Mississippi River Anatomy

THE MISSISSIPPI RIVER

This is the mighty force that has formed the largest delta in North America.  The Mississippi drains 41% (1.25 million sq mi) of the continental U.S., including 31 states and two Canadian provinces.  It is 1200 mi by river from Cairo, IL, to the mouth of the river, and only 600 mi as the crow flies; also, it is 569 mi from the Arkansas/Louisiana line to the Gulf by river, but only half that in a straight line distance.  Over this 569 mi length, its elevation drops 115 ft, or 2.5 in/mi.  Rufus LeBlanc, an early expert on the geology of coastal Louisiana, said “No pool table is that flat.”

A few years ago, I was visiting the Jamestown Nature Center in the snow belt country of eastern New York.  The grounds were covered with 30 in of snow and the ponds were all thickly iced over.  As we stood in the cold, far from New Orleans, we watched a trickle of water flow from below the ice, over a weir, and back under the ice of an adjacent creek.  Jim Yaich, then the nature center director, said, "This water runs from our ponds, down this stream into Conewango Creek, then into the Allegany River, on to the Ohio River, and finally into the mighty Mississippi."  This struck home:  Here I stood in 30 in of snow watching water molecules flow from beneath ice that were potentially destined to be my drinking water at home!  The Mississippi truly has a vast drainage basin.

There are presently no major tributaries in Louisiana, but the Red River was connected to the Mississippi River at one time.  The only current distributary is the Atchafalaya River (other than the short channels near the river's mouth:  Southwest Pass [where all commercial shipping travels], South Pass [lots of crew boats and big time sports fisherpeople fishing out of Port Eads], and Pass a Loutre [small vessels and some barges]).  There were many channels that during historic times functioned only during high water periods when they would become navigable.  The last three that were separated from the river, and their last date of connection, are as follows:

Bayou Plaquemines - 1770

Bayou Manchac - 1826

Bayou LaFourche – 1902 – now slightly reunited

 

ANATOMY OF A RIVER

from Kniffen, F.B. 1968. Louisiana, its land and people. LSU Press

 

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.

 

Rivers will take the path of least resistance toward the sea.  As they pass through an area, they tend to move toward areas where the soil is more friable (such as sands) and away from more dense soils (such as clays).  This causes the characteristic meander that we associate with river beds.  They actually tend to do this more above their distributaries.  The lower portions of rivers (below their distributaries) tend to have less meandering because:

-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.

Back to the word meander.  The derivation of this word is from the Maiandros (now the Menderes) River of Phrygia (now northwest Turkey) that had this characteristic meander.  If one maps all the former paths of a river where it has changed over time, one sees a meander belt as follows:

from Fisk, 1947

The figure below illustrates the anatomy of a meandering river:

from Strahler

-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.

Question:  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 middle image below.

An oxbow lake on the Rio Apure in Venezuela.

 

-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 far right image below:

Towheads, islands, & chutes on the Mississippi River above Baton Rouge, LA.

 

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 shallower.  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 “cannibalizes” its own sediment (i.e., it cuts into the bottom sediment and moves it down stream, so it gets deeper at that point).  

 

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 riprap, dredging, digging new channels, reinforcing banks, etc.

The principal section model in Vicksburg is that of the Old River Control Complex (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!

 

HOW MUCH WATER DOES THE MISSISSIPPI RIVER HANDLE?

Stream flow is measured in cubic feet per second (cfs).  This measure means that one cubic foot of water passes a point each second.  Imagine a creek in your yard that is one foot wide and one foot deep.  You place a string across the creek, drop a cork on the surface at the string, and one second later mark where the cork has reached downstream.  Let's say it traveled one foot.  During this one second, the amount of water flowing in your creek past the string was one cubic foot (remember, one foot wide, one foot deep, and it traveled one foot distance), so the flow rate is 1 cfs.

The Mississippi River is much bigger!  Its flow rate varies from annual averages of over 700,000 cfs to around 200,000 cfs.  Flow rates are higher in the spring (especially April) when the northern snows are melting and the spring rains abound, and they are lowest in the fall (especially September and October).   The following graph shows monthly averages over a 30 year study period:

Most of us have trouble visualizing 450,000 cfs.  I do, too, so I called my friends at the Audubon Zoo and asked how much water an elephant can snoot into its snoot at one time.  They told me the average is about 3 gal.  I calculated that 450,000 cf of water is equal to 3,366,000 gal, so it would take just over 1 million elephants per second to keep the Mississippi River drained. 

 This, of course, would be impossible since 1 million elephants could never change places in a second!

Another way to visualize this amount of water is that it will take just 4 min to completely fill the Superdome!  (The dome is 273 ft high and covers 9 acres.  9 ac = 391,500 sf, so the volume of the dome is 273 ft x 391,500 sf = 106,879,500 cf.  106,879,500 cf / 450,000cfs = 237.5sec, or, about 4min).

Yet one more way is to see how the flow relates to the size of Louisiana.  Our state is 48,523 sq mi, or, 1.35 trillion sq ft.  Using the average flow rate of 450,000 cfs, this expands to 106.434 trillion gal/yr.  These numbers tell us that the average flow rate of the Mississippi River could cover our entire state in 10 ft of water in one year!

The following graph shows the average yearly flow rates of the Mississippi River over a 30 year study period.

  Now we see that although the trend is to year after year have the high water in the spring and the low in the fall, the annual averages from year to year vary enormously.  The above graph presents data accumulated over a 30 year (1935-1964) study.  The dotted line represents the 30 year average flow rate - 450,000 cfs.  This means that, over this 30 year study, the average was that each second of the 30 years, 450,000 cfs of water flowed down the river!!!

 

The lowest recorded flow rate was 85,000 cfs (November 4, 1939).  The highest recorded flows in the Mississippi River include:

1927    2,345,000 cfs

1973    2,261,000 cfs

1983    2,150,000 cfs

1945    2,123,000 cfs

1950    2,054,000 cfs

1979    2,005,000 cfs

1937    1,977,000 cfs

1975    1,927,000 cfs

[These flow rates are based on latitude flow.  Latitude flow is a designation used by the U. S. Army Corps of Engineers to designate the combined flows of the Mississippi, Red, Ouachita, and Black rivers.  Imagine a line drawn across the Old River Control Structure near Red River Landing at about latitude 30°58'N.  All the water flowing across that line is contained in the measurement, regardless of its source.]

 

In 1945 and 1950, the year average was 700,000 cfs.

 

HOW MUCH SEDIMENT IS CARRIED IN THE MISSISSIPPI RIVER?

 

The amount of soil being carried is termed the river's sediment load.  It is divided into two portions:

-suspended load - that which is suspended in the water column.

-bed load - that which is moving along the bottom.  Studies have shown that particles in the bed load actually somersault (move by saltation) along the bottom.

Though sediment loads in the river had been occasionally studied, the annual data collection and reporting began in 1949 for the Lower Mississippi, and 1951 for the Atchafalaya.  The data are compiled on 26 samplings between October and September of each year.  It is very important to understand that these data, collected and maintained by the U. S. Army Corps of Engineers, are extrapolations based on 26 collections, coupled with estimates of water flow in the river.  They are the best information available, and the methods followed in their accumulation are rigorous.  However, one should be aware of the variability that may (probably) exist in a system when several variables are estimated based on occasional samples.  This is not stated to focus on a weakness, but instead to caution about the true nature of current information.  

Before March 1958, the Mississippi River samples were taken in Baton Rouge.  Between March 1958 and June 1963, they were taken at Red River Landing, which is located at mile 302.5 on the Mississippi River.  When the Old River was closed in 1963, the sampling site was moved to Tarbert Landing (mile 306.3, at 31º00”N), in order to stay just below the point where the Mississippi splits into the Lower Mississippi and the Atchafalaya.  In the Atchafalaya, the measurements are taken at a gauge in Simmsport, La.

Since 1949, the total sediment load has varied from a combined low of 92,909,000 tons in 2005-06 (lowest for Lower Mississippi: 2005-06 - 69,746,000 tons; lowest for Atchafalaya: 2005-06 – 23,163,000 tons) to a combined high of 604,850,000 tons in 1951-52 (highest for the Lower Mississippi: 1950-51 - 575,280,000 tons; for the Atchafalaya: 1956-57 - 225,474,000 tons).  It does, however, fluctuate greatly as shown in the following chart:

 

Sediment Loads in the Mississippi River System

 

LOOK AT THE NUMBERS BY YEAR CLOSELY: Note how variable the loads are from year-to-year.  There is a troubling trend toward less sediment. There is obviously less sediment carried in the river today, but huge annual changes have not been unusual (see 1972-73, 352 million tons; 1973-74, 340 million tons; 1974-75, 322 million tons; then 1975-76, down to 171 million tons). These variations are presumably caused by most of the easily mobilized top-soil of the Mississippi River drainage basin being gone, agricultural practices are now in place to protect and hold top-soil more efficiently, soil is now trapped in a myriad of dams along the tributary system, and there are other activities that result in sediment reduction in the Mississippi River.

The following list of decades shows the number of years in which the sediment loads in the Lower Mississippi River system dropped below 200 million tons during the year.  The last two decades have witnessed marked declines.
1997-2006 – 8 times
1987-1996 – 6 times
1977-1986 – 1 time
1967-1976 – 2 times
1957-1966 – 3 times
1951-1956 – 1 time (6 years only)

 

HOW MUCH SEDIMENT CAN THE RIVER CARRY?

Since it is the movement of water (its energy) that supports the load, slower moving water cannot carry as much sediment as faster moving water.  Studies in the early 1960s showed that the Mississippi River's sediment load is a minimum of 0.7 tons/day/cfs at low stage in October.  The maximum is 2.6 tons/day/cfs at flow rates of 700,000 cfs and above (once the 700,000 threshold was reached, increased flow rate did not result in increased sediment load).  The daily average sediment load at the time was about 1.8 tons/day/cfs.

 

How much sediment per year?  1.8 tons/day/cfs average X 450,000 cfs average flow rate = 810,000 tons/day or about 300,000,000 tons/year.

 

Play time:  Dirt weighs about 3375 lb/cu yd (125 lb/cu ft).

300,000,000 T = 177,777,777 cu yds

It would take 11,111,111 dump trucks (average 16 cu yd variety) to move this soil.  These trucks are about 20 ft long, so if we placed them bumper to bumper, they would form a line 42,087 miles long, or about twice around the world.  That's a lot of dirt!!