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Water Resources Investigations
The USGS has the principal responsibility within the Federal Government to provide the hydro
logic information and understanding needed by others to achieve the best use and management of the Nation's water resources. To accomplish this mission, the Water Resources Division, in cooperation with State, local, and other Federal agencies, • Systematically collects and analyzes data to evaluate the quantity, quality, and use of the Nation's water resources and provides results of these investigations to the public. • Conducts water-resources appraisals describing the occurrence, availability, and physical, chemical, and biologic characteristics of surface and ground water. • Conducts basic and problem-oriented hydrologic and related research that aids in alleviating waterresources problems and provides an understanding of hydrologic systems sufficient to predict their response to natural or human-caused stress. • Coordinates the activities of Federal agencies in the acquisition of water-resources data for streams, lakes, reservoirs, estuaries, and ground water. • Provides scientific and technical assistance in hydrologic fields to other Federal, State, and local agencies, to licensees of the Federal Energy Regulatory Commission, and to international agencies on behalf of the Department of State. • Administers the State Water Resources Research Institutes Program and the National Water Resources Research Grants Program.
(Facing page, top) View upriver toward the northwest of the alluvial valley of the Mississippi River a few miles above Ste. Genevieve, Mo. (Facing page, bottom) View of Lake Cumberland, a reservoir on the Cumberland River in Kentucky, showing sediment-laden river water (brown) flowing into the upper end of the lake. (Above) View upriver (west) showing mixing of the brown, sedimentladen water of the Ohio River (right) with the clearer water of the Mississippi River near Cairo, Ill. The photograph was taken on March 22, 1988, during the spring runoff of the Ohio River before the time of peak runoff in the upper Mississippi River. (Photographs by Robert H. Meade and Terry F. Rees.)
Sediment-Transported Pollutants in the Mississippi River
By Robert H. Meade
The Mississippi River and its tributaries drain about 40 percent of the conterminous United States (fig. 1), and their waters have been used for a wide range of purposes-municipal, industrial, and agricultural — by the time they reach the Gulf of Mexico. Although an exceptional range of known pollutants has been identified in the Mississippi River system, and the river itself is perceived by scientists and the public alike to be polluted, there have been some signs of improvement in recent years.
Large cities have curtailed discharges of untreated sewage into the Mississippi River. The city of Memphis, Tenn., for example, which until 1977 discharged at least 100 million gallons of raw sewage per day into the Mississippi, now discharges only the treated effluent water from its secondary-sewage plants. Another improvement is the recent decrease in the quantity of lead transported by the Mississippi, a result of the
nationwide change from leaded to unleaded gasoline. On the other hand, new chemicals and compounds are being introduced to the river every year. And, as pollution from point sources is gradually curtailed, the pollutants from nonpoint or diffuse sources assume greater importance. In 1987, to ascertain whether the water quality of the river was improving or deteriorating, USGS scientists began a comprehensive assessment of the Mississippi River between St. Louis, Mo., and New Orleans, La.
In addition to providing a present assessment of the water quality of the Mississippi, the study enables USGS scientists to pursue a program of multidisciplinary research on how pollutants are transported by large rivers. Research questions of interest include: How are pollutant substances partitioned between true solution in river water and adsorption onto sediment particles? Which specific fractions of the suspended sediment (the tiniest clay particles versus the coarsest silt grains, for example) are most important to the transport of specific pollutants? How do the various pollutants repartition themselves between adsorbed and dissolved phases when two great rivers, such as the Mississippi and Ohio, each with its own distinctive load of sediment and assemblage of pollutants, converge and gradually mix? How long can we expect a given pollutant adsorbed on a given sediment particle to remain in storage in any given part of the river system?
Figure 1. The Mississippi River drainage basin includes about 40 percent of the conterminous United States and a small area of Canada. A new study begun in 1987 is assessing the water quality of 1,200 miles of the Mississippi River and its major tributaries between St. Louis, Mo., and New Orleans, La.
Although the Mississippi River discharges an average of 420 billion gallons per day to the Gulf of Mexico, not all parts of the drainage basin contribute water in equal measure. Nearly half of the total water in the Mississippi River comes from only one-sixth of the total drainage area—the Ohio River basin, which includes the drainage basins of the Tennessee, Cumberland, and Wabash Rivers (fig. 2). In contrast, the Missouri River, which drains 43 percent of the total area, contributes only 12 percent of the total water. The Mississippi, upriver
Another improvement is
the recent decrease in the quantity of lead transported by the
of the confluence with the Missouri, contributes only 15 percent of the total water that is discharged to the Gulf.
The most striking feature of the lowermost part of the Mississippi River system is the diversion of part of the discharge out of the mainstem and down the distributary Atchafalaya River. This diversion began more than a century ago as part of the Mississippi River's natural tendency to change its lower course to the Gulf of Mexico every 1,000 years or so. Today, the proportion of the water discharge that goes down the Atchafalaya is held at 30 percent of the total (Mississippi River plus Red River) by the Old River Control Structures, completed by the U.S. Army Corps of Engineers in 1963 and 1987.
Mississippi upriver (St. Louis) and downriver (Vicksburg, Miss.) from the mouth of the Ohio. This estimate, however, depends on the assumption that no sediment is being remobilized from storage in the banks and floodplains of the Mississippi between the Ohio River and Vicksburg, an assumption that needs to be tested.
The sediment picture in the Mississippi River was not always as shown in figure 2. In its natural state, the Missouri River was one of the world's most prodigious transporters of sediment. The journals of the Lewis and Clark Expedition of 1804–1806 contain numerous references to shifting sandbars, caving banks, and muddy waters in the Missouri. As recently as 1950, the Missouri River transported sediment in quantities nearly four times greater than it transports to the Mississippi today. Likewise, the Arkansas River before 1950 contributed nearly 10 times as much sediment as it contributes today. Sediment in both the Missouri and Arkansas Rivers was reduced drastically when large dams were constructed across them for hydropower and navigation purposes after World War II. The large reservoirs that formed behind these dams now trap much of the sediment that formerly reached the Mississippi.
Figure 2. Discharges of water (left) and suspended sediment (right) in the lower 1,200 miles of the Mississippi River during an average recent year. Suspended-sediment data refer to average conditions prevalent around 1980 and were compiled by the U.S. Army Corps of Engineers. (Ohio River sediment contribution has a dashed outline to denote uncertainty.)
Sediment and River Quality
Suspended sediment is an important determinant of river quality because many of the pollutants, such as metals, hydrocarbons, and insecticides, are carried in the adsorbed state. The principal sources and quantities of suspended sediment transported by the Mississippi River in an average recent year are shown in figure 2. The present-day discharge of suspended sediment to the Gulf of Mexico by the Mississippi (including the RedAtchafalaya) averages about 210 million tons per year. Most of the principal tributary sediment loads have been measured to a reasonable degree of accuracy, but the estimate of the sediment discharge of the Ohio River represents a major uncertainty. No comprehensive suspended-sediment data have ever been collected from the Ohio mainstem. Instead, the Ohio's contribution has been estimated from the difference between the sediment discharges measured in the
RIVER DISTANCE, IN HUNDREDS OF MILES DOWNRIVER
ment load on a schedule that is episodic and discontinuous. Sediment particles may be transported for a distance downriver and then, if the river stage begins to fall, deposited on the river bed or along its banks. If the river is high enough to overflow its banks, sediment may be deposited on the floodplain. The length of time a particle spends in storage before the next episode of movement is highly variable and ranges from a few days for a particle stored on the river bed to tens or hundreds of years for a particle stored on the floodplain.
River-Quality Assessment: Procedures and Equipment
U.S. Geological Survey chemist steadies apparatus for collecting water and suspended sediment while R/V Acadiana is being positioned for river sampling. Orange poles on the river bank (background) mark the alignment of the section to be sampled, Mississippi River at Helena, Ark. (Photograph by Robert H. Meade.)
Sediment in the Ohio River, on the other hand, has increased over natural levels. Sediment discharges in the Ohio River basin were probably lower during the years before European settlement because the basin was covered by nearly continuous forests, which held the soil in place and retarded erosion. Two centuries of deforestation, crop farming, and coal mining has led to accelerated erosion and an increase of sediment in the Ohio River. On balance, however, the decrease of sediment in the western tributaries has been greater than the increase in the eastern parts of the basin, and the Mississippi now carries to the Gulf of Mexico only about half the sediment it carried before 1950.
The reach of the river being studied in detail begins near the confluences of the Missouri and Illinois Rivers with the Mississippi near St. Louis and continues downriver to New Orleans. Samples are collected from the 57-foot Research Vessel Acadiana, operated by the Louisiana Universities Marine Consortium. The ship is equipped with sufficient laboratory and bunk space to accommodate a 3to 4-week sampling trip along 1,200 miles of the Mississippi River and its principal tributaries. Two trips were completed in 1987, another one in 1988, and an average of two trips per year is planned for the next several years. At each of 16 stations, 9 on the Mississippi mainstem and 7 on tributaries, a comprehensive series of samples is collected to characterize the river water, the sediment carried in suspension, the materials resting on the river bottom, and (on one trip so far) the fish that feed on the bottom. Samples are given preliminary analysis and treatment aboard the research vessel and are shipped to the USGS laboratories in Denver, Colo., for comprehensive analysis of the full range of constituents, organic and inorganic, natural and man-made.
Sediment in the Ohio River, on the other hand,
has increased over
Sediment storage is one further factor that confounds our understanding of suspended sediment and the fate of the pollutants attached to sediment particles in the Mississippi River. A river like the Mississippi transports much of its sedi
Although analyses are still in progress and results are preliminary, USGS chemists W.E. Pereira, C.E. Rostad, and T.J. Leiker have reported several