Setting: Geology, Hydrology, Sediments, and Engineering of the Mississippi River

By Robert H. Meade

Geologic Settings

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Figure 4. -- The Mississippi River follows the trend of an ancient continental rift system down the center of North America, flowing through different landscapes that record different geologic histories (Redfern, 1983). Three vignettes (A, B, and C) in this figure exemplify some of the different processes that shaped the landscapes during the latest ice ages and during the 10,000 years since the ice melted away.

A

The Upper Mississippi River ("Upper" is conventionally assigned to the Mississippi above its confluence with the Ohio) flows for much of its length through a valley 1-10 km wide between bluffs that stand 50-100 m above the river and its fringing flood plain. The painting, done in 1844 by John Casper Wild, shows the bluffs above the confluence of the Minnesota (left) and Mississippi (right) Rivers, at the site of Fort Snelling, which is now included in the city of Minneapolis. The bluff-bordered river valleys here and farther down the Upper Mississippi were formed by a combination of glacial and riverine processes. The great ice ages of the Pleistocene epoch gave the basin of the Upper Mississippi much of the shape that we consider characteristic today. Pre-existing river valleys were widened and deepened by the ice as it pushed its way south. Between and after the ice ages, the rivers transported and rearranged the sediments in the valleys by meandering across them and constructing islands and flood plains. The main valley in the painting (upper left to lower right) formed mostly when the ice-age predecessor to the Minnesota River (called Glacial River Warren) was the main outlet of a large lake, Lake Agassiz, that was dammed along its northern margin in southern Canada by the retreating ice sheet. The small notch out of which the Mississippi River is flowing (upper right in painting) was cut, mostly after the demise of Glacial River Warren, by a retreating headcut that can be seen today at St. Anthony Falls, 13 km upriver of the confluence with the Minnesota River.

B

The Lower Mississippi River flows along and through a wide alluvial plain formed by the river and its predecessors. Vignette B (taken from R.T. Saucier, 1991) shows the contrast in stream patterns between those formed during the latest ice age (left) and those formed since the ice ages. Typical of glacial meltwater rivers heavily laden with coarser sediments are the so-called "valley trains" like those shown in the western half of the vignette. Even though no great river has flowed there for thousands of years, the braided and anastomosing pattern still shows on the landscape and is clearly visible from the air. The present pattern, in which the Mississippi River meanders through a belt 20-30 km wide defined by the traces and remnants of older meanders through which the river once flowed, is typical of most of the length of the lower river.

C

Where the Mississippi River meets the Gulf of Mexico, its delta has a complex history that has been described in classic papers by C.R. Kolb and J.R. Van Lopik (1958, 1966). The succession of different river channels and delta lobes during the last 5,000 years are numbered from oldest (1) to youngest (7) in Vignette C. Let John McPhee (1989, p. 5-6) recount the story:

"Southern Louisiana exists in its present form because the Mississippi River has jumped here and there within an arc about two hundred miles wide, like a pianist playing with one hand-frequently and radically changing course, surging over the left or the right bank to go off in utterly new directions. Always it is the river's purpose to get to the Gulf by the shortest and steepest gradient. As the mouth advances southward and the river lengthens, the gradient declines, the current slows, and sediment builds up the bed. Eventually, it builds up so much that the river spills to one side. Major shifts of that nature have tended to occur roughly once a millennium. The Mississippi's main channel of three thousand years ago is now the quiet water of Bayou Teche [3], which mimics the shape of the Mississippi......Eight hundred years before the birth of Christ, the channel was captured from the east [4]. It shifted abruptly and flowed in that direction for about a thousand years. In the second century A.D., it was captured again, and taken south, by the now unprepossessing Bayou Lafourche [5], which, by the year 1000, was losing its hegemony to the river's present course, through the region that would be known as Plaquemines [6, 7]."

Water Discharge

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Figure 5. -- The Mississippi River discharges an average of 520 cubic kilometers of water each year past the cities of Vicksburg and Natchez, Mississippi. This represents the greatest amount of water discharged by the river while it still is confined to a single channel, but it does not represent all water that the Mississippi River system discharges to the Gulf of Mexico. The river bifurcates 77 km below Natchez, and the lesser of its two main distributaries is joined by another significant tributary, the Red River. The two outlets of the Mississippi River eventually discharge a combined average of 580 cubic kilometers per year (or about 420 billion gallons per day) of freshwater to the Gulf of Mexico. This discharge ranks seventh in the world, being exceeded only by those of the Amazon, Congo (or Zaire), Orinoco, Yangtze, the combined Ganges-Brahmaputra, and Yenisey Rivers.

A

Not all parts of the Mississippi River drainage basin contribute water in equal measure. Nearly one-half the water discharged to the Gulf is contributed by the Ohio River and its tributaries (including the Tennessee) whose combined drainage areas constitute only one-sixth of the total area drained by the Mississippi. By contrast, the Missouri River drains 43 percent of the total area but contributes only 12 percent of the total water. As the Mississippi flows southward from its headwaters in the northern Midwest, its discharge is more than doubled by the waters it receives from the Illinois and Missouri Rivers. This combined discharge, in turn, is more than doubled again as it joins the waters of the Ohio River.

B

Just as the spatial distribution of sources of water in the Mississippi River is uneven, so is the temporal distribution of river flow. Shown here are the yearly flows of the Mississippi since 1930 at Keokuk, Iowa, and since 1931 at Vicksburg, Mississippi. Keokuk is at the Iowa-Missouri State line, 235 and 270 km, respectively, up the Mississippi from its confluences with the Illinois and Missouri Rivers. Vicksburg is downriver of the mouth of the Arkansas River and upriver of the Atchafalaya diversion (see fig. 10B), and the discharges recorded at Vicksburg represent the largest and most integrated flows measured in the Mississippi River system. The two graphs are drawn so that the long-term mean discharges at the two stations are represented equally; although the mean discharges at the two stations differ by a factor of nine, the scales and ranges of variability can be compared directly by simple visual inspection. Wet years and dry years at the two cities generally coincide. The range of flow variation is somewhat less extreme at Vicksburg than at Keokuk, which is a reflection of the damping influence of flows from the intervening large tributaries, especially the Illinois, Missouri, Ohio, and Arkansas Rivers. The ratio between the extreme maximum and minimum yearly discharges shown here (1973 compared to 1934) is 5.5 at Keokuk but only 3.1 at Vicksburg.

C

Average seasonal differences in river discharge are on the same order as the extreme annual differences between wet and dry years. Shown here are mean monthly discharges at Keokuk and Vicksburg for the same periods of record as shown in B. In nearly all years along the length of the Mississippi River, mean discharges during the high-water months can be expected to be about three times the discharges during the low-water months. At Keokuk, spring runoff usually begins quickly, in response to the melting of ice on the river. At Vicksburg, the usually high flows from the Ohio River during the months of December through March give a more gradual beginning to the annual peak of spring runoff.

Suspended-Sediment Discharge

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Figure 6. -- The Mississippi River now discharges an average of about 200 million metric tons of suspended sediment per year past Vicksburg and eventually to the Gulf of Mexico. This sediment discharge to the ocean ranks about sixth in the world today, being equaled or exceeded by those of four rivers of Asia (the Yellow and Yangtze Rivers of China, the Ganges-Brahmaputra of India and Bangladesh, and probably the Irrawaddy River of Burma) and two rivers of South America (the Amazon River of Brazil and possibly the Magdalena River of Colombia).

A

The suspended-sediment loads carried by the Mississippi River to the Gulf of Mexico have decreased by one-half since the Mississippi Valley was first settled by European colonists. This decrease has happened mostly since 1950, as the largest natural sources of sediment in the drainage basin were cut off from the Mississippi River main stem by the construction of large reservoirs on the Missouri and Arkansas Rivers (see fig. 8). This large decrease in sediments from the western tributaries was counterbalanced somewhat by a five- to tenfold increase in sediment loads in the Ohio River-an increase that has resulted from deforestation and rowcrop farming. Further complicating the picture today is the controlled diversion of part of the water and sediment from the Mississippi River below Vicksburg into the Old River Outflow Channel and the Atchafalaya River (see fig. 10B). The average suspended-sediment discharges portrayed for 1980-90 are taken mainly from the extensive compilations of M.P. Keown and his colleagues (1981, 1986) and of R.S. Parker (1988).

B

Temporal variation of suspended-sediment discharge in the Mississippi River is more pronounced than that of water discharge (compare with fig. 5B). Shown here are the yearly totals of suspended sediment discharged past three long-term monitoring stations: Burlington, Iowa, 335 km upriver of the confluence with the Missouri River; St. Louis, Missouri, 25 km downriver of the confluence with the Missouri River; Tarbert Landing, Mississippi, 13 km downriver of the Atchafalaya diversion at Old River. Sediment discharges shown for Burlington and Tarbert Landing are based on data of the U.S. Army Corps of Engineers (Rock Island and New Orleans Districts). Although records of sediment at all three stations began some years prior to 1959, only partial records are shown here to eliminate the confusion that might have been introduced by including pre-reservoir sediment discharges at St. Louis (see fig. 8) and pre-diversion sediment discharges at Tarbert Landing (see fig. 10B). The three graphs are drawn so that the long-term mean sediment discharges at all three stations are represented equally and the scales and ranges of variability may be compared by simple visual inspection. Years of high and low sediment discharges generally coincide at all three stations, but the range of year-to-year variation is more extreme in the upper river than in the lower river. The ratios between extreme maximum and minimum yearly suspended-sediment discharges for the periods of record shown here are 11.7 at Burlington, 9.2 at St. Louis, and only 2.8 at Tarbert Landing. Even during 1988 and 1989, when sediment discharges in the upper river were especially small, the Ohio River contributed enough sediment to damp the extremes of year-to-year variation at Tarbert Landing.

C

Average seasonal differences in river-sediment discharges are of the same order as extreme annual differences between wet and dry years in the Upper Mississippi, and they exceed the range of year-to-year differences in the Lower Mississippi. Shown here are the monthly average suspended-sediment discharges at Burlington, St. Louis, and Tarbert Landing for the same periods of record as shown in B. In the Upper Mississippi River (Burlington), average suspended-sediment discharges during the high-water months following ice breakup are nearly ten times greater than discharges during midwinter months when the river usually is covered with ice. In the Lower Mississippi (Tarbert Landing), the late winter-early spring runoff from the Ohio River contributes sediment during the months of December through March. Even with this temporal offset in tributary contributions, the maximum monthly sediment discharge (March) in the lower river averages five times greater than the minimum monthly discharge (September).

Particle Sizes of Sediments

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Figure 7.-- Sediment particles in the Mississippi River range in size from the very finest clays or colloids to coarse sand and gravel. Different sizes of particles are found in suspension and on the river bed, and the interrelations between the sediments being transported in suspension and those stored or being transported along the riverbed are complex and variable. The finest particles play the largest role in the transport and storage of toxic contaminants.

A

In the freely flowing reaches of the Mississippi River downstream from St. Louis, part of the suspended-sediment load interacts with the channel bed and part of it is independent of any such interaction. The examples shown here are data collected from the Mississippi River at Thebes, Illinois, on June 10, 1989, and at Vicksburg, Mississippi, on March 27, 1989. The channel bed at these two sites (and through most of the 1850-km length of the Mississippi between St. Louis and the Gulf of Mexico) consists almost entirely of sand and fine gravel, with few particles, if any, finer than 0.063 mm (millimeter) in diameter. Some of the finest sand (mostly 0.125-0.25 mm) is mobilized from the channel bed to become part of the suspended sediment; hydraulic engineers refer to such sand in suspension as "bed-material load" because it usually represents an exchange of sand particles between the river waters and the beds over which they flow. Most of the sediment in suspension, however, is finer than sand. To aid in understanding the chemistry of the suspended matter, we have divided the fine suspended sediment into two fractions called "silt" and "colloid." The division between the two fractions is here defined arbitrarily at about 0.001 mm. The relative volumes of silt and colloid shown in the figure represent the sizes of the individual particles after they have been disaggregated in the laboratory with a dispersing agent. In the river itself, most of the colloid-size particles are found in aggregates that are large enough to be transported and deposited as silt particles.

B

In the Upper Mississippi River, which has been dammed in order to form a series of lakes to provide depth sufficient for barge navigation (see fig. 10A), the sizes of the particles in both the bed sediments and suspended sediments are distributed differently from those in the unimpounded reaches of the lower river. The examples shown here are data collected from the Mississippi River at Hastings, Minnesota, in the upper end of navigation Pool 3 on October 10, 1991, and from the nonchannel areas of lower Pool 3 on October 11, 1991; and from the Mississippi River near Winfield, Missouri, at the upper end of Pool 26 on July 24, 1991, and from the nonchannel areas of lower-middle Pool 26 on November 1, 1991. In the navigation channels, the bed sediments consist largely of sand, as they do in the channels of the freely flowing lower river. Suspended sediments, however, consist almost entirely of silt and colloidal particles and contain very little sand except during floods. In the shallow nonchannel areas of the navigation pools, which cover the former flood plains of the upper river, the bed sediment is typically intermediate in size-finer than the bed material in the main channels but generally coarser than the bulk of the sediment in suspension.

Effects of Reservoirs

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Figure 8.-- Reservoirs reduce the sediment discharges of the Mississippi River and its tributaries by trapping sediment that otherwise would have been transported downriver.

A

The Missouri River has been the principal supplier of sediment to the Mississippi River since the end of the last ice age. The graphs show the annual discharges of suspended sediment measured by the U.S. Army Corps of Engineers and the USGS at three gaging stations on the Missouri River and two gaging stations on the Mississippi River over a period of about four decades. After five large dams were completed for hydroelectric power and irrigation above Yankton, South Dakota, between 1953 and 1963, the discharge of sediment from the Upper Missouri River Basin virtually was stopped. Following the closure of Fort Randall Dam and Gavins Point Dam in 1953, downriver sediment discharges were diminished immediately, and the effect could be observed all the way down to the mouth of the Mississippi River. Sediment discharges to the Gulf of Mexico in 1992 were less than one-half of what they were before 1953.

B

In this downriver view (looking west-southwest) of Lake Cumberland, a reservoir on the Cumberland River in Kentucky, sediment-laden brown water can be seen flowing into the upper end of the lake during late winter (February 28, 1988); the sediment gradually settles out to leave blue water farther down the lake.

C

Deltas form where rivers flow into reservoirs, especially if the inflowing rivers transport substantial amounts of sand. This large sand delta has formed where the Canadian River flows into Lake Eufala, a large reservoir in eastern Oklahoma near where the Canadian River joins the Arkansas River. The view is down Lake Eufala (looking east-northeast) in early spring (March 25, 1988).

Engineering Activities

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Figure 9. -- For more than 270 years, the banks and the channel of the Mississippi River have been engineered for various purposes: originally, starting about 1720, for flood control; eventually, starting about a century later, for navigation.

A

The Lower Mississippi River from Cairo, Illinois, to the Gulf of Mexico is constrained by a system of flood-control levees that is longer than the Great Wall of China. The Z-shaped levee in this photograph (view north) separates the active flood plain of the Mississippi River (left of levee) from intensively cultivated cropland in the fertile "Delta" region of Mississippi about 30 km upriver of Vicksburg.

B

Dikes and wing dams are constructed to focus the main flow of the river into the navigation channels, and they encourage sediment to deposit in areas of the river that lie outside the navigation channels. These dikes have allowed sandbars to grow and stabilize into permanent islands along the right (western) bank of the Mississippi River about 10 km upriver of Cape Girardeau, Missouri. The sediment stored behind and between dikes such as these is virtually immobilized and is unlikely to be resuspended for transport downriver during the foreseeable future.

C

Bank-protection measures are applied along most of the Mississippi to impede erosion and to maintain the shape of the navigation channel. Shown here is an articulated concrete mat that is laid like carpet on the riverbank by a special machine. Other banks along the Mississippi are stabilized by boulder-size rock fragments that are quarried from bluffs near the banks of the Upper Mississippi and brought downriver by barge. One effect of the bank stabilization is to prevent the remobilization of sediment previously deposited on the flood plains of the river.

D

Despite the controls on water flow and sedimentation that are provided by dikes and other engineering works, some reaches of the river require periodic dredging to maintain the depth of water necessary for navigation. In the Lower Mississippi, as shown in the photograph, the dredged material is frequently piped out to the fast flowing part of the river to be discharged. In the Upper Mississippi, where sand is frequently the material dredged, large spoil banks and artificial islands have been built alongside the main navigation channel.

Major Engineering Works

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Figure 10.-- The two most prominent examples of river engineering on the Mississippi are the lock-and-dam system on the upper river and the Atchafalaya diversion on the lower river.

A

The entire 1080-km reach of the Upper Mississippi River between Minneapolis, Minnesota, and St. Louis, Missouri, is controlled for barge navigation by a series of 29 lock-and-dam structures (O'Brien and others, 1992). One of these structures-the first to be completed, in 1913, at Keokuk, Iowa-was built to impound water to generate hydroelectric power. The other 28 structures were built, mostly during the 1930s, to maintain a minimum river depth of 9 feet (2.7 meters) for barge navigation.

Before the dams were built, navigation during low-water periods was extremely hazardous, if not impossible, across rapids such as those at Keokuk and Rock Island, and it was difficult in many other reaches of the upper river. The lower reaches of the navigation pools, such as the one shown in the photograph, are shallow lakes in which former flood plains, previously inundated for infrequent short periods, are now permanently under water. These shallow lakes are storage areas for fine-grained sediments and the contaminants adsorbed to them.

B

About 500 km upstream from its main outlet to the Gulf of Mexico, the Lower Mississippi River is partly diverted into the Atchafalaya River. About one-fourth, on average, of the water that flows down the Mississippi River past Vicksburg is diverted at a place called "Old River" to join the waters of the Red and Ouachita Rivers in forming the Atchafalaya River (McPhee, 1989). The diagram on the lower right is an excerpt of figure 5A that has been enlarged and rotated 90 degrees so that the direction of flow is to the right. The accompanying photograph (view east, June 1991) shows the Mississippi River flowing from center left to upper right. In the foreground is the Old River Outflow Channel (flow west toward the viewer), an artificial channel that joins the Red River just off the lower edge of the photograph to form the Atchafalaya. Three artificial channels, each containing a control structure, divert water from the Mississippi River into the Old River Outflow Channel: (1) the original channel, in the center, contains the Old River Control Structure, completed in 1963; (2) the southernmost channel (upper right in photograph) contains the Old River Auxiliary Control Structure, completed in 1987; and (3) the northernmost channel (left center) contains a low-head hydroelectric power dam, completed in 1990 and having a rated capacity of 194 megawatts, which supplies electricity to communities in Louisiana.

Keown, M.P., Dardeau, E.A., Jr., and Causey, E.M., 1981,

Characterization of the suspended-sediment regime and bed-material gradation of the Mississippi River Basin: U.S. Army Engineer Waterways Experiment Station Potamology Program (P-1) Report 1, 2 vols., 62 p., 7 app.

___ 1986,

Historic trends in the sediment flow regime of the Mississippi River: Water Resources Research, v. 22, no. 11, p. 1555-1564.

Kolb, C.R., and Van Lopik, J.R., 1958,

Geology of the Mississippi River deltaic plain, southeastern Louisiana: U.S. Army Engineer Waterways Experiment Station Technical Report 3-483, 120 p.

___ 1966,

Depositional environments of the Mississippi River deltaic plain-Southeastern Louisiana, in Shirley, M.L., and Ragsdale, J.A., eds., Deltas in their geologic framework: Houston Geological Society, p. 17-61.

McPhee, John, 1989,

Atchafalaya, in The control of nature: New York, Farrar Straus Giroux, p. 3-92.

O'Brien, W.P., Rathbun, M.Y., O'Bannon, Patrick, and Whitacre, Christine, 1992,

Gateways to commerce-The U.S. Army Corps of Engineers' 9-foot channel project on the Upper Mississippi River: National Park Service, Rocky Mountain Region, 238 p.

Parker, R.S., 1988,

Uncertainties in defining the suspended sediment budget for large drainage basins, in Bordas, M.P., and Walling, D.E., eds., Sediment budgets: International Association of Hydrological Sciences Publication 174, p. 523-532.

Redfern, Ron, 1983,

Fluvial plain, in The making of a continent: New York, Times Books, p. 159-178.

Saucier, R.T., 1991,

Geomorphology, stratigraphy, and chronology, inAutin, W.J., and others, Quaternary geology of the Lower Mississippi Valley, in Morrison, R.B., ed., Quaternary nonglacial geology: Geological Society of America, The Geology of North America, v. K-2, p. 550-564.