Ecology of the Mississippi River Delta Region
WHAT IS CAUSING COASTAL EROSION?
As a Native American, I belief is that when the place where I live sinks under the sea,
the Houma Nation, my tribe, ceases to exist
- Steve Cheramie, 1999
Introduction
I. Subsidence
II. Levees
III. We've reached the edge of the continental shelf
IV. Reduction in available sediment in the river
V. Sea level rise
VI. Saltwater intrusion
VII. Toxic effects of sulfide accumulation in the wetland soils
VIII. Produced water
IX. Hurricanes and other storms
X. Canals and channelization
XI. Spoil banks
XII. Filling, drainage, and development
XIII. Loss of barrier islands
XIV. Life cycle of the coastal marshes
XV. Herbivory by wildlife
XVI. Summary
One would hope that if the cause of Louisiana's coastal wetland loss is simple, the solution might also be simple. Unfortunately, this is not so. Oh, yes, each of the many causative factors is rather simple, but for any given location, different combinations of factors are at work and each different combination demands different solutions.
The following are 15 of the problem causing factors that we recognize today.
I. SUBSIDENCE
Subsidence is simply a decrease in the level of the soil's surface. The level of the soil's surface is recorded relative to mean sea level (MSL), that is,
a house slab level may be recorded as -1 MSL, meaning that it is one foot below sea level. The actual scale used today is NGVD (National
Geodetic Vertical Datum), a fancy name that is equivalent to MSL. The old measure for stating the height of any surveyed object was Cairo Datum,
based on a bench mark at a Corps of Engineers facility in Cairo, Illinois. That bench mark was 20.43 ft above MSL, so one had to always subtract 20.43
from each Cairo Datum number to equate it to MSL. Aren't you glad they changed? Don't you wonder why they ever did it that way?
Many people seem to think that subsidence is a recent phenomenon that is quite simply a problem associated with house construction. They know that there are areas where, over time, the soil will recede below the slab, driveway, and sidewalk, causing cracking and the need to add more soil. Actually, subsidence began when dirt was "invented." It has always occurred and it will continue as long as soil exists! The major effect of subsidence along Louisiana's coastal zone is that as the soil subsides, the sea creeps further inland, covering valuable resources and places where we live.
There are two categories of subsidence:
- Surface subsidence: This type of subsidence, i.e., soils near the surface sinking, was not a problem in the not too distant past.
Though subsidence has always occurred in the delta, each year's high water would overtop the low, natural levees and spread new soil across
the river's floodplain. In most places, the river added more soil than that which had subsided during the year. This resulted in net growth of the
delta, so subsidence was not considered a villain, but just a natural process.
What causes surface subsidence?
- When soil particles fit closer together, they occupy less space so the surface sinks _ or subsides. The particles are very rough edged,
so it is easy to imagine how this might happen. Envision a family activity such as working on a jigsaw puzzle. When you lay out all the
pieces, the puzzle covers a relatively large space.
After being put together (=making the "rough edges" fit closer), the puzzle occupies much less space.
The same may be true of soils.
- When water is removed from soil, the spaces where the water was are now filled with air so the soil particles move closer together and the surface subsides.
- When organic matter (such as leaves, peat, etc.) decomposes, soil subsides as particles move into the spaces where the organic matter was.
- When plants die and their roots decay, soil subsides as the particles move closer together.
- When soil particles fit closer together, they occupy less space so the surface sinks _ or subsides. The particles are very rough edged,
so it is easy to imagine how this might happen. Envision a family activity such as working on a jigsaw puzzle. When you lay out all the
pieces, the puzzle covers a relatively large space.
- Geologic subsidence: This is subsidence caused by events deep in the earth. The delta consists of great quantities of sediment laid down over a great layer of salt (called the Louann salt) located some five miles below the surface. Huge chunks (fault blocks) of this salt (usually in the shape of polygons) subside, thus causing the surface to subside. Below a line running between New Orleans and the northwest side of Houma, the rate has been 3-4 in per century. This is a phenomenon that is out of human control.
II. Levees
Before levees, there were predictable variations in the alluvial environment - plants and animals (including native humans) adapted to and lived by those cycles.
Each spring, our mighty Mississippi River swells as a result of melting snow and spring rains in its drainage basin _ 1.25 million sq mi, or 41% of the continental U.S. (31 states and two Canadian provinces). In days gone by, the river frequently swelled above her natural levees and spread out across her flood plain, becoming a very wide river and flooding much of the area between New Orleans and Lafayette. Since the founding of New Orleans in 1718, people have tried to control flooding by building levees to protect their property. Originally, this was not done on a grand scale, but block_by_block or plantation_by_plantation.
The great flood of 1927 was the final straw. Louisiana citizens demanded that Congress allocate money to build a levee parallel to the river that would protect their property from future flooding. Congress mandated that the U.S. Army Corps of Engineers provide protection and a system of levees and upriver structures were built. Yea! Louisiana was now safe from floods and we could expand our settlements and associated facilities such as highways, railroads, industrial plants, power lines, etc. Life was wonderful and all rejoiced!
But guess what! As discussed above, soil in coastal Louisiana continued to subside! In fact, subsidence increased because of the absence of water in the soil and faster decomposition of organic matter. Before the human_made levees, the river would flow over the natural levees and spread out and deposit millions of tons of new soil in coastal Louisiana each spring. It deposited so much, in fact, that the coastal wetlands of the Mississippi River Delta grew and expanded.
Today, the levees prevent the sediment_laden water from reaching the wetlands, and subsidence continues, and the wetlands disappear as land becomes open water. We saved ourselves from floods, but we also directed the valuable sediments away from the coast. We didn't know that our valued levee protection would cause the loss of our coastal wetlands.
III. WE'VE REACHED THE EDGE OF THE CONTINENTAL SHELF
The logical question: Why doesn't the Delta just continue to grow into the Gulf? The Delta has reached the edge of the continental shelf.
The river's sediments are being deposited into the Gulf's depths, and they are not returning to the shore. If the Mississippi River is to build new
wetlands, it must change its course to flow toward the available continental shelf.
IV. REDUCTION IN AVAILABLE SEDIMENT IN THE RIVER
In the 1960s, the Mississippi River transported 600,000,000,000 lb of soil per year. If we loaded that much soil in 16 yard/load dump trucks and lined
them up bumper to bumper, they would extend around the earth almost two times _ a distance of about 50,000 mi. In 1983, the Mississippi moved only
366,000,000,000 lb, about one half as much as 20 yr earlier.
Presumably, much of this decrease in sediment load was a product of upstream soil conservation activities such as dams and soil erosion projects. Farmers in the heartland of America value their top_soil and use many erosion_control techniques to prevent its loss. This is good for them, but bad for us because we have less soil to enrich our coastal wetlands.
We don't know how much soil was transported in 1066, 1492, or 1776, and we can't project how much will be moved in 2000, but we are sure that big changes can definitely occur.
V. SEA LEVEL RISE
There is a finite amount of water on earth, but the term sea level is a relative phrase and it can change:
Tectonic changes don't appear to be at issue today, so let's first discuss glaciers.
During the height of the ice ages (the Pleistocene), lots of ocean water was trapped in glaciers, so sea level dropped and more land was exposed. There were three major glacial advances during the Pleistocene. During periods between glaciations, the ice caps melted and sea level rose, perhaps as much as 600 ft. In Louisiana, the coast of some 15,000 ybp (the height of the Wisconsin Glaciation, the largest of the three) extended 50 mi further south than Grand Isle of today (to the edge of the Mississippi Canyon). What is now the habitat of stingrays, sardines, pogie, sea turtles, and dolphins was once the domain of egrets, snakes, lizards, rabbits, and deer.
The most recent ice sheets began to melt 15,000 ybp. Geologists believe that sea level rose rather rapidly, but stabilized and has remained that way for the last 6,000 yr. See the adjacent figure "Gulf Coast Biostratigraphy" (last column) for sea level variation since the Eocene (last 40 million yr).
The normal, cyclical changes that happen in nature may be quite drastic in their extremes, but they are usually slow, orderly processes that all healthy components of the ecosystem can adapt to and thus survive (see the earlier discussion about the sea level rises and standstills of the Holocene transgression under “How Has the System Evolved”). Such is the case with the rise in sea level. It may be, as many say, that the eustatic (=world_wide) sea level rise is Mother Nature at work. But instead of the relatively smooth transition (with alternating spurts of rise and standstill) to a higher sea level, we find that the ice sheets are melting faster than they should, possibly due to the greenhouse effect. Atmospheric warming is causing the polar ice caps to melt faster than they normal. Additionally, warmer temperatures have caused the ocean's water to expand (steric expansion: as molecules of sea water components are heated, the arrangement of their atoms is adjusted so that they simply occupy more space). The oceans would occupy more space, consequently they will eventually cover more coastal wetlands.
While examining tidal gauges along the Louisiana coast, it was noted that there was a period between 1962 and 1975 when eustatic sea level rise may have occurred at a rate of 3 cm/yr (Penland et al., 1996: 5). This value is equal to the projections of sea level rise between the Holocene transgression standstills, meaning that what has been termed the 1960/1970 eustatic event was a large contributor to the rapid wetland loss during that period.
Sea level rise affects the world's coasts. If you lived on the White Cliffs of Dover that rise some 125 m above the sea, you wouldn't be concerned if told that world sea level will rise 6 inches by the year 2040. It would have, however, special significance along Louisiana's coastal lowlands. Louisiana is blessed with the most extensive coastal wetlands in America. There is little grade (elevation) change in some areas, and a sea level increase of 6 in might cover miles of existing marsh. But that's not all! The White Cliffs of Dover are standing sturdy with the water creeping up 6 in. The marshes of Louisiana, while being assaulted by the sea, are themselves sinking due to subsidence. Current estimations are that by the year 2040, the water along Louisiana's coast will be 30 in deeper than it is today: the sea goes up, the soil goes down, and we go under!
VI. SALTWATER INTRUSION
Each marsh plant species has its own tolerance for salinity (amount of salt in the water). Oyster grass and other species of the salt marsh can withstand sea
water, while species from freshwater wetlands require salt-free water.
If saltwater enters the fresher environs, the freshwater plants will die. Once dead, the marsh soil that their roots held together may erode away. If seeds of salt-tolerant plants are available, they may vegetate the newly salty environment. The main concern is that once fresh marsh areas die and turn to open water, the open water areas frequently enlarge, with no revegetation.
Do you remember making homemade ice cream? Did you ever pour the very salty water on your lawn when you were finished? What happened?
But there is another insidious problem associated with saltwater intrusion. Once an area is changed to saltwater, it becomes subject to tidal activity. The cyclical tides, regardless of height, move water into and out of marshes. Each time they move water out, organic materials are swept into open canals and ultimately to the sea. Very low tidal change may allow existing vegetation to filter out the organic matter and retain it in the marsh, but elevated tides will rob the marshes rapidly. While this is very good for our estuaries, it increases the rate of marsh loss.
A blow out is a place (usually narrow) where tidal water flows into a deteriorating oil well key hole from adjacent marshes. Since a blowout carries a lot of organic material, it is a great place to fish, but it causes the marsh from which it flows to change to open water surface since it continually removes organic matter.
VII. TOXIC EFFECTS OF SULFIDE ACCUMULATION IN THE WETLAND SOILS
In normal low salinity wetland systems, soils may cyclically stack up. Under natural circumstances, these soils will systematically be removed by natural
processes or, if they remain, the sulfides that occur will be gradually detoxified. If natural removal and detoxification are prevented by human activities such
as construction and maintenance of impoundments and canals (and their spoil banks), then toxic sulfide levels may increase and vegetation may die - possibly
never returning.
VIII. PRODUCED WATER
Water is produced from an oil well along with the oil. It is saltwater, usually 4_6 times more salty than the sea water (35 ppt) of the Gulf of Mexico.
Produced water is separated from oil through a number of processes. It is then usually disposed of by one of two methods:
- Deep well injection: it is injected deep underground. In the U.S. as a whole, 90% of produced water is injected this way.
- Discharged into surface water: In Louisiana coastal wetlands, 90% of produced water is discharged into surface waters, bayous, bays, canals, etc. In 1986, just 14 oil fields discharged 150,000,000 barrels of produced water.
The discharge of this water and its relationship to wetland loss has recently become a concern of the Louisiana State Legislature and others. Scientists at the Louisiana Universities Marine Consortium are studying this relationship.
Preliminary findings have not shown a significant impact to vegetation near a discharge site when the produced water is discharged directly into a water body. Historical aerial photography confirms these findings.
However, produced water discharged directly onto vegetated marsh instead of to a water body will have a serious impact. The plants will die.
In the late 1980s, it was estimated that about 730 million barrels (almost 31 billion gallons) of produced water were discharged annually in Louisiana waters. Not only is the brine potentially harmful, but there may be many other toxic substances present. Produced water has been shown to contain up to 2800 picocuries per liter of Radium 226 (the maximum allowable for the Riverbend Nuclear Plant near St. Francisville is 30 picocuries of Radium 226).
IX.HURRICANES AND OTHER STORMS
The heavier than normal pounding of waves on a beach or marsh washes soil and plants away. In SW Terrebonne Parish, Hurricane Andrew (1992)
left marshballs (big chunks of marsh vegetation with their roots and soil) scattered about and floatant marsh pushed up like a blanket. Some areas looked
the same as before the storm, but the floatant marsh had been moved 150 ft. This same process recreated 1500 acre lakes during the Hurricane of 1915.
Floatant marsh folded on itself during that storm formed ridges that were high enough so that upland plants, like Iva, began to grow and totally changed the
overall aspect of the marsh.
If a storm pushes large quantities of saltwater into a marsh, the effects may cause immediate plant die_back, but this impact is seldom long lasting, yet it may weaken the marsh. Saltwater that stays on the marsh will eventually completely kill the vegetation.
The greenhouse effect may cause more frequent and more powerful storms in the future by adding more warmth to the atmosphere.
On a positive note, some studies have shown that hurricanes may be important in bringing enriching sediments to wetlands. There are no simple situations!
Surges are not always uniform throughout the marsh. Hurricane Andrew had the following surge levels in a rather small area:
Cypremort Point - 0
Chauvin - 5.9 ft
Cocodrie/LUMCON - 8.3 ft
Just N Isle Dernier - 12.0 ft
X. CANALS AND CHANNELIZATION
These have four effects:
- Direct loss of marsh from canal construction. Canal surfaces made up 141.3 sq mi of our 4580.7 sq mi of coastal wetlands that existed in 1978.
- In order to be efficient, canals are usually deep and straight; they do not meander like natural bayous. Some (such as the Mississippi River_Gulf Outlet) are like straws that draw saltwater straight into freshwater habitats, thus killing the plants. Another aspect of deepened canals is that they allow tides to impact deeper into the marsh. This happens when tidal flows move into areas formerly not affected by them, pick up organic materials, and suck them out to the open sea, thus removing them from the marshes.
- Since canals are unnatural waterways, they distribute water differently than Mother Nature, thus upsetting the balanced natural flow within the complex marsh ecosystem. As an example, natural ridges protect freshwater marshes behind them. If we put canals through the ridges, salt and/or floods get through and harm the freshwater marsh.
- As boats travel through canals, their wakes slosh against the bank and slowly but surely wash the soil away. As time goes by, plants are washed away and the channel widens.
XI. SPOIL BANKS
Spoil banks are the piles of soil placed in ridges on the banks of a canal when it was constructed. They provide upland habitat in the marsh and enhance
production of certain wildlife, but there are problems.
Spoil banks are thought to harm wetlands in the following ways:
- The marsh they are built on is covered and lost. In Louisiana, spoil banks covered 169.2 sq mi of the 4580.7 sq mi of wetlands that existed in 1978.
- The flow of sediment_laded water into the marsh is blocked, the spoil banks serving as small but efficient levees.
- Spoil banks change wetland's hydrology (water movement). They directly block the flow of surface waters and their weight on the soil blocks the flow of subsurface water.
- Wetland drying cycles, caused by a natural, periodic absence of water, may be increased, thus promoting decomposition within the soil and causing subsidence.
- Soil flooding duration may be increased because spoil banks may block the draining process. This may result in marsh plants being under water too long or waterlogging of the soil, thus promoting changes in water chemistry.
Spoil banks may not always cause serious problems in the wetlands, but when they cross natural levees they may form ponds that lead to wetlands loss.
XII. FILLING, DRAINAGE, AND DEVELOPMENT
Though actual filling and drainage of wetlands is not presently a major problem (due only to the strict enforcement of laws and regulations), it has been in the
past and the threat still exists.
Be sure to question proposed levee alignments. They've been frequently abused _ at taxpayers expense for the benefits of a few! (see adjacent figure on the Ormond levee boondoggle).
XIII. LOSS OF BARRIER ISLANDS
Barrier islands protect our coastal wetlands from the constant wave action of the sea and from storm damage.
Hurricanes can cause devastating damage,
as discussed above. Hurricane Camille (1969) cut Cat and Timbalier Islands in two. Hurricane Andrew (1992)
virtually destroyed Timbalier Island.
Sea level rise is gradually flooding our barrier islands. As they are gradually inundated, they are more susceptible to natural storm damage and erosion.
XIV. LIFE CYCLE OF THE COASTAL MARSHES
It has been proposed that deltaic wetlands grow and degrade on a regular cycle. Garden Island Bay, located at the mouth of the Mississippi River, is
believed to have developed and degraded over a 150 year period. Will it build again? (see the adjacent map of changes in
Garden Island Bay from Gagliano, Light and Becker, 1973, p. 25).
What percentage of our present wetland loss might be attributed to these poorly understood marsh life cycles?
XV. HERBIVORY BY WILDLIFE
As discussed elsewhere, nutria, muskrat, and geese can and will eat all the vegetation in localized areas. When they consume all the surface vegetation
and then the roots, this is called an eat out. Nutria are the biggest problem at present because there is no market for them and they are severely
overpopulated. They also damage cypress trees by stripping the bark from the lower portions of the trunk during the winter. Large trees seem unaffected,
but smaller trees may die (they actually bite seedlings off at the base).
XVI. SUMMARY
A RECENT FURTHER EVALUATION
A recent study (Penland et al., 1996), initiated and funded by the Gas Research Institute through the Argonne National Laboratory,
sought to evaluate, using Geographic Information Systems (GIS) technology, the human and natural causes of coastal land loss.
They focused on localized impacts, and thus did not evaluate percentages for such huge contributors as river control, subsidence,
and eustacy (worldwide sea level rise). See adjacent table “Individual GIS Class Rankings of the
Localized Processes of Coastal Land Loss.”
Individual GIS Class Rankings of the Localized Processes of Coastal Land Loss¹
| Altered Hydrology² | 26.96% |
| Storms Shoreline Erosion³ | 23.81% |
| Sediment Loading³ | 15.40% |
| Oil and Gas Channels² | 11.01% |
| Waterlogging³ | 6.34% |
| Navigation Scour² | 4.76% |
| Mineral Extraction² | 4.07% |
| Failed Land Reclamation³ | 2.38% |
| Navigation Channels² | 1.65% |
| Borrow Pits² | 1.25% |
| Natural Scour³ | 0.61% |
| Drainage Channels² | 0.39% |
| Engineered Structured² | 0.15% |
| Lightning Burnout³ | 0.11% |
| Sweage Ponds² | 0.05% |
| Drainage Transport² | 0.03% |
¹ does not include the contribution of regional processes
² human processes: percent of total - 53.73%
³ natural processes: percent of total- 46.27%
Individual GIS Class Rankings of the Localized Processes of Coastal Land Loss¹
| Oil and Gas Canals² | 31.23% |
| Storms Shoreline Erosion³ | 21.81% |
| Sediment Loading³ | 15.40% |
| Navigation² | 13.15% |
| Waterlogging³ | 6.34% |
| Minerals Extraction² | 4.07% |
| Failed Land Reclamation² | 2.38% |
| Borrowed Pits³ | 1.25% |
| Other Natural Processes² | <2% |
| Other Human Processes³ | <2% |
¹ The indirect and direct contributions of oil and gas navigation are integrated in the GIS ranking.
² human processes
³ natural processes
Note that each of these values are percentages of direct cause. Thus, Oil and Gas directly caused 11.01% of localized loss. Human causes are quantified at 53.73%, and natural causes are 46.27%.
This table takes the same data and integrates the direct plus indirect impact of Oil and Gas and Navigation upon localized coastal land loss. These data show that Oil and Gas overall contribute to 31.23% of the loss.
SUMMARY OF REASONS FOR OVERALL LOSS OF COASTAL WETLANDS
Mother Nature
- natural sea level rise (especially the 1960/1970 eustatic event)
- subsidence
- storms/hurricanes
- barrier island loss
- marsh life cycles
- muskrats and friends
Human Activity
- increasing sea level rise
- sediment reduction
- accelerated barrier island loss
- levees
- channelization/canalization
- spoil banks
- filling, drainage, development
- saltwater intrusion
- increasing sulfide toxicity
- produced water
- barrier island erosion
- controlling the channel so the river mouth empties at the edge of the continental shelf
- introduced nutria!