periods has produced the lower mudflats between ridges that have been subsequently colonized by salt marsh grasses. Thus, sandy, shelly eroding beaches and muddy accreting tidal flats and marshes have characterized the coast of this region through the Holocene.
The U.S. Geological Survey’s report (Barras et al., 2003) on coastal Louisiana land changes describes a complex mix of land losses and gains for this region, with land loss dominating. However, Roberts et al. (2002) and Bentley et al. (2003) demonstrate that westward movement over the inner shelf of sediments supplied by the Atchafalaya River mouth, about 100–150 km (62–93 mi) to the east, has contributed to episodic progradation of the Chenier Plain. Draut et al. (2005) conclude that the eastern portion of the Chenier Plain is the ultimate sink for about 7 percent of the sediment presently discharged by the Atchafalaya River. The westward transport of this sediment apparently takes place in the form of current-driven migration of a fluid mud layer (Bentley et al., 2003). According to Bentley et al. (2003), the passage of cold fronts causes both westward and onshore movement of the mud. The resulting coastal deposition is rapid but intermittent. Therefore, there is reason to believe that sediment bypassing the current delta system, as river flow is maintained in channels leading to Head of Passes, would be distributed naturally to the west, increasing the sedimentation and accumulation rates in areas west of the current channel.
THE MODERN, ANTHROPOGENICALLY MODIFIED RIVER AND DELTA
Significant anthropogenic changes to the Mississippi River Delta began with European settlement because the natural deltaic environment was not sufficiently stable for safe or comfortable European habitation. The initial changes to the land were local and relatively small compared
with modifications that commenced in the mid-nineteenth century and continue now (Syvitski et al., 2005). Changes in land use and dams upstream of Louisiana today have less of a direct impact on wetlands because levees and distributary closures have significantly separated the river from its delta.
External Changes to the Mississippi River Delta
Beginning in the nineteenth century, dams were constructed along the Mississippi River and its tributaries to improve navigation, control floods, and provide water for irrigation and electric power generation (Meade, 1995). These dams, especially in the Missouri River Basin, trap much of the suspended and bedload sediment within reservoirs. Changes in the river’s hydrologic profile have led to erosion of some portions of the banks and riverbed, which remain a source of coarser material transported as bedload and delivered to the delta. Similarly, artificial levees and armoring (constructed along riverbanks for flood protection) prevent sediment introduction from bank erosion and river meandering. Countering these changes, the longitudinal profile of the river was steepened by eliminating meanders, and artificial levees kept sediment in transport within the river rather than allowing it to escape to floodplains. Published assessments suggest that the suspended sediment load of the Mississippi River and the percentage of sand in the suspended load have decreased significantly since the mid-1800s (Kesel, 1988) (Figure 2.2; Table 2.1).
For example, the suspended-sediment discharge in the Mississippi River near Baton Rouge decreased from 500 million metric tons to 200 million metric tons between 1950 and 1982 (Figure 2.2). Although organic material derived from vegetation is an important constituent of deltaic wetlands, inorganic (mineral), river-derived sediment deposition is the most important means to build new land to the level where plants can thrive. Less sediment to build land is now available than in the historic past (Kesel, 2003).
As population growth and agricultural and industrial activity have increased within its drainage basin, the nutrient load of the Mississippi River has also increased (Antweiler et al., 1995). The flux of nutrients, primarily nitrates (NO3−) and orthophosphates (PO4−3), increases downstream where various tributaries enter the Mississippi River. This flux is the result of numerous inputs: (1) sewage treatment, industrial wastewater treatment, and stormwater discharge; (2) automobile exhaust and fossil fuel power plant emissions; and (3) agricultural runoff from animal waste and fertilizer (National Research Council, 2000a; Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, 2001). The nutrients are generally not at a high enough concentration to cause problems in the
TABLE 2.1 Estimates of the Suspended Sediment Load in the Lower Mississippi River (at New Orleans)
Suspended Sediment Transport (millions of tons per yr)
SOURCE: Kesel, 1988; with kind permission of Springer Science and Business Media.
river. However, when this dissolved plume—in particular NO3−—enters the Gulf of Mexico, it leads to phytoplankton blooms and creates a serious problem for the nearshore ecosystem (Turner and Rabalais, 1991, 1994), and when the dead plankton sink through the nearshore water masses, which are stratified by vertical salinity and temperature variations, the decay of the plankton depletes the bottom waters of oxygen and harms pelagic and benthic communities over a large area on the continental shelf. Evidence from cores indicates that this problem did not exist before 1900 and became acute after 1950 (Rabalais et al., 2002a).
Relative Sea Level Rise
Eustatic sea level rise occurs with respect to an absolute datum (and is reported as a worldwide average) as opposed to relative sea level rise, which occurs with respect to benchmarks established on land surfaces that may themselves be sinking or rising. Relative sea level rise, which is most relevant to the problem here, includes both true sea level rise and decreases or increases in land elevation from subsidence or uplift, respectively. Relative sea level rise in much of the Louisiana coastal area is approximately one order of magnitude greater than the eustatic rate. A component of relative sea level rise is due to eustatic sea level rise. Recent studies confirm that the Earth is in a period of global warming (National Research Council, 2000b). As the world’s oceans grow warmer, the ice caps will melt, increasing the volume of the ocean. Also, even modest warming increases the volume of water in the world’s oceans through thermal expansion (steric effects). During the twentieth century, eustatic sea level rise occurred at a rate of 1–2 millimeters (mm) per yr (0.04–0.08 inches [in] per yr), increasing the amount of coastal land that is submerged subject to erosional pressures and increasing the duration of flooding (U.S.
Army Corps of Engineers, 2004a). Relative sea level rise, which includes this and other components, contributes to erosional pressure and, therefore, to land loss—further complicating coastal restoration efforts. The causes and possible policy implications of eustatic sea level rise are beyond the scope of this report; however, restoration and protection efforts must address the problem, albeit indirectly through their treatment of relative sea level rise.
Internal Changes to the Mississippi River Delta
During the ninetieth and twentieth centuries, artificial levees were constructed almost to the mouth of the river and enlarged for flood protection. At the present time, about 3,620 km (2,250 mi) of levees prevent flooding of populated and agricultural areas in Louisiana and maintain navigation channels (U.S. Army Corps of Engineers, 2004a). Levees prevent overbank flooding and deny most of the delta region nutrient-rich river water and sediment. Instead, significant portions of sediment pass through the delta and accumulate on the outer continental shelf and in deeper waters of the Gulf of Mexico. The loss of this inorganic sediment from annual floods is a major underlying cause of land loss in coastal salt marshes experiencing rates of local, relative sea level rise up to 1 centimeter (cm) per yr (0.4 in per yr) (DeLaune et al., 1983, 1994; Reed, 1995).
Levees not only prevent widespread flooding; they inhibit crevasse splay formation. Crevasses occurred as large breaks through the natural levees and were a mechanism through which the river built land rapidly to the side of the main distributary channel. The average area of a crevasse splay was estimated at 1,683 km2 (650 mi2) (Davis, 2000), and as many as 20 were active in the period 1750–1927 (Figure 2.3). After the flood of 1927, higher levees, coupled with construction of the Bonne Carre Spillway that allowed flood waters to empty into Lake Ponchartrain, eliminated most crevasses above the Head of Passes near the river mouth. (A significant exception is the Bohemia Spillway and recently developed crevasse splays near Fort St. Phillip.) New and planned river diversions, which attempt to mimic natural processes of land building, are located largely at sites of former crevasses.
Canals and pipelines with associated spoil banks represent another major engineering impact on the delta wetlands. There are 10 major navigation canals and 14,973 km (9,300 mi) of pipelines in coastal Louisiana servicing approximately 50,000 oil and gas production facilities (U.S. Army Corps of Engineers, 2004a). The direct loss of land from these human modifications is large, but Turner (1997) suggests that most wetland loss results from secondary effects caused by canal and pipeline dredging. Canals dredged perpendicular to the coast allow saltwater to intrude
into and degrade freshwater wetlands (Day et al., 2000). The Mississippi River Gulf Outlet (a 122-km [76-mi] long, man-made navigational channel connecting the Gulf of Mexico to the city of New Orleans) is often cited as a prime example of this phenomenon (Day et al., 2000; National Research Council, 2004a). Canals and spoil banks approximately parallel to the shoreline lead to ponding of water and drowning of wetlands through altered hydrology. Turner (1997) argues that most indirect wetland loss is caused by these effects. However, Day et al. (2000) claim that the influence of dredging on wetland loss varies spatially, and only 9 percent of the nondirect loss is due to altered hydrology and saltwater intrusion. These differing points of view highlight the knowledge gaps that exist with regard to the effectiveness of specific and localized wetland protection and restoration efforts.
Many towns and several large cities, including New Orleans, have been developed on the delta plain. These communities occupy large areas of former wetlands, and some are situated on the high ground of the natural levees. Where they are protected by artificial levees and by forced surface drainage (Snowden et al., 1980), no land is being lost, but urban impact on the natural landscape ranges from freshwater consumption and sewage disposal to levee construction for protection from storms and floods. Forced drainage in urban areas leads to extreme local subsidence and a need for higher levees. Industrial centers, principally those associated with oil and gas extraction and refinement, are often separate from population centers and require independent protection. Wetlands drained to allow agriculture represent lost wetlands but not necessarily lost land. Road and rail networks connecting human populations and industrial centers also destroy wetlands and require maintenance and protection from flooding. The protection of existing infrastructure from flooding was considered during the development of the LCA Study, but the impacts of Hurricane Katrina have drawn greater attention to the role wetlands play in protecting key infrastructure and urban development.
Large-scale human activities have taken many decades to have an impact, and it is difficult to separate their influence from many smaller-scale, shorter-term actions. The Old River Control Complex, for example, was completed by the U.S. Army Corps of Engineers in 1963 to prevent the Atchafalaya River from completely capturing the discharge of the Mississippi River. The structure apportions about 30 percent of the overall discharge of the Mississippi and Red Rivers to the Atchafalaya River and the remainder to the Mississippi River. Without the structure, the Atchafalaya River may have already captured most of the overall discharge. Although the Atchafalaya River would be building land faster as a result, the Birdsfoot Delta would be in a state of severe erosion, and navigation and the water supply of New Orleans area would be affected.
Much of its sediment mass would have been dispersed to the west, and a barrier island system would have begun to form from its channel, bar, and levee sands. Large hydrologic and salinity changes would also have accompanied this natural change.
Growth faults have existed in the Mississippi River Delta plain for millions of years and are a natural component of the landscape (Gagliano et al., 2003). Land sinks along growth faults on geologic time scales and can locally accelerate the rate of relative sea level rise. Morton et al. (2002, 2003a) attribute high rates of wetland loss to fault reactivation caused by pressure reductions in petroleum fields. Although the amount of land loss resulting directly from fluid withdrawal for oil and gas production is uncertain, there is a close spatial and temporal association between changes in petroleum field pressure reductions and land losses in nearby wetlands (Morton et al., 2002). As much as 43 percent of the marsh loss in southern Louisiana occurs in “hot spots” that Morton et al. (2003b) suspect are related to hydrocarbon extraction.
Shore protection structures, such as seawalls, jetties, groins, and breakwaters, are a more recent human influence on the coast and are limited to a few barrier islands and the Chenier Plain. These structures were intended to control the movement of sand and the position of the shoreline but sometimes have had unintended consequences. Areas of erosion downdrift from jetties (termed erosion shadows) are cited as a major cause of beach erosion on the Chenier Plain (Penland et al., 2004). Some prominent construction, such as seawalls near East Timbalier Island and jetties near Empire Pass Inlet, are no longer influencing shoreline sand movement because land loss has made them isolated, open water structures. The behavior of hard engineering structures during large storms in terms of their success in protecting land and property has never been proven.
The cumulative impact of human activities on sediment dispersal has generally been one of reducing sediment contributions from the Mississippi River to the delta wetlands. Instead of spreading its sediment and nutrients over a vast delta plain, the river discharges its reduced sediment load directly onto the outer continental shelf and upper slope. Land building (outbuilding) and delta maintenance (upbuilding) (Coleman et al., 1998), the large sedimentary processes through which the Mississippi River Delta was built, no longer occur. Instead of following the past history of delta growth, avulsion, and destruction, the modern Birdsfoot Delta has extended across the continental shelf.
The coastal physical oceanographic regime of the Louisiana shelf is presently scaled and constrained by the elongated Birdsfoot Delta, which creates a barrier to east-west currents (Wiseman and Dinnel, 1988). By blocking alongshore flow, this active delta lobe causes shelf circulation to be divided into two cells: one to the east and one to the west of the
Birdsfoot feature. The most recent and comprehensive analysis of the shelf circulation in this region is that of Smith and Jacobs (2005) who use extensive sets of current observational data with the governing dynamic shallow water equations. Results show that depth-averaged flows to the west of the Birdsfoot Delta during spring and summer are weak and variable but generally set from west to east. Stronger and more persistent flows from east to west prevail during autumn and winter. These flows are essential to the delivery of suspended sediments to the disappearing wetlands of Regions 2 and 3 (Figure 1.1). Research indicates that east-to-west migration of a layer of fluid mud on the Louisiana inner shelf plays an important role in nourishing the coast of southwestern Louisiana and in moderating the impact of storm waves on the coast (Huh et al., 2001; Bentley et al., 2003; Sheremet and Stone, 2003; Stone et al., 2003). Human intervention, particularly by maintenance activities or potential changes in management strategy of the Birdsfoot Delta system, has a significant influence on the coastal circulation and transport processes in this region.
Future Scenarios: Desirable Versus Attainable
The ideal future condition of the Mississippi River Delta would be one that achieves the goal of Coast 2050: Toward a Sustainable Coastal Louisiana (Coast 2050)—a sustainable “coastal ecosystem that supports and protects the environment, economy, and culture of southern Louisiana” (Louisiana Coastal Wetlands Conservation and Restoration Task Force and the Wetlands Conservation and Restoration Authority, 1998). To be “sustainable” requires that Mississippi River water, nutrients, and sediment be effectively spread across the wetlands of the delta. Modern human occupation requires that the sediment flow occur without river flooding or meandering in inhabited areas. Restoration to achieve this goal will require a new and significant system to convey water and sediments from the river water around inhabited areas to where it is needed. In the absence of natural avulsion, this conveyance system will require numerous large diversion structures, or a controlled—but more natural—evolution must be allowed to occur; both approaches require that pathways for water flow be created or expanded.
River sediment is essential to allow wetlands to keep pace with subsidence and sea level rise, and freshwater is needed to maintain salinity gradients. To a degree, these objectives are achieved through diversions, but canals and channels that allow saltwater to penetrate into freshwater and brackish water wetlands must be blocked. Since many of these channels are related to essential navigation needs, in an ideal world a new waterway system that separates fluvial processes from navigation would be required to protect salinity gradients.
Under conditions ideal for wetland maintenance, Mississippi River waters will disperse through wetlands before entering the ocean. Wetlands contribute to the removal of nutrients by plant uptake of soluble material and flow reduction, which enhances the deposition of organic and inorganic sediments. In surface layers, nitrification occurs and is followed by denitrification as the nitrates are carried to anoxic zones in sediments or in suspended sediment regions near the bottom. Phosphates are bound to sediments, and both phosphates and nitrates are taken up by plants. Wetlands also promote flow reductions, which enhance nutrient removal and the deposition of organic and inorganic sediments.
In the normal course of delta evolution, barrier islands move and eventually drown, and the marshes landward of them erode, supplementing the sediment supply for islands down-current through the alongshore drift. This is a part of the natural system that is not considered desirable from the perspective of wetlands restoration if the rate of wetland loss exceeds the formation of new wetlands. Thus, some maintenance of the existing barriers, or other structures, is necessary to protect some areas and to slow the loss of wetlands in other areas. Ideally, these would be self-sustaining or require little human maintenance (Reading, 1978), a characteristic that may be difficult to achieve.
With a larger and growing delta to buffer populated areas from storm waves and surges, public safety would be enhanced. Although this ideal state is a goal of the LCA Study and follows on the broad suggestions of Coast 2050, “restoration,” in the strictest sense, cannot be achieved by what is proposed in the LCA Study or any plausible group of projects today. The Mississippi River Delta is inherently dynamic and large. Even maintenance of the status quo would require unreasonable quantities of sediment to travel great distances at unreasonable cost. No reasonably scoped effort will bring back the Mississippi River Delta of historic times. Those responsible for restoration efforts in Louisiana have to clarify that if these projects are executed successfully, the future delta will contain all of the landform types that exist today; however, those landforms will be smaller in size, and some will be located in different places than today. To conserve resources and focus effort where it will be most beneficial, some presently inhabited regions may have to be abandoned or relocated. If this is undertaken in a carefully planned manner that views processes on the scale of decades rather than years, the impacts to individuals and communities can be minimized.
Although the future map of Louisiana likely will not look like earlier versions, many currently nonfunctional deltaic processes can be restored, and people can live safely in a more sustainable environment than exists today. In short, Louisianans can draw their own future map of the state that enhances natural processes, ensures sustainability over decadal time
scales, and protects the key values and infrastructure. The Chenier Plain, for example, can never be restored to a previous state and sustain a large human population. Growth of the Atchafalaya River Delta will eventually bring mud to the west, changing the essential character of the coastal areas in the area (i.e., sandy beaches replaced by finer-grained, muddier systems). Some areas in Regions 2 and 3 (Figure 1.1) that are too far from sediment sources to be rebuilt may become the bays and open water of tomorrow so that new land can be built to protect preferred ecosystems and population centers elsewhere. Furthermore, erosion and coastal inundation by marine water represents an important phase of delta development when submerged coastal habitats may be at their peak in productivity. Most of the seaward areas of Region 1 (Figure 1.1) will apparently be abandoned since no land-building efforts are directed to the Chandeleur Islands or the wetlands landward of them. The selection of projects now, and in the future, should be viewed as an effort to balance specific mechanisms that will draw the new map of Louisiana and not efforts that will bring back the past.
THE FUTURE LOUISIANA COASTAL SYSTEM
The Mississippi River Delta has changed constantly throughout history but experienced net growth in land area until human activities inhibited delta-building processes. The extent of human activities and population growth in Louisiana has exacerbated land loss associated with natural processes and placed the current population and its infrastructure at risk from storms and land erosion. The natural and anthropogenic processes contributing to net land loss in coastal Louisiana are significant and pervasive and have been operating for decades. Furthermore, achieving no net loss may be problematic because of the limited sediment supply; the large affected area; and the extensive social, political, and economic impediments. It is not possible to restore the earlier extent of the delta or to maintain the present status of coastal Louisiana. However, through the selection of appropriate projects and with abandonment of areas that economically cannot be saved, a sustainable delta environment and management strategy can be achieved. Additionally, the LCA Study needs to convey the message that the map of Louisiana will change in the future. To achieve this, the development of an explicit map of the expected future landscape of coastal Louisiana should be a priority as the implementation of the LCA Study moves ahead. Such an explicit declaration of the proposed “end state” of restoration efforts in Louisiana provides an important performance metric. Development of such a map will also require meaningful stakeholder involvement and the commitment of decision makers at all levels of local, state, and federal government.