Ecology of the Mississippi River Delta Region

See the adjacent figures “Anatomy of a River Basin” and “Anatomy of a River." The key terms are as follows:

• Drainage, or Catchment, Basin - the zone in which the river derives its water via drainage; the channels that contribute to the river are called tributaries.
• Alluvial Plain - the zone that is the principal path of the river, where it can spread out and deposit its sediment (which is called alluvial soil).
• Deltaic Plain - the zone that is built entirely of sediment carried by the river and deposited in the area of its distributaries (the channels that “distribute” the water).
• Receiving Basin - the zone (a lake, Gulf, ocean, sea, etc.) into which the river’s water flows at its mouth.
• Channel - the main bed of a river, stream, bayou, etc.
• Thalweg - the deepest point in a channel; the shallowest in the Mississippi River in Louisiana is south of Baton Rouge at 35 ft; the deepest is off the Moon Walk in the French Quarter of New Orleans where it is over 200 ft.
• Backslope - the slope down the back of a levee (natural or human-made).
• Backswamp - places that hold water behind levees after the river recedes back into its channel.
• Batture - the area between the inside base of a levee and the river's edge.
• Natural levee - the high ground at the margin of a river that resulted from the natural processes associated with the river overflowing its banks and depositing soil. Humans have modeled nature by constructing very high levees that prevent normal, natural overflow.

• there are more clays and they are harder to scour.
• the annual fluctuation of the river level is less due to more outlets being available to the sea.

The middle illustration on the same (see the above figure). shows the anatomy of a meandering river.

• Point bar - a growing section due to water slowing down as it turns a corner. When it slows, it has less energy, so it drops any sediment it is carrying, thus the water on a point bar is shallower than elsewhere. If growth of the point bar proceeds smoothly, then the point bar is smooth. If growth stops and starts, then ridges may form on the point during no growth periods and, as it begins renewed growth, swales form.
• Cutbank - an area where the water is turning and cutting into the bank, so the bank is being lost. The thalweg is near the cutbank since this is where the current is fastest, so it cuts into the channel. One finds the broadest natural levees on the cutbank side because that is where overflow most often takes place. [If you were buying a farm near a meandering river, would you choose property on the cutbank side or the point bar side?]
• Cutoffs, oxbows, blind, or horseshoe lakes - when two cutbanks approach one another too closely, a neck forms and there is a strong likelihood of a crevasse occurring. When the crevasse occurs, the river prefers to run straight, so, over time, it abandons the old route. Since no water is regularly passing through, sediment falls out and vegetative growth adds more organic matter until, finally, the ends fill up and a oxbow lake is formed. Ultimately, the lake fills in, though one can see the evidence of a past oxbow for decades. (see the figure).

  How an Oxbow Lake Forms

  

• Islands and towheads. If a river becomes too wide (i.e., for any reason becomes so wide that its water speed is insufficient to carry its sediment), then it drops the sediment and forms islands (high with vegetation growing on them) or towheads (low, sandy exposed area with virtually no vegetation). If a narrow water pass occurs between them and the land, the water is called a chute. (see the figure).

Islands and Towheads

Can you identify the islands and towheads? Note the oxbow lake to the left.

WHY ARE RIVERS SHALLOWER AT THEIR MOUTHS THAN UPSTREAM?
Rivers usually bifurcate as they travel downstream, especially near their mouths. With each bifurcation, there is less energy in each new stream than in the original stream. Since there is less energy, sediments more readily settle out, and the stream gets more shallow. As deltas move forward due to growth, the depth of the river at a given point increases. This is because the river continues to bifurcate during deltaic growth, and the “friction” resulting from these new, smaller streams causes the water pressure at the designated point to increase, so it “canabalizes” its own sediment (i.e., it cuts into the bottom sediment and moves it down stream, so it gets deeper at that point). (see the adjacent figure).

MODELS OF THE MISSISSIPPI RIVER
Since rivers can be very dynamic, one might ask how do agencies such as the U.S. Army Corp of Engineers know how the river flow is impacted by more water, changes in vegetation, new channels, caved-in banks, and the like. The answer is amazingly simple. They use models, one complete river model in Clinton, MS, and one highlighting smaller sections in Vicksburg, MS.

The Clinton (MS) model was built during World War II (supposedly by German prisoners). It is about 100 yds long and made of concrete. The model consists of a impression in the concrete that serves as the basic mold. Known flow rates can be varied proportionately to the size of the model. If engineers want to see what happens if more trees grow in flow areas, they simply fold hardware cloth and place it appropriately. They can block the flow in certain areas, and put peg in the bottom to create drag on the flow. The impact of their changes are recorded by a myriad of water-level gauges, each connected to computers. When the chosen flow rate is introduced, the water-level gauges note the altered water-level and the computers correlate that data. The computer out-put is then analyzed and the engineers learn how to adjust when the river and its anatomy changes - as it inevitably does. They can then force desirable changes by adding rip-rap, dredging, digging new channels, reenforcing banks, etc.

The principal section model in Vicksburg is that of the Old River Control Structure (see later in notes). It is a scale model and the engineers simply add scale models of rocks, walls, channels, etc., and note their change. When desirable alterations are discovered, they go out and, at the real structure, implement the program at real size. As strange as this may be, it works!

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