Geography of the Mississippi Delta

Abstract

The Mississippi delta is located in the south-central United States coast, at sea level, coordinates 29.5 N, 89.5 W.  It is the drainage outlet for the largest river on the North American continent, covering approximately 1500 square kilometers, with a drainage area of nearly 3 million square kilometers. 

Generally, a Mesothermal climate, or humid subtropical, maximum temperatures tend to remain relatively constant, ranging from 20 °C to 30 °C annually.  Minimums do show more of shift during the winter months.  The average coldest is 6 °C in December, up to as high as 26 °C in July. (Christopherson, 289)  Surface albedo ranges from approximately 2.7 kWh/m2/day in the winter, up to 6.1 kWh/m2/day, while top-of-atmosphere albedo ranges from 5.6 kWh/m2/day in the winter to 11.3 kWh/m2/day in the summer.  

Precipitation is consistent year-round, averaging 3-5 mm/day, with a low point typically in October. Water surpluses occur primarily in the winter months, and during the summer, precipitation and evaporation are closely balanced.  Deficits tend to occur, for only brief period in May, just prior to hurricane season.

The geology is almost purely sedimentary in nature.  Soils tend to be of the Entisols, Inceptisols, and Vertisols nature, as with most coastal wetlands.   Aquifers are only present in the upper delta region, and then are layered, alluvial, and prone to salt-water intrusion.

The region supports a diverse selection of tropical flora and fauna, which thrive in the marsh and wetlands that are the dominant feature.  Unfortunately, human intervention has changed, and continues to change once thriving populations.  Many species have become extinct as a direct consequence, though efforts are being made to alleviate the stress on the biogeography that is the Mississippi Delta.

Location

The Mississippi River Delta is located in the south-central United States coast, 29.5 N, 89.5 W at sea level.  The average temperature is 10.5 °C, receives 12.13 hours of sunshine per day, and about 13.5 cm/day of precipitation annually.  There are very few humans living in the area, because the land is not particular stable, as the mucky squishy drainage outlet for the largest river on the North American continent.  The delta is where the Mississippi river finally empties in to the Gulf of Mexico, covers approximately 1500 square kilometers, with a drainage area of nearly 3 million square kilometers. (1)  

I chose this area because it has always been of interest to me, culturally, historically and technically.  I have visited many different areas on the delta plain itself, including Jean Lafitte Historic Park and Preserve several times, and have always been amazed at the differences in geology, geography and culture of the region.  It is certainly one of the most diverse geographic systems on the continent but is in grave danger of disappearing forever due to constant interference by man and his need to tame the untamable. 

Climate

The Mississippi Delta is generally a Mesothermal climate, or humid subtropical, and is mostly warm and humid year-round.  There is very little variation in temperature, even from season to season.  Maximum temperatures tend to remain relatively constant, ranging from 20 °C to 30 °C annually.  Minimums do show more of shift during the winter months.  The average coldest is 6 °C in December, up to as high as 26 °C in July. (Christopherson, 289)

The area is frequently cloudy, of the cumulonimbus variety, and rain is equally as frequent.  As with any tropical to sub-tropical region, the rain can be quite unexpected, typically in heavy amounts.  In addition, it is subject to tropical low-pressure systems, bringing with them the possibility of hurricanes.  In the spring and the fall, usually for a month or so, conditions mirror those traditionally felt in regions dominated by these types of systems, with clear skies and dry conditions.  This is likely due to the shifting jet streams, subtropical high-pressure systems, and sub polar low-pressure cells.  (Christopherson, 148-240)  (Weatherly)

Positioned just south of the Sub-Tropical High Pressure belt, and influenced by the Subtropical Jet Stream, the area consistently receives large quantities of warm, moist air collected by the trade winds, as they return through the Hadley circulation cell.  In the fall and the winter, down-coast along-shore winds prevail. In the spring and the summer months, the Bermuda-Azores high, brings warm tropical air northward, and up-coast.  (Christopherson, 155-160) (7) (8)

The Maritime tropical air mass is in charge most of the time, causing the area to be subjected to pressure instabilities.  It is in a bit of a transition zone though, and at times, during the winter months, Rossby waves force low-pressure systems southward, subjecting the delta to colder than normal conditions, though rarely below freezing. (Christopherson, 209) (Weatherly) 

While inversions do occur in the area, they are typically due to the river water dumping into the Gulf of Mexico.  Inversions experienced follow the increase or decrease in discharge rates, being more significant in the spring and the fall, and less so during summer and winter months. (Gyory)

Convergent and convectional lifting are the primary mechanisms in this area.  Heating of the surrounding landmasses, produce convergence with onshore winds from the Gulf of Mexico and afternoon convection showers are frequent in the summer months.  Being a flat and level delta plain, there is no orographic lifting, but frontal lifting does occur occasionally in the winter, at such times when the Continental polar air mass moves nearer. (Weatherly)

Weather on the delta is also affected by eddies from an extension of the Gulf Stream, known as the Loop Current.  This current is variable in position, though typically flows northward out of the Yucatan Current, and into the Florida Current.  Frequently during the spring and summer months, it extends as far north as the Mississippi Delta, though it is not directly tied to seasonal changes.  The along-shore currents at the delta are influenced by this Loop current and local surface winds, shifting down-coast during the winter months, bringing in cooler, dryer air, and up-coast during the summer, keeping things warm and tropical.  (Gyory)

Overall, positioned under a shifting subtropical jet stream, the subtropical high, and primarily influenced by the mostly unstable Maritime tropical air mass, the entire region’s weather systems are consistently unstable.  Cyclones, lightening storms, tornadoes, tropical storms that sometimes develop into hurricanes, are all considerations in this region of the United States.  In addition, random visits by sub polar low-pressure cells can bring with them the possibility of ice storms. (Christopherson, 141-242) (Weatherly)

The insolation represented is typical of most areas at this latitude, just above the Tropic of Cancer, in the western hemisphere.  The below graph presents a 10-year average, including data from 3.5 El Nino years, 2 La Nina years and 4.5 “near-average” years, taken on a 3-hourly basis and calculated using the Pinker and Lazlo Algorithm.  This particular algorithm uses input from the World Climate Research Program’s International Satellite and Cloud Climatology Program (ISCCP) and takes into account radiance, water vapor and ozone.  Primarily, the data is collected by the NASA Data Assimilation Office’s Version 1 Goddard Earth Observation System (GOES-1) and the Total Ozone Mapping Spectrometer (TOMS).  Aerosol and cloud optical depths are used as tuning parameters.  (Kusterer) (5)

There does not seem to be a great amount of variation in albedo, primarily due a landscape consisting of sea and marsh areas and minimal urban areas, though there are numerous petroleum manufacturing facilities and intense industrial water traffic.  The increases during the winter months, is directly related to the significant cloud cover that accompanies rainy season in the region during the respective months, specifically at the height of hurricane season, during the month of September. (Kusterer) (5, 6)

 Hydrology

The Mississippi delta is the result of the Mississippi River dumping into the Gulf of Mexico and is home to hundreds of lakes, streams, bayous, swamps and creeks.  Two other dominant bodies of water in the region are Lake Pontchartrain and the Atchafalaya River.  Water budgets for the area tend to follow the seasonal rains, with a deficit appearing just before the beginning of the hurricane season. (4)

 

Gosselink, James.  The Ecology of Delta Marshes of Coastal Louisiana:  A Community Profile.  May 1984.  Louisiana State University:  Center for Wetland Resources.  http://www.nwrc.usgs.gov/wdb/pub/others/84_09.pdf (accessed 18 NOV 2007)

While there is no aquifer underlying the Mississippi delta, shown in white on the map), it does somewhat overlay the Alluvial Aquifer system, confined by varying layers of silt and clay.  This aquifer recharges by direct infiltration of rain, lateral and upward movement from adjacent and underlying aquifers, and flooding.  The level of water fluctuates with the seasons, changes in river stage, and precipitation.  Natural flow is down gradient toward rivers and streams.  The water from this source is used primarily for agricultural needs, tends to be heavy with hard calcium and magnesium bicarbonate, and due to its proximity to the Gulf of Mexico, there is a high probability of salt-water infiltration. (9)

 

Because this particular delta is the primary drainage outlet for most of the North American continent, there is a variety of concerns with ground water.  Reviewing the most recent Ground Water Assessment Report from the state of Louisiana, contaminants were found at all levels, from a variety of sources including, but not limited to:  inorganic and organic pesticides, solvents, petroleum compounds, nitrates, fluorides, metals, radionuclides, bacteria, viruses, protozoa, sulfates, as well as high salinity near the Gulf of Mexico.(9)

Geology

The above map, from the Louisiana Geological Survey, indicates that the Mississippi River delta, hosts a geology beginning in the Holocene epic.  There are extensive alluvium and coastal marshes, composed of fine-grained and organic sediment deposited by the river over the past 10,000 years.  In more detail, the following map shows natural levees created by the river, as well as differentiating between fresh and saline marshes in the region.

 The rock in this area is primarily fine-grained and organic sediment.  As the end of the line for North America’s largest drainage basin, the origins have the potential of being from generally anywhere east of the Rocky Mountains.  It was constructed from spreading along the Caribbean, North American, and South American plates in the early Jurassic, and has no history of orogeny.  The primary mechanism for construction is simply millions of years of sediment deposition from the North American Craton, and other orogenesis activity around it, though no orogeny is immediately present. (11) (12) (Roberts) 

There is no plate boundary; the area is in a state of tectonic stability, and has been since the late Jurassic period.  This has allowed the area to develop a stable continental shelf, with ramps along basin margins.  Recently, there has been “Mesozoic rifting”, resultant from excessive accumulation of sediment and outbuilding of the same. (11)  This rifting is associated with a number of minor faults, predominantly the recently discovered Michaud fault.  Located on “the hanging wall of a large, listric, normal fault system that forms the northern boundary of a 7-10 km think allochthon that is detached from stable North America,” the fault is the primary suspect for major subsidence in the area (Dokka)

The USGS has two earthquakes listed in a 25-mile radius of the delta.  The first listed is only mentioned as being “slight”, on December 15, 1927.  The most recent, on August 10, 2007 was a magnitude of 3.4.  It is suspected that these minor quakes might increase due to subsidence in the area, along the Michaud fault.  It is suggested that subsidence might accelerate in the near future, leading to more mass movement of sediment in the immediate gulf region, and on land.  This will obviously lead to more earthquakes as well. (Stevenson) (13) 14)  In addition, due to the ease with which seismic waves travel through the sedimentary deposits that dominant the area, earthquakes from as far as 200 miles away have been felt, and may cause more sedimentary shifting in the alluvium. (Stevenson) (13, 14) 

There is no volcanic activity presently associated with the region, though there is evidence of a possible volcanic past that can be found in the Andes of South America, as part of the rifting of the Precordillera Terrane away from North America. (15)  As well, there is no karst data available for this region.  The Karst landscapes and limestone regions map from Christopherson’s book, do not indicate that there would be any in this region.  (413)

 Weathering in the area is primarily through chemical processes, specifically in the form of water, through sediment transport and release, as well as tidal influences.  The biggest weathering factor in the region is human-induced, from extensive hydrologic and geomorphic alterations to the area, causing subsidence, and loss of wetland forests through erosion. (Chambers) (11)

 

SURFICIAL PROCESSES

The meandering Mississippi River splits out on the delta plain, draining to a classic “bird’s foot delta.”  Prior to interference by man, it was maintained by sediment deposition and avulsion.  This had the effect of causing the delta to build up and prograde towards the Gulf of Mexico.  In combination with the compaction of fine-grained sediment, it would normally subside slowly into the Gulf.  However, heavy regulation of the river by man, in the area of the delta, and further upstream, is causing the Mississippi delta to subside faster than usual, causing the associated marshlands to sink into the Gulf, and inducing the shoreline to retreat northward.  Current base level of the delta is sea level and the stream profile is extremely gentle, at 0.2 degrees, making floods an extreme possibility at any time of the year. (Parker) (16)

Flooding of the Mississippi delta is way of life though, but the incidents have decreased, due to human intervention.  The most significant flooding of the Mississippi delta took place shortly after Hurricane Katrina, in 2005.  Storm surges from the hurricane inundated the area and large-scale damage, loss of land and life were witnessed throughout the region.    

Exempt from the area are eolian processes, excepting deposition from other regions.  As well, there are no desert features in the delta, though early explorers considered the region to be a desert, because it appeared to be unsuitable for anything, agriculturally.  No glaciers exist either, past or present, though positioned south of the maximum Pleistocene glacial extent, the impact was still somewhat significant.  As the glaciers melted, runoff and sediment was deposited in the area, by way of the Mississippi River, and surrounding tributaries, adding to the creation of the delta and surrounding flood plain. (17)

As previously mentioned, the biggest concern for the delta is coastal wetland loss. Human intervention in the natural processes of the area have caused significant reductions in sediment deposition and created huge areas of open water, where none was before.  Canal dredging, and over-used shipping lanes in the area have accelerated the process.  This has also increased the amount of salt-water intrusion, causing massive dieback of marsh regions.  

Degrading Quality.  UN Environment Programme.  GEO:  Global Environment Outlook.  http://www.unep.org/geo/yearbook/yb2003/073.htm (accessed 10 DEC 2007)

The region is also surrounded by a natural hypoxia zone.  Unfortunately, because of over-regulation of the river, scientists have recently discovered that the zone is both increasing in size and frequency, due largely to the manner in which drainage patterns have been altered, as well the excessive amounts of herbicides and pesticides entering the system from all over the Mississippi River Basin.  The image below shows the hypoxia in light blue, as still being persistent in January, coupled with the attached graph, which shows the increase in hypoxia over the last 20 years. (Rabalais) 

Plaquemines Parish, Louisiana. US Department of Agriculture. http://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx (accessed 08 DEC 2007)

Soil Properties

 According to a soil survey of the Plaquemines parish that was done in 1988, the region “had a total area of 901,817 acres, of which 590,160 acres is land and 271,657 acres is large water areas” including “streams, lakes, bays, sounds, and the Gulf of Mexico.”

Christopherson’s book indicates that the soil taxonomy soil order of the Mississippi Delta region is primarily histosols. (588)  However, this was not evident in the samples taken using the USDA Soil Survey web site, which indicated a variety of soil orders consisting of Entisols, Inceptisols, and Vertisols.  One such sample is shown in the map above.  The suborders of soil appearing in the selected area are primarily composed of the AT (Aquents), BA (Balize and Larose) and CV (Carville Cancienne, and Schriever) series.  These indicators are more consistent with my knowledge of the area, and are silty and fluid loam types, with stratified very fine sandy loam to silty clay.  The parent material is thin herbaceous organic material over fluid clayey alluvium. (18, 19, 20)

The formation of soils in the region is primarily sediment deposition from the Mississippi River, in combination with decomposing plant material.  Creation of channels for streams and rivers, as well as the construction of levees and dams in the area, and further upstream have restricted sediment, and influenced natural wetland development. Wetland plants produce most of the organic material for maintaining the coastal marshes. (Ko) (20, 21)  

The soil in this area is not particularly worth using, being characterized as “mucky”, and subject to drying out to a consistency of brittle concrete.  In addition, salt water intrusion present difficulty for the only biogeography particularly suitable to this type of soil, wetland marshes. Attempts are being made to control the situation though, to preserve and restore.  Terraces are being built, and planted in open with only a few feet of water, in the hopes the hopes of reducing persistent erosion from hydrological processes. As well, new canals are being prohibited; ship traffic is being directed to assist in alleviating some of the conditions that they produce. (Ko) (21)(22)

Ecology

The delta region is zoologically Neoarctic, and is primarily Boreal, botanically, though it is part of a transition zone (or ecotone), assuming some characteristics of the adjacent Neotropical zone. (651) The terrestrial biome that this region is a part of is the “Midlatitude broadleaf and mixed forest”, as characterized by the map of “The 10 major global terrestrial biomes” in Figure 20.3 of Christopherson’s book (654).  While the region is distinctly different than it was 1000 years ago, it is somewhat similar.  The major difference is the loss of the extensive system of coastal wetlands, which are only a shadow of their former self, thanks to humans.

The delta provides habitat and a source of food, which is critical to finfish, oysters, blue crabs, crayfish, and shrimp.  In addition, one-half of the United States fur harvest comes from the area.  It is also a nursery and feeding ground for a good portion of the migratory birds of North America, which use the “Mississippi flyway.”  Of course, there is a multitude of snakes, alligators, amphibians, multiple varieties of fresh and salt-water fish and shellfish.   There are up to 93 species of plant life in the coastal wetlands and marsh areas.

Barrier islands contact mostly salt-tolerant Xeric grasses and succulent herbs implanted in dunes that grade into salt marsh vegetation.  Some barrier islands were previously home to live oak forests, though most have disappeared, due to salt-water intrusion.  The salt marshes are almost completely composed of Smooth Cordgrass, with minor occurrences of a variety of other salt-tolerant species, including the Black Mangrove. Brackish marshes usually contain varieties of freshwater plants that can tolerate random spikes of brackish water, as well as Wiregrass. The freshwater marshes are dominated by Maidencane and Arrowhead, most of which are found in floating mats of vegetation, where Waxmyrtle tends to colonize and cause the mats to sink. On the landward edges of the freshwater marshes, and extending along the high ground formed by ancient levees, are the forested wetlands, containing tree species such as Water Elm, Black Willow, Water Hickory, Water Tupelo, Bald Cypress and Hackberry. (23) (24)

Six species no longer occur in the wild of Louisiana.  Attwater’s Greater Prairie Chicken, Eskimo Curlew, Ivory-billed woodpecker and the Red Wolf are now considered extinct.  Whooping Crane and Bachman’s Warbler may also have occurred in the more recent past.  As well, currently there is only a single record of the American Burying Beetle within the state.  Indications are that it may be, or has already become extinct within Louisiana. (Ko)(24)(25)

Numerous human alterations, in the region and outside of the region, are affecting the ecosystem of the area.  A system of levees and dams along the river channel water and sediment into the Gulf of Mexico, rather than depositing them on the coastal wetlands, are causing land loss at a rate of 25-30 square miles per year.  Habitats are literally wasting away in coastal marshes, changing swamps and freshwater marshes to open salt-water regions.  In addition, oil spills from transport and drilling are making most of the region uninhabitable. (Ko)

It is because of the impacts that the Mississippi delta is great danger of disappearing under the waves of the Gulf of Mexico.  It appears that unless drastic measures are taken immediately, in all aspects of the geography of the region, it could very well be relegated to the fate of being nothing more than an industrial wasteland.  It is not very surprising that it has earned the reputation of being the “toilet bowl of the United States.”

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Geography 315: World Physical Geography