Humans and the Louisiana Coastal Wetlands
| Taming the Mighty Mississippi
The quest to tame the Mississippi began as the result of a large flood
in 1927: Congress initiated a massive plan to levee the river, seal
off crevasses, and cut off meanders to increase slope and accelerate runoff.
Because of this the delta has moved out into the Gulf to the edge of the
continental shelf. This plan also included the construction of numerous
dams along the Mississippi and its tributaries. The Old River Control
Structure was placed 480 km upstream from the mouth to send a maximum of
1/3 of the flow into the Atchafalaya river which also has extensive levees.
The Mississippi River Gulf Outlet was a major canal dug in the 1960's
to facilitate the passage of ships from the Gulf of Mexico into the Mississippi.
The dredging directly destroyed 23,000 acres of wetland and has since
grown 250% in size, costing the state 7.6 million dollars/year to maintain.
|Loss of Sediment
Sediment discharge was historically approximately 270 million cubic meters/year
of suspended load and 130 million cubic meters/year of bedload. This
has decreased 80% since 1850 and can be divided into three periods:
historical period (pre 1900), pre-dam period (1932-1952), and post dam
(1963-1982). Suspended sediment loads declined 43% between the historical
and pre-dam and 51% from pre-dam to post-dam periods. The size of
sediment also decreased drastically including a 72% decrease in the sand
fraction. Most of this is due to dams on the tributaries acting as
sediment traps primarily for the coarser sediments. Large-scale land
clearing for agriculture contributed to increased sediment loads in the
Canals have been built for several purposes: mainly for transportation,
oil and gas exploration, and well maintenance. Canals and their spoils
account for 10-30% of the direct wetland loss and secondary effects such
as expansion and hydrologic changes add significantly to the impact of canals.
Navigation canals cause serious degradation due to the depth to which they
are dredged. Because they are significantly deeper than the surrounding
marsh, water is pulled in from the marshes leading to salt water intrusion
deep into the marshes. A complex system of medium sized canals were
dredged and continue to be dredged to facilitate oil and gas exploration.
These canals change circulation patterns and increase salt-water intrusion.
Natural channels such as tidal creeks are greatly impacted by canals
due to the loss of flow. These natural canals represent some of the
most productive areas of the marsh, while man-made canals tend to be among
the least productive areas. Canal dredging creates spoil levees on
either side which impact water flow through the marsh and create semi-lagoons.
All of the stresses associated with canals can be modeled in a
linear relationship between marsh loss and canal density.
Agricultural development has also led to the large scale destruction
of wetland. More than 800 square kilometers were dredged and drained
to create agricultural and suburban lands. These reclamation projects
experienced rapid subsidence as the soils oxidized and compacted and have
since been flooded by levee breaches to create vast tracks of rectangular
Resulting Land Loss
|Land loss is the dominant process
occurring in the Louisiana coastal marshes, and a majority of this is due
to anthropogenic alterations. Flooding is essential to the health of
the marshes and represents the primary accretionary processes through deposition
of mineral sediments from fluvial and marine processes, which occur during
the construction phase of deltaic cycle. These sediments control plant
growth more through introducing nutrients than through structural means.
Flooding also flushes toxic sulfides from the anoxic soils. The anthropogenic
alterations have virtually eliminated both flooding and sediment deposition
which has resulted in staggering land loss rates. The land loss has
been described as creating a rapidly rising potential for system collapse
due to the interconnectedness of the ecosystem. The land loss rates
are projected to increase in response to the impacts of global climate change.
A conservative estimate has placed the public uses losses at over 37
billion dollars by the year 2050.
| Current land loss is approximately
66 square kilometers/year which is down from the high of 107 square kilometers/year
in the early 1980’s
- Global climate change is resulting in rising sea levels and
more intense winter storms and increased hurricanes
- The levees along the Mississippi contain the river and prevent
the flow of nutrients, sediments, and water into the marshes during spring
floods. The levees extend all of the way to the continental shelf
- The only areas where sediment and water are reaching the
marsh are at the mouths of the Atchafalaya river and the Balize delta.
| This balance has since been
tipped to result in the net conversion of 4000 square km to open water. Land
loss rates are very differential due to several processes such as:
sediment thickness, deltaic patterns, fossil fuel withdrawls, subsurface
faulting, vegetation types, salt water intrusion, wind direction, tidal
surges. These differential loss rates have resulted in some areas
of the marsh experiencing a majority of the loss. Shoreline loss due
to tidal action and boat wakes accounts for 31% of the loss while interior
marsh loss due to canals and subsidence accounts for 67%, primarily in the
brackish interior marshes where small ponds form and then gradually collapse
into large lakes
The combination of logging and subsidence has resulted in the permanent
loss of the diverse natural levee habitat throughout most of the marsh.
The loss of deep roots due to logging causes the levees to disintegrate
and eventually sink into open water. The rate of barrier island retreat
in Louisiana is 4.2 meters/year which is the worst in the nation, the area
has decreased 41% in the past century
Infrastructure and economy
|Population along the Louisiana
coast has reached 2 million and is continuing to grow. Much of the economy
and infrastructure associated with this growth is threatened by the direct
and indirect effects of land loss. One of the biggest concerns is the loss
of storm surge protection associated with marsh deterioration. Ports
along the coast are first in the nation in shipping tonnage, much of which
is petrochemical. The wetlands contain over 3000 miles of navigable
canals. It is projected that between 20-80 billion dollars will be
needed to prevent structural failure in coastal roadways over the next 5
years. Major ports and cities are built along the coast and the potential
impacts of two examples are discussed on this page.
|The community of Lafourche is
an example of settlement on the Chenier plain: The Community of Lafourche
was founded in the early 1800’s and populations grew based on the success
of fishing and agriculture. A large fishing town named Caminadiville
was destroyed by a hurricane in 1893. The survivors resettled further
inland and created two new towns and made livings primarily off of oyster
harvests. Another hurricane hit in 1909 causing significant damage but
the towns were rebuilt. A major hurricane struck in 1915 innundating
the two town in 20 feet of water. The few survivors headed futher north
and formed new towns. Oil was discovered in 1930 and the abandoned
towns were resettled and booming by 1937. With the discovery of offshore
oil reserves the area was converted into the largest deep water port in the
Gulf handling over 10% of the nation's oil supply. The offshore drilling
is projected to bring 500 million dollars and 5000 jobs to the region in the
next 9 years. It is now recognized that the port of Lafourche is under
major threat from the land loss. It is projected that 70% of the marshes
currently in the region will be gone by 2050. The loss of this land
will result in the loss of Highway 1, the only land connection to the port.
The loss of the wetlands will directly increase the impacts of storms
which is of particular concern due to the fact that Highway 1 is the only
evacuation route for the more than 35,000 residents of Lafourche. The
area was abondoned several times over the course of human settlement due
to storm damage, and this will only get worse.
| New Orleans is home to over
1.2 million people and over 40 billion dollars in assets. The city has
a rich culture but it also has a slight problem of almost 45% of the land
being below sea level. 520 miles of levees with 270 flood gates and
92 pumping stations are designed to keep the city dry. New Orleans is
virtually an island bordered by the Mississippi River and the sinking coastal
marshes. The same subsidence affecting the marsh is causing the city
to sink. This great city is already battered by hurricanes, losing
the coastal wetlands will greatly accentuate this.