Louisiana Lowland Geomorphology
by Tim Saksa
Louisiana has many different natural regions with in its borders. There are five such natural regions that make up is called Louisiana. These regions are called: 1. Costal marches, 2. The Mississippi Floodplain, 3. The Red River Valley, 4. Terraces, and 5. Hills. These features are the main reasons why the state of Louisiana are shaped the way it is.
Much of the state is below the 500 foot above sea level mark. The elevation range of Louisiana is from 5 feet below sea level at parts of New Orleans to 535 feet at Mount Driskill. There are only three places that are considered high lands in La. They are in the western, north western, and north central regions of the Florida Parishes. The rest of the state is in the lowland category of elevation.
One of the reasons for so much of La is within the lowlands category is because there are so many rivers running through La. Over time the rivers have cut thought the land and lowered the elevation around the rivers. The other reason is because the state of Louisiana is slowly sinking into the Gulf of Mexico. About 24 sq acres a year of the coastland is sinking to the Gulf.
Rivers, Lakes, Waterways, And
(different river maps)
THE MISSISSIPPI AND OTHER DRAINAGE SYSTEMS
Louisiana owes its existence to the Mississippi River that now runs though the body of the state. All of the land that makes up La was brought down the Miss River thousands of thousands of years ago. During the defrosting of the last ice age much of Louisiana was born. Even thought the icebergs did not cover that far south, sediments were brought down the Miss River. Then as time passed more and more sediments were brought down the river and they built up so much they started to make landforms. Thus the beginning of Louisiana was created.
The Mississippi River is also created with much more than just creating La. It also is responsible for the drainage of more than half of the states in the United States, including La. The Miss River also touches two Canadian provinces. The river spans over 1.245,000 square miles of from Canada to the Gulf. Thought its entire span from Canada to the Gulf the Miss has very few tributaries. On the other hand it has several distributaries.
Even thought the Miss River is the largest river in La it is not the only drainage system it has. On a much smaller scale the Red River is the second largest system in La. Unlike the miss river the Red has many tributaries and very few distributaries. The Ouachita or Black river flows from the northeast and goes southward from Arkansas to the Red.
The rivers that are in the central west and southwest part of the state act as the drainage for the area. Rivers such as the Sabine, Calcasieu, Mermentau, and Vermilion rivers all drain straight to the Gulf of Mexico on there own.
Another drainage system in La is not even a river
it is by the lakes. In the Florida Parishes of central northeast La follow
under this pattern. >From the Mississippi streams such as the Amite,
Tickfaw, Tangipahoa, Tchefuncte, Bogue Chitto, and Pearl flow southward to
Lakes Maurepas, Pantchartrain, or Borgne. After the water goes into the lakes
then it is let out into the Gulf of Mexico though channels between the lakes
and the Gulf.
Alluvial valleys come in many different shapes and sizes. One of the many factors that come with forming an alluvial valley is the size of the river that forms it.
The larger the river is the larger and more spread out the alluvial valley will become. A good example of this is in Louisiana with the Mississippi river, the Red River, and the Ouachita River. The radius of the alluvial valleys says it all. The radius of the Mississippi is close to 2 miles, the radius of the Red is close to ¾ of a mile and the radius of the Ouachita is less than ½ of a mile.
Another factor in shaping is the material that is at the edges of the valley. If the rock is soft and dissolvable then the water will slowly wash and erode the rocks away until it finds a rock that is more resistant. On the other hand if the rock that it runs into does not dissolve with water and is resistant to erosion, then he valley cannot spread past it.
The Mississippi River’s eastern valley walls stretch form the bluffs that Vicksburg, Natchez, St. Francisville, and Baton Rouge occupy. Macon Ridge, Sicily Island, Avoyelles Prairie and the bluff lying between Opelousas and Franklin make up part of the western wall of the alluvial valley.
The valley walls of the Red river are the bluffs that were created by the Red river when it cut through the up warped lands of north La, and where it cut though the Nacogdoches and Kisatchie wolds. The Ouachita used the same techniques to cut out the western walls of its valley. The eastern walls are more distinct and more complex. The Bastrop hills, Macon ridge, and the Ouachita Gorge form parts of the eastern wall. The Ouachita Gorge lies between the eastern end of the Kisatchie Wold and Sicily Island.
Within many of the floodplains or alluvial valleys there are many meander belts. The Mississippi has many meander belts along its path. Several of the meanders got cut off because the Miss wants to take the shortest path to the Gulf of Mexico. When the meander gets cut off from the river it will become a sort of lake called a cutoff lake. There are many of these lakes with in the state of Louisiana an example is the False River.
The way these lakes form is very complex and takes a long time to do. First, it starts with a river. As known steams and rivers are very curvy, but they have a tendency to want to flow in a straight line to the main body of water that it flows into. This is true for the Mississippi River as well. When the Miss River twists and turns sometimes it will have a tendency to run into itself. On occasion when this happens the river will relies that it is quicker to not go all the way around the turn and just start flowing down the quicker way. In doing this the water that was in the turn will sometimes get cut off form the main flow of the river which then will cause a lake or pond to form depending on the size of the bend in the river.
Regardless of the size of the streams or rivers in Louisiana they all share certain forms. This is determined mainly by the energy of the flowing water. Water naturally flows downhill under the attraction of gravity. Normally the course that provides the direct path to lower elevation becomes the principal channel. This channel in virtually all streams in La meanders during its decent to lower elevation. Because of the meandering the flow of the river will often change direction. When one looks at a steam they speak of right and left banks while viewing the stream downstream. This shows how Homer lies on the right bank of Bayou D’Arbonne, while Lake Charles stands on the left bank of the Calcasieu. Another example of this is with New Orleans. The people who live in new Orleans often call the side of the river that Gretna stands on the "west bank" where as it actually stands on the south bank of the Mississippi.
Along the course of a river/stream, each curved segment is a meander, and the direction of the flow reverses in each meander; water enters flowing east and exits the curve flowing west, always downstream. When there are two meanders, a more or less straight segment, the reach, joins the two. The reach is also called a crossing because the principal current crosses from right to left and left to right in theses straight segments. Inside of a meander, the main current flows near the outside bank, or cutbank, making that side steep because the energy of the moving water undercuts the natural levee there. Opposite the cutbank lies the point bar, so called because it protrudes or points, and it is an area of shallow water, where bar comes in.
During normal flow it is so gentle that the lack of stream energy there leads to deposition of sediment, gradually building up silt and sand. On the other hand when the stream/river is in flood stages the cutbank will often collapse because of the current eroding so much of the underside out. This will allow the steam/river to spread out laterally in the direction of the cutbank, but because all fluvial features are always proportional in size to the steam/river that made them, the point bar will grow out towards the cutbank. This process will happen every time the stream floods, thus forming a new natural levee each time. After a period of time one could look at the new levees and say they look like small rolling hills. This feature of the point bar is called ridges and swales.
During normal flow stages the current usually flows above the deepest part of the bed, the thalweg. The thalweg lies near the cutbank, where it has cut the most deeply into the valley floor. In the crossing, the thalweg follows a shallower course becoming deep again at the head of the next cutbank downstream.
During high stages, the principal current tends to straighten, cutting across the point bars and moving the stored silt and sand down-valley to the next point bar. During these flood periods, a meander my get cutoff as the stream shortens its course, when this happens a cutoff lake or oxbow lake is formed. Where the cutoff was formed between the cut banks, a neck cutoff occurs, while one that occurs across a point bar forms a chute cutoff. Once the stream/river shifts away form the cutoff, the lake becomes a body of quiet, low energy, water. Fine clays will begin to fill the ends of the lake where the clay plugs form.
When the river/stream flood an extensive amount, the water will flow far back past the natural levees this area is called the backswamp. Only the smallest soils make it to the backswamps.
CONTROLING THE MISSISSIPPI
Controlling the Mississippi river did not come into peoples’ minds until they saw that the Mississippi was slowly starting to change its course again and totally avoid New Orleans. This first started to get serious after Captain Henry Shreve cleared a large logjam in 1831 that was blocking water from going down the Atchafalaya River. After he had cleared the raft (logjam) the waters from the Miss started to flow down the Atchafalaya more and more. This started to worry the people that owned businesses in New Orleans and the politicians in the state. Not only was New Orleans a popular place for tourists to go, it was also one of the major ports in the United States. In an attempt to slow or even stop the natural process the river wanted to take the Army Corp of Engineers built a structure that would monitor and control the amount of water that flowed into the Atchafalaya from the Mississippi. This structure is called the Old River Structure. With this new structure built the Army Engineers can also stop major floodwaters from reaching New Orleans. If a flood that would threaten New Orleans would start to flow down the Mississippi river the engineers could open up the locks that control the waters form going down the Atchafalaya and let the waters go down toward Morgan City instead of New Orleans.
Another way man has attempted to control the Mississippi River is by building man made levee systems down the river’s sides. Since more and more companies have put power plants or factories next to the river, the owners felt they needed more protection from the river’s floodwaters (see chapter 18). The floodwaters that would normally rise over the natural levees and flow into the valleys next to the rivers, where the plants and factories now are, would cause enormous amounts of damage to the factories and plants.
In the mid to late 1800’s the local governments started to build manmade levee systems along the dangerous parts of the Mississippi. In 1872 the Federal Government started to supervise the production of the levees but leaving most of the power to the local governments. Even with the supervision the levee system was not sufficient to prevent the flood of 1927. After that year the Army Corp of Engineers took over the planning and construction of the levees. The levees were built much higher and there were extensive studies done on the river during flood and normal stages to see what other measures should be taken in the instance of a flood. With this study they started work on the spillways or emergency outlets. The levees were built higher, broader, and stronger. New techniques were put into effect that made production much faster
There were also two spillways made to prevent the Mississippi River from flooding into New Orleans from flooding into New Orleans, the Bonnet Carre, which goes into Lake Pontchartrain, and the Atchafalaya spillway or Old River Structure. The Atchafalaya spillway is much larger than the Bonnet Carre spillway. It is 13 miles wider than the Bonnet, which allows 350,000 more cfs of water form the Mississippi River to the Atchafalaya.
Mississippi River Delta System
The lower Mississippi River is created by the addition of the Missouri and Ohio rivers to the upper Mississippi. The present course of these two great tributaries approximates the southward limit of the great continental ice sheets during the most recent ice age. About six thousand years ago when the ice melted and made the sea level raised a considerable amount. Only three thousand years ago did the Gulf of Mexico level out and more or less stabilized. When the Mississippi meet up with the Gulf a great embayment formed by the tremendous load of sediments dumped into the Gulf.
Seven generalized delta complexes have been postulated, although one study suggests a far more intricate system of sixteen delta lobes within five delta complexes. The seven general delta systems are named, in chronological order of years before present (B.P.): (1) Sale-Cypremort, 5000-4500 years B.P.; (2) Cocodrie, 4500-3500 years B.P.; (3) Teche, 3500-2500 years B.P.; (4) St. Bernard, 2600-1500 years B.P.; (5) Lafourche, 1500-700 years B.P.; (6) Plaquemines; 1200-500 years B.P.; and (7) the present Balize, 500 years B.P to the present. Each of these deltas was responsible for creating what are now the south-central parts of Louisiana.
The present day delta has been built up for the past 500 years. Its fifty-mile extension beyond the main shoreline attests to the great load of sediments with which the river battles the sea in forming new land. There are three main distributary channels in these Balize Delta: the southwest pass, the south pass and the pass Loutre. The present delta’s shape is called "bird’s foot," from the shape of its distributary channels.
Each of the prior deltas was in different places along the coast of Louisiana. About 5,000 years ago the main channel of the Mississippi lay approximately along Bayou Grosse Tete. At about the present location of Lower Grand River, the main channel diverted toward the southeast. Passing over what would later be Lake Verret, it turned generally eastward to form a delta whose center stood more or less were New Orleans would someday be. At the time that the Cocodrie delta was begun, of course, there was only a region of marsh, bays, and open Gulf where Lake Pontchartrain and New Orleans are today.
While the Mississippi discharged into the Pontchartrain area, the western part of its lower floodplain was a swamp that received little sediment. About 3,500 years ago the river moved westward, cutting along the eastern margin of Avoyelles Prairie and taking a new route along what is today Bayou Teche. In the northern part of the Atchafalaya Basin, it left its trace in, for example, Bayou Jack, Bayou Rouge, Negro Foot Bayou, and Dry Bayou. Palmetto in St. Landry Parish occupies the natural levees of that route. From Palmetto, the Teche Mississippi moved almost next to western valley wall and formed a course that arced southeastward to a delta centered under the present surface of Terrebonne Parish. Some of the remnants of Teche delta can be seen in the Bayous that radiate south of Houma.
About 2,800 years ago, another diversion placed the main course of the river approximately in its present location and sent its delta to a pint centered in St. Bernard Parish. The St. Bernard delta, in reaching the Gulf, Passed over and buried the Cocodrie delta. The St. Bernard delta’s branching remnants can readily be seen in bayous Sauvage, la Loutre, and Terre aux Beoufs. Lines of dying live oaks trail across the marshes of St. Bernard Parish, showing the old courses of the St. Bernard delta’s distributaries.
The platform of that delta is being carried downward bye down warping, while the Gulf’s waves and currents gradually wear it down. The Chandeleur Islands, formed form the coarser sediments of the delta’s natural levees and channel loads, are repeatedly re- organized bye these Gulf forces. These and other barrier islands are the shifting evidence of a deteriorating delta.
About 100 years ago, the river, or at least a large part of it, diverted at what is now Donaldsonville, to spread its delta again over Terrebonne Parish. In a sub-diversion, it later branched to the east a bit to Thibodaux to form the course of lower Lafourche. The Lafourche delta has two parts: an earlier part in Terrebonne Parish and reaching into Terrebonne and Timbalier bays and a younger part forming the spine of Lafourche Parish down to Belle Pass and Bay Marchand.
The Plaquemines delta and the Balize delta developed into what is now Plaquemines Parrish. The Balize delta in particular is peculiar in it protrusion beyond the general arc formed by older deltas. His notable extension carries some of the Mississippi’s annual load of 300 million tons of sediment beyond the edge of the continental shelf and into the deeper part of the Gulf. It also exposes the delta to a greater frequency of hurricanes.
Finally, a long delayed diversion of the Mississippi into the sediment-starved Atchafalaya Basin became increasingly evident in the late nineteenth century. The present route of the Atchafalaya had not received major sediment contributions for more than 5,000 years. A consequence, it provided a basin of seriously low elevation that once the opening was provided, actively attracted Mississippi water and sediment. That, together with the advanced deterioration of the Sale-Cypremort delta, provided a shorter, steeper route to sea level. As a consequence, we can confidently recognize the Atchafalaya delta as the newest in the sequence.
All of Louisiana lies with in the general region of a continent known as the Gulf Coastal Plain. Three sub regions exist. The Florida Parishes are the part of the East Gulf Coastal Plain, and western Louisiana is a part of the west Gulf Coastal Plain. Separating them is the Mississippi Alluvial Plain.
A coastal plain contains low relief and overall low elevation, generally less than five hundred feet. The plain consists of sedimentary layers deposited by rivers and by the ocean itself along its shifting coastline during the past seventy million years. At the same time, the eroded materials have been laid down along valleys and the shore. Thus, as one moves for shoreline inland, on generally moves from newer sediment to older, even as one moves into areas of higher elevation.
The most recent physiographic area of Louisiana is located along the coast and interior south of the ancient terraces and Mississippi alluvial valley, this region of prairie, swamp, and marsh, largely flat with natural levees, cheniers, and salt domes punctuation its surface, represents the deposition of sediments by the Mississippi and other streams over the past six thousand years.
At five pints on the coast, salt domes have thrust so near the surface to form remarkable local elevations. These Five Islands form a line lying from northwest to southeast in Iberia and Ts. Mary parishes. Jefferson Island (Anse la Butte) and Avery Island in Iberia Parish lie somewhat inland, while Weeks Island, and Cote Blanche Island, and Belle Isle in Ts. Mary Parish stand on the shores of Vermilion and Cote Blanche bays. Especially the more southerly four stand in the mist of the marsh. Because they have elevations ranging to a height of 171 feet (Weeks), they are covered with forest or in other wise marsh region. For that reason and because they are more or less circumnavigable, they came to be called "islands."
This elevated character has also made the Five Islands attractive sites for human settlement. In deed, some of the oldest evidence of human habitation in Louisiana comes form Weeks Island. Avery Island long ago became the celebrated stronghold of one family who, among others contributions, created Tabasco sauce and an important wildlife refuge.
The landforms made during some storms are beach ridges. Because they stand like islands of slightly higher land in the marsh, they are visually notable for having live oak dominated woods growing along their crests, earning them the local name, "chenier."
They often lay dozens at a time, more or less parallel to each other and to the shore. Each chenier, then, represents an earlier shoreline.
The region of these beach ridges is the Chenier Plain, an area of about 15,00 square miles. It is the marsh region west of Marsh Island and lying between the Gulf and the outer edge of the Southwest Prairies. More promenade cheniers have proper names. Form west to east, some of the well known cheniers are Smith Ridge, Holly Beach, Back Ridge, Font Ridge, Little Chenier, Grand Chenier, Chenier Perdue, hackberry Ridge, Pecan Island, and Chenier au Tigre. In fact, however, there are several hundred of these ridges, giving much of the Chenier Plain an appearance of having been combed.
The most interior of the set is Little Chenier, which is also the oldest, having been formed about 3,300 years ago (on the basis of carbon-14 dates). In as much as storms would otherwise have destroyed it, we know that the shore retreated Gulf ward after Little Chenier was formed.
Nearly all of the coast of Louisiana is retreating before the advance of the Gulf. Of course, subsidence and down warping underlie nearly all of this current loss of Louisiana. The loss of land is directly related to the rise of sea level 6000 years ago with the melting of the ice during the end of the ice age. The shore is washing away at an alarming rate. In some places Louisiana can lose up to 25 feet a year, and in other up to 15 feet a year.
The arc from Lake Borgne to lower Terrebonne Parish is doomed to progressive, irreversible transformation into ever widening bays. For an image of the out come, one might recall that firm land once extending all the way from New Orleans to the Chandeleurs. The higher pars of old natural levee ridges will be the last to sink beneath the waters of the Gulf.
The weight of sediments deposited by the
Mississippi River over millions of years greatly depressed the floor of the
Gulf or Mexico. Down warping of the sea floor in response to sediment loading
is know as geological subsidence. This down bending has generally kept pace
with the deposition, so that deltaic deposits have been allowed to grow to at
thickness of over four miles, while the sediment surface remains near sea
level. This along with sediment compaction is causing the coastal plains of
Louisiana to sink
into the Gulf of Mexico.