Mississippi River Delta

This information was taken from: http://academic.emporia.edu/aberjame/wetland/mississippi/miss_delta.htm

Mississippi Delta

James S. Aber

Wetland Environments
Emporia State University

The Mississippi Delta region comprises much of coastal Louisiana and adjacent Mississippi, stretching from the Atchafalaya Bay on the west to the Chandeleur Islands on the east, and includes the metropolitan area of New Orleans. The delta complex contains major river channels and levees, numerous bayous, swamps and marshes, lakes, tidal flats and channels, barrier islands, and shallow sea environments. Water chemistry grades from fresh to brackish to marine. The climate is subtropical; freezing conditions occur rarely, almost never near the coast. The major climatic events are hurricanes that strike the region every few decades, and floods derived from upstream runoff. Economic activities include shipping, traditional fishing and farming enterprises, as well as oil production and petrochemical processing. Recreation and tourism are likewise quite significant for the local economy.

The foundation of the modern Mississippi Delta was constructed during the Pleistocene (ice age), when melt water from ice sheets poured down the Ohio, Missouri and Mississippi valleys. These melt-water floods transported huge volumes of sediment that accumulated in the delta. Since the end of glaciation, the delta has undergone continual change during the past several millennia. As distributary channels (passes) build farther and farther into the sea, river gradient is reduced. Eventually the channel gradient becomes too low for water to flow readily. When this happens a new channel, with a shorter route and steeper gradient, may be carved during flooding, and then a new delta lobe begins to build. In this way, the water flow within and across the delta shifts back and forth, and many delta lobes have been constructed during past centuries.

Coastal Louisiana has experienced rapid loss of wetland habitats, in which wetlands become open-water environments. Rates of loss exceeded 100 km² per year in the early 1980s. These rates have declined to 60-70 km² yearly loss more recently, which remains the greatest wetland conversion for any state in the nation. The losses are due to a combination of natural and human factors, including land subsidence, rising sea level, and human drainage modifications. For more information, see Guide to wetlands (Dugan 2005, p. 84-86).

Barrier islands

When a lobe of the delta is abandoned by a shift in drainage, that portion begins slowly to subside into the sea and is further reduced by erosion. Some of the sediment may be reworked by wind and waves into barrier islands; the Chandeleur Islands are an excellent example of this situation. Another example is Grand Isle; it is the only barrier island accessible via highway in Louisiana. Grand Isle has a long history of agricultural, recreational, and industrial development. First colonized by the Spanish in the 1700s, the island was shifting sand without any tree cover. French Creole settlers took over in the 1800s with agriculture and fishing. Gradually woodland vegetation became established on the dune sands--oaks and oleander. Salty meadows, marshes and lagoons occupy the lower terrain. The island remained relatively inaccessible until offshore oil production began in the late 1940s. Electricity and telephones arrived in the 1960s along with a recreational boom.

Hurricanes

Hurricanes are a way of life in the Mississippi delta and adjacent Gulf coast region. For example, major hurricanes impacted Grand Isle in 1893 and 1965. The hurricane season of 2005 was the most devastating in recent times, when Hurricane Katrina struck New Orleans and Hurricane Rita came onshore in western Louisiana. Some 100 square miles (260 km²) of marshes were converted into open water, as a consequence of erosion during storm surges. Most of this wetland loss was attributed to Hurricane Katrina and took place east of the Mississippi River in St. Bernard and Plaquemines parishes and in Breton Sound. Hurricane Rita caused marsh loss west of the Mississippi River to the Texas border.

The eye of Hurricane Katrina passed directly over the Chandeleur Islands, a chain of delicate barrier islands east of the modern delta (see above). These islands were heavily damages by the storm; initial estimates indicate the land area of the islands was reduced by about one half. Hurricane Katrina was the latest of five hurricanes to impact the Chandeleur Islands during the brief period 1998 to 2005. Grand Isle also was heavily affected by Katrina's storm surge and high winds that caused widespread structural damage to most buildings on the island.

The long-term wetland retreat in the Mississippi delta region (see above) may have contributed to severity of damage during the hurricane season of 2005. Loss of coastal wetlands has reduced the capacity to absorb storm surges. Flooding has become more frequent and deeper in some delta areas. The exact contribution of wetland loss to storm damage is difficult to quantify, but it is undoubtedly significant. Efforts to mitigate this situation will be quite expensive ($2 billion) and require decades to put into place, as detailed in the Coast 2050 plan.

The following information was taken from this site:

Delta

Deltas have long played an important role in human history. These fertile areas where rivers flow into large bodies of water have served as fishing, farming, and living sites. Of the great deltas around the world, perhaps none has had a greater role in civilization than the delta of Egypt's Nile River. Greek historian Herodotus (c. 484–c. 425 B.C.E.), considered by many as the "Father of History," studied this great geologic feature. He is credited with coining the term "delta" for this type of landform because its triangular shape reminded him of the Greek letter ∆ (delta).

The shape of the land

A delta is a body of sediment deposited at the mouth of a river or stream where it enters an ocean or lake. Unlike other landforms affected by running water, a delta is not created primarily by water cutting into or eroding the landscape (erosion is the gradual wearing away of Earth surfaces through the action of wind and water). Water does not tear down a delta; instead, it builds up a delta.

A river creates a delta by laying down sediment or rock debris such as gravel, sand, silt, and clay that it has picked up and carried along its course. Alluvium (pronounced ah-LOO-vee-em) is the general term for sediment deposited by running water. A river's depth, its width, and its speed determine how much sediment it can carry. The Mississippi River flows at an average surface speed of about 2 miles (3 kilometers) per hour. Yet it drains between 1.2 and 1.8 million square miles (3.1 and 4.6 million square kilometers), which is more than 40 percent of the total area of the continental United States. Over the course of a year, it moves an average of 159 million tons (144 million metric tons) of sediment.

In general, deltas are similar in shape to another type of landform deposited by flowing water, alluvial (pronounced ah-LOO-vee-al) fans.

A river creates a delta, like that of the Colorado River, seen here, by laying down sediment or rock debris that it has picked up and carried along its course. PHOTOGRAPH REPRODUCED BY PERMISSION OF THE

Found typically in desert and other arid (dry) environments, these fanlike deposits of sediment form where an intermittent, yet rapidly flowing canyon or mountain stream spills out onto a plain or relatively flat valley. An alluvial fan is a landform that forms on land. A delta is a landform that forms in water. (For further information on alluvial fans, see the Dune and other desert features chapter.)

A delta may be divided into three main zones: upper delta plain, lower delta plain, and subaqueous (pronounced sub-AY-kwee-us) delta plain. The upper delta plain is that part of the delta that is farthest inland. It lies above the high tide mark and is not affected by the action of waves or tides. (Tide is the periodic rising and falling of water in oceans and other large bodies of water in response to the gravitational attraction of the Moon and the Sun upon Earth.) The river or stream that forms the delta begins to divide in the upper delta plain into smaller channels called distributaries, which carry sediments toward the delta's edges. Immediately seaward of the upper delta plain is the lower delta plain. It occupies the area between high and low tides and, thus, periodically lies underwater. The landscape is affected by the action of distributaries, tides, and waves. Finally, the subaqueous delta plain is that part of the delta that lies below the low tide mark and, as a result, lies completely underwater.

The tug of war between land and water determines a delta's shape. It is a battle that pits the strength of a river's flow and the amount of sediment it carries against wave and tidal currents. Deltas build outward from a coast only if the slope from the shore is gentle and ocean currents are not strong enough to carry away the sediment deposited by the river. The three main varieties of deltas based on shape are the arcuate (pronounced AR-cue-et), the bird's foot, and the cuspate (pronounced KUSS-pate).

Delta: Words to Know

Alluvial fan: A fanlike deposit of sediment that forms where an intermittent, yet rapidly flowing canyon or mountain stream spills out onto a plain or relatively flat valley.

Alluvium: A general term for sediment (rock debris such as gravel, sand, silt, and clay) deposited by running water.

Bottomset bed: A fine, horizontal layer of clay and silt deposited beyond the edge of a delta.

Dissolved load: Dissolved substances, the result of the chemical weathering of rock, that are carried along in a river or stream.

Distributaries: The channels that branch off of the main river in a delta, carrying water and sediment to the delta's edges.

Erosion: The gradual wearing away of Earth surfaces through the action of wind and water.

Foreset bed: An inclined layer of sand and gravel deposited along the edge of a delta.

Suspended load: The fine-grained sediment that is suspended in the flow of water in a river or stream.

Topset bed: A horizontal layer of coarse sand and gravel deposited on top of a delta.

Arcuate deltas are the commonest form of delta. They are fan-shaped, with the wide portion of the fan farthest from the mainland. Crossed by many short, well-defined distributaries, these types of deltas are composed of relatively coarse sediments. Wave and river activity are fairly well balanced. The seaward edge of the delta is rather smooth because strong waves push the sediment back against that edge. The Nile Delta is an example of an arcuate delta.

Where the action of waves is weak and that of a river is strong, an irregular-shaped delta forms that extends out into the water well beyond the local shoreline. Resembling the spread claws of a bird's foot, this type of delta is called a bird's foot delta. Fine sediments and shifting distributaries mark this river-dominated delta. Bird's foot deltas are not common along ocean coasts because the action of ocean currents and waves is often as strong if not stronger than that of rivers. The Mississippi Delta, on the Gulf of Mexico, is a bird's foot delta.

Cuspate deltas form where a river drops sediment onto a straight shoreline with strong waves that hit head-on. The waves force the sediment to spread outwards in both directions from the river's mouth, making a pointed tooth shape with sides that curve inward. Few distributaries are found in cuspate deltas. The Tiber Delta in Italy is a classic example.

Forces and changes: Construction and destruction

Deltas are found throughout the world, except at the poles. Most of the world's great rivers—the Amazon, the Ganges-Brahmaputra, the Huang He, the Mississippi, the Nile—have built massive deltas. All have a few characteristics in common: they drain large land areas, they carry large quantities of sediment, and they empty at coasts that are geologically quiet (no earthquake or volcanic activity).

Deltas are geologically young landforms. Present-day deltas began forming no more than 7,000 years ago, when sea levels stopped rising after the last ice age ended. Over Earth's history, as sea levels have risen and fallen in response to glacial periods, deltas have formed and have been covered over. The current deltas of some rivers are built on the remains of numerous deltas stretching back millions of years. Yet their surface can change rapidly and significantly. The key to the creation of a delta, and its continual formation, is a river and the sediment it transports.

Running water

Water is a natural force of erosion everywhere on Earth. As it surges over a landscape, water picks up and transports as much material from the surface as it can carry. Gravity and steep slopes aid rushing water in carrying increasingly larger and heavier objects. Erosion by water begins as soon as raindrops hit the ground and loosen small particles. During heavy rains, sheets of water flow over the ground, loosening and picking up even more particles. This water quickly concentrates into channels, which then become streams that flow into rivers.

The amount and size of the material that a river can transport depends on the velocity, or speed, of the river. A fast-moving river carries more sediment and larger material than a slow-moving one. A river that is turbulent or agitated can also lift and carry more rocks and sediment than one that flows gently.

The sediment load in a river consists mainly of two parts. The first part is the coarse material that moves along the bed or bottom of the river. This is known as the bed load. As it is carried along, this coarse sediment acts as an abrasive, scouring and eating away at the banks and bed of the river. The river then picks up any newly loosened and eroded material. The second part is the fine-grained material that is suspended in the flow of water as the river moves downstream. This is the suspended load. Rivers also carry a dissolved load. These dissolved substances are the result of the chemical weathering of rock, which alters the internal structure of minerals by removing or adding elements.

A river will continue to carry its load as long as its velocity remains constant or increases (if it increases, it can carry an even larger load). Any change in the geography of the landscape that causes a river channel to bend or rise will slow the flow of water in a river. As soon as a river's speed decreases, it loses the ability to carry all of its load and a portion will be deposited, depending on how much the river slows down. Particles will be deposited by size with the largest settling out first.

Laying it down in a delta

When a river meets the standing water of an ocean at a coast, it quickly loses velocity and the heaviest particles drop out. The fine suspended load may be carried farther out into the water before it settles out and sinks to the bottom. Sediments deposited in a delta are laid down in layers known as beds. Bottomset beds are those nearly horizontal or flat layers of fine clay and silt that form underwater farthest from the mouth of the river. Closer to the mouth, yet still underwater, are foreset beds of sand and gravel that slope steeply down toward the bottomset beds at an angle up to 25 degrees. Thin, horizontal layers of coarser sand and gravel that are deposited on the surface of the delta are topset beds. As a delta increases in size and advances farther out into the water, the topset beds cover the foreset beds, which in turn cover the bottomset beds.

As more sediments are brought by the river to the delta, especially in times of flooding, the main river may become choked with sediment. When this occurs, the river branches into distributaries, finding the least resistant path to the shoreline. When sandy deposits block the distributaries, they then become inactive, and smaller, active distributaries branch off. As the process continues, distributaries constantly shift position across the surface of the delta.

The Greatest Sediment Load

The Huang He is the second longest river in China. It begins in the highlands of Tibet and flows eastward for 3,000 miles (4,830 kilometers) before it empties into Bohai Bay. Along its course, it drains more than 290,000 square miles (751,100 square kilometers) of land area. It is the muddiest river in the world, carrying more sediment than any other. Each year, it transports an estimated 1.6 billion tons (1.45 billion metric tons) of sediment. Because that sediment colors the water of the river yellow, the river is also known as the "Yellow River." Much of that sediment is deposited in a delta that has formed at the mouth of the Huang He. It increases in size by as much as 20 square miles (50 square kilometers) each year.

A delta is often a patchwork of marshes, swamps, lakes, and tidal flats (muddy or marshy areas that are covered and uncovered by the rising and falling tides). During the normal flow of the main river in a delta, all the water is guided out to the ocean by the active distributaries. Sediment is deposited in these channels and immediately offshore to them. The areas between the channels receive no sediment. In times of flood, water flows out of the distributaries over the delta surface, depositing sediment. Coarse sandy particles are deposited first, producing low ridges or embankments along the banks of the distributaries. These are known as natural levees (pronounced LEH-veez).

When the balance between a river and ocean is shifted, the delta will either enlarge or decrease in size. If waves and currents are not strong enough to carry most of the sediments away, those sediments will collect over time to form landmasses laced with distributaries that extend a delta farther and farther out to sea. Floods and periods of heavy rain bring more sediment to a delta, building it up. Periods of drought, however, have the opposite effect. Human activity may also affect the size of a delta. If forests or similar types of land upstream are cleared, increased erosion may occur, sending more sediment downstream to build up a delta. If a dam is built on or water is otherwise diverted from a river, the velocity of the river and the amount of sediment it can carry will decrease. Consequently, the delta at its mouth will shrink.

Spotlight on famous forms

Mississippi Delta, Louisiana

The waters of almost half a continent flow through the Mississippi River. About 159 million tons (144 million metric tons) of sediment—70 percent of which consists of clay, silt, and fine sand—are carried by the river annually. Where it empties into the Gulf of Mexico in southern Louisiana, the river slows and drops its sediment load, forming the Mississippi Delta. The giant bird's foot delta, featuring a large middle toe, marks the seaward growth of land into the gulf.

The delta, the most fertile area of Louisiana, covers about 13,000 square miles (33,670 square kilometers), roughly 25 percent of the state's land area. It measures about 12 miles (19 kilometers) long and 30 miles (48 kilometers) wide. In the delta, the Mississippi River breaks into a number of distributaries, the most important of which are the Atchafalaya (pronounced uh-cha-fuh-LIE-uh) River and the Bayou Lafourche (pronounced BYE-oo luh-FOOSH). The main river continues southeast through the delta to enter the gulf through several mouths, including Southeast Pass, South Pass, and Pass à Loutre.

Geologists believe the present delta has been built outward into the gulf over the last 600 years. The river has built its unique shape because it carries so much sediment and the Gulf of Mexico has such a limited tidal range. One result of this is that the river's distributaries travel very long distances to reach the gulf. Over time, the river switches its route to the sea, taking a shorter and more energy-efficient route. The Mississippi River has done this at least five times in the last 5,000 years.

The Mississippi River Delta is formed where the river empties into the Gulf of Mexico in southern Louisiana. PHOTOGRAPH REPRODUCED BY PERMISSION OF THE CORBIS CORPORATION

When Rivers Run Into the Ocean

Where rivers meet the ocean is called the mouth of the river. Soil and dirt carried by these rivers is deposited at the mouth, and new land is formed. The new, soil-rich land is known as a delta.

Aerial View of the Mississippi River Delta Scattered across the delta at the end of the Mississippi River is the city of New Orleans. By the time the mighty Mississippi winds its way south through America's center it becomes a force well over a mile wide. It's a very dramatic sight to watch as this mammoth river spills into the ocean.

Because the Mississippi watershed drains much agricultural land, it has high sediment content. The mud spilling from the river into the Gulf of Mexico is clearly visible in this picture.

This information is from: http://www.physicalgeography.net/fundamentals/10z.html

Within a single stream we can often recognize three different channel types. These unique channel types develop in response to changes in stream velocity, sediment texture, and stream grade.

Channels located in the upper reaches of many streams tend to be narrow with flow moving at high velocities (Figure 10z-1). The high flow velocities found in these streams are the result of a steep grade and gravity. Within these stream systems, erosion is a very active process as the channel tries to adjust itself to the topography of the landscape. Deposition occurs primarily during periods of low flow. As a result, floodplain deposits are very limited, and the stream bed is very transient and shallow.

Streams with high sediment loads that encounter a sudden reduction in flow velocity generally have a braided channel type (Figure 10z-2). This type of stream channel often occurs further down the stream profile where the grade changes from being steep to gently sloping. In a braided stream, the main channel divides into a number of smaller, interlocking or braided channels. Braided channels tend to be wide and shallow because bedload materials are often coarse (sands and gravels) and non-cohesive.

Meandering channels form where streams are flowing over a relatively flat landscape with a broad floodplain (Figure 10z-3). Technically, a stream is said to be meandering when the ratio of actual channel length to the straight line distance between two points on the stream channel is greater than 1.5. Channels in these streams are characteristically U-shaped and actively migrate over the extensive floodplain.

Within the stream channel are a variety of sedimentary beds and structures. Many of these features are dependent upon the complex interaction between stream velocity and sediment size.

Streams carrying coarse sediments develop sand and gravel bars. These types of bars seen often in braided streams which are common in elevated areas (Figure 10z-4). Bars develop in braided streams because of reductions in discharge. Two conditions often cause the reduction in discharge: reduction in the gradient of the stream and/or the reduction of flow after a precipitation event or spring melting of snow and ice.

Point bars develop where stream flow is locally reduced because of friction and reduced water depth (Figure 10z-5). In a meandering stream, point bars tend to be common on the inside of a channel bend.

In straight streams, bar-like deposits can form in response to the thalweg (red arrows Figure 10z-6) and helical flow. Figure 10z-6 below shows an overhead view of these deposits and related features.

In this straight channel stream, bars form in the regions of the stream away from the thalweg. Riffles, another type of coarse deposit, develop beneath the thalweg in locations where the faster flow moves vertically up in the channel. Between the riffles are scoured pools where material is excavated when the zone of maximum stream velocity approaches the stream's bed. The absolute spacing of these features varies with the size of the channel. However, the relative distance between one riffle and the next is on average five to seven times the width of the channel (exaggerated in diagram). Both of these features can also occur in sinuous channels.

Dunes and ripples are the primary sedimentary features in streams whose channel is composed mainly of sand and silt. Dunes are about 10 or more centimeters in height and are spaced a meter or more apart. They are common in streams with higher velocities. Ripples are only a few centimeters in height and spacing, and are found in slow moving streams with fine textured beds. Both of these features move over time, migrating down stream. Material on the gently sloping stoss-side of these features rolls and jumps up the slope under the influence of water flow. Particles move up the slope until they reach the crest of the feature and then avalanche down the steeper lee-side to collect at the base of the next dune or ripple. This process is then repeated over and over again until the material reaches a location down stream where it is more permanently deposited.

Alongside stream channels are relatively flat areas known as floodplains (Figure 10z-7). Floodplains develop when streams over-top their levees spreading discharge and suspended sediments over the land surface during floods. Levees are ridges found along the sides of the stream channel composed of sand or gravel. Levees are approximately one half to four times the channel width in diameter. Upon retreat of the flood waters, stream velocities are reduced causing the deposition of alluvium. Repeated flood cycles over time can result in the deposition of many successive layers of alluvial material. Floodplain deposits can raise the elevation of the stream bed. This process is called aggradation.

Figure 10z-7: The following Landsat 5 image taken in September 1992 shows a section of the Missouri River at Rocheport, Missouri. The oblique perspective of this image is looking westward or upstream. This image has been color enhanced and modified to show an exaggerated topographic relief. Bare soil and plowed land appears red, vegetation appears green, and water is dark blue. A flat river flood plain can be seen in the center of the image. Because of the season, most of the farmland located on the rich and fertile soils of the floodplain is plowed and devoid of vegetation. (Source: NASA Scientific Visualization Studio).

Floodplains can also contain sediments deposited from the lateral migration of the river channel. This process is common in both braided and meandering channels. Braided channels produce horizontal deposits of sand during times of reduced discharge. In meandering streams, channel migration leads to the vertical deposition of point bar deposits. Both braided and meandering channel deposits are more coarse than the materials laid down by flooding.

A number of other geomorphic features can be found on the floodplain. Intersecting the levees are narrow gaps called crevasses. These features allow for the movement of water to the floodplain and back during floods. Topographical depressions are found scattered about the floodplain. Depressions contain the some of the finest deposits on the floodplain because of their elevation. Oxbow lakes are the abandoned channels created when meanders are cut off from the rest of the channel because of lateral stream erosion.

Streams flowing into standing water normally create a delta (Figure 10z-8 and 10z-9). A delta is body of sediment that contains numerous horizontal and vertical layers. Deltas are created when the sediment load carried by a stream is deposited because of a sudden reduction in stream velocity. The surface of most deltas is marked by small shifting channels that carry water and sediments away from the main river channel. These small channels also act to distribute the stream's sediment load over the surface of the delta. Some deltas, like the Nile, have a triangular shape. Streams, like the Mississippi, that have a high sediment content and empty into relatively calm waters cause the formation of a birdfoot shaped delta.

Most deltas contain three different types of deposits: foreset, topset and bottomset beds. Foreset beds make up the main body of deltas. They are deposited at the outer edge of the delta at an angle of 5 to 25 degrees. Steeper angles develop in finer sediments. On top of the foreset beds are the nearly horizontal topset beds. These beds are of varying grain sizes and are formed from deposits of the small shifting channels found on the delta surface. In front and beneath the foreset beds are the bottomset beds. These beds are composed of fine silt and clay. Bottom set beds are formed when the finest material is carried out to sea by stream flow.

An alluvial fan is a large fan-shaped deposit of sediment on which a braided stream flows over (10z-10). Alluvial fans develop when streams carrying a heavy load reduce their velocity as they emerge from mountainous terrain to a nearly horizontal plain. The fan is created as braided streams shift across the surface of this feature depositing sediment and adjusting their course. The image below shows several alluvial fans that formed because of a sudden change in elevation.

Figure 10z-10: Alluvial Fans - Brodeur Peninsula, Baffin Island, Canada. (Source: Natural Resources Canada - Terrain Sciences Division - Canadian Landscapes).

This information from: http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=9304

As the Mississippi River enters the Gulf of Mexico, it loses energy and dumps its load of sediment that it has carried on its journey through the middle of the North American continent. This pile of sediment, or mud, accumulates over the years building up the delta front. As one part of the delta becomes clogged with sediment, the delta front will migrate in search of new areas to grow. The area shown in this false-color image is the currently active delta front of the Mississippi. The migratory nature of the delta forms natural traps for oil. Most of the land in the image consists of mud flats and marsh lands. There is little human settlement in this area due to the instability of the sediments. The main shipping channel of the Mississippi River is the broad stripe running northwest to southeast. This scene covers an area of 54 by 57 km, and was acquired on May 24, 2001 by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) aboard NASA’s Terra satellite.

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