Rising Tide

by John M. Barry

  In 1850 he had just finished both the Wheeling bridge and a survey of the Ohio River. He had developed theories about the Ohio he believed applicable to the Mississippi as well, and now sought the assignment already given to Humphreys.

  The entire civilian engineering profession and its supporters in Congress demanded that the government give Ellet the job. The War Department and its allies lobbied bitterly and intensely to allow Humphreys to proceed. In the end, President Millard Fillmore directed that the $50,000 appropriation for the survey be divided between the two men. Each was to operate independently and produce a separate report.

  Humphreys, representing not only himself but the entire Army, was in a competition. He was determined to win it.

  “AT THE MOUTH of the Missouri, the Mississippi river first assumes its characteristic appearance of a turbid and boiling torrent, immense in volume and force…[which] impart to it something of sublimity,” wrote Humphreys, describing the survey’s goals, “yet the Mississippi is really governed by laws, the development of which was the first object of these investigations.”

  The force did seem sublime in its immensity. Mass and velocity determine the force of any moving object. Volume determines a river’s mass. Slope, chiefly, determines its velocity. The steeper the slope to the sea, the steeper the fall and, hence, the greater the speed, or the velocity, of the current. The Corps of Engineers defines the starting point of the lower Mississippi River as the confluence of the Mississippi and the Ohio at Cairo, Illinois, 290 feet above sea level. The river in its natural state flowed 1,100 miles from there to the Gulf (its many curves lengthen the straight-line distance of 600 miles), giving it an average slope equal to 290 feet, the height, divided by 1,100 miles, the distance, or slightly over 3 inches to the mile. In long stretches the slope drops below 2 inches a mile. The Mississippi, and even more so the lower Mississippi, runs through some of the flattest land in the world. This gentle slope that moves the tremendous volume of water in the Mississippi to the sea suggests that the river moves sleepily through the belly of America. The suggestion is false.

  The river’s characteristics represent an extraordinarily dynamic combination of turbulent effects, and river hydraulics quickly go beyond the merely complex. Indeed, studies of flowing water in the 1970s helped launch the new science of chaos, and James Gleick in his book on the subject quotes physicist Werner Heisenberg, who stated that on his deathbed he would like to ask God two questions: why relativity? and, why turbulence? Heisenberg suggested, “I really think God may have an answer to the first question.”

  Anything from a temperature change to the wind to the roughness of the bottom radically alters a river’s internal dynamics. Surface velocities, bottom velocities, midstream and mid-depth velocities—all are affected by friction or the lack of friction with the air, the riverbank, the riverbed.

  But the complexity of the Mississippi exceeds that of nearly all other rivers. Not only is it acted upon; it acts. It generates its own internal forces through its size, its sediment load, its depth, variations in its bottom, its ability to cave in the riverbank and slide sideways for miles, and even tidal influences, which affect it as far north as Baton Rouge. Engineering theories and techniques that apply to other rivers, even such major rivers as the Po, the Rhine, the Missouri, and even the upper Mississippi, simply do not work on the lower Mississippi, which normally runs far deeper and carries far more water. (In 1993, for example, the floodwaters that overflowed, with devastating result, the Missouri and upper Mississippi put no strain on the levees along the lower Mississippi.)

  The Mississippi never lies at rest. It roils. It follows no set course. Its waters and currents are not uniform. Rather, it moves south in layers and whorls, like an uncoiling rope made up of a multitude of discrete fibers, each one following an independent and unpredictable path, each one separately and together capable of snapping like a whip. It never has one current, one velocity. Even when the river is not in flood, one can sometimes see the surface in one spot one to two feet higher than the surface close by, while the water swirls about, as if trying to devour itself. Eddies of gigantic dimensions can develop, sometimes accompanied by great spiraling holes in the water. Humphreys observed an eddy “running upstream at seven miles an hour and extending half across the river, whirling and foaming like a whirlpool.”

  The river’s sinuosity itself generates enormous force. The Mississippi snakes seaward in a continual series of S curves that sometimes approach 180 degrees. The collision of river and earth at these bends creates tremendous turbulence: currents can drive straight down to the bottom of the river, sucking at whatever lies on the surface, scouring out holes often several hundred feet deep. Thus the Mississippi is a series of deep pools and shallow “crossings,” and the movement of water from depth to shallows adds still further force and complexity.

  High water—a flood—makes river dynamics more volatile and enigmatic. In some parts of the river high water raises the surface seventy feet above low water. By raising the surface in relation to sea level, high water can thus increase the slope of the river by 25 percent or more. And velocity depends upon the slope. The river’s main current can reach nine miles an hour, while some currents can move much faster. During floods, measurable effects of an approaching flood crest can roar downriver at almost eighteen miles an hour.

  And, for the last 450 miles of the Mississippi’s flow, the riverbed lies below sea level—15 feet below sea level at Vicksburg, well over 170 feet below sea level at New Orleans. For this 450 miles the water on the bottom has no reason to flow at all. But the water above it does. This creates a tumbling effect as water spills over itself, like an enormous ever-breaking internal wave. This tumbling effect can attack a riverbank—or a levee—like a buzz saw.

  But the final complexity of the lower Mississippi is its sediment load, and understanding it was the key to understanding how to control the river.

  Every day the river deposits between several hundred thousand and several million tons of earth in the Gulf of Mexico. At least some geologists put this figure even higher historically, at an average of more than 2 million tons a day.

  By geological standards the lower Mississippi is a young, even infant stream, and runs through what is known as the Mississippi Embayment, a declivity covering approximately 35,000 square miles that begins 30 miles north of Cairo to Cape Girardeau, Missouri—geologically the true head of the Mississippi Delta—and extends to the Gulf of Mexico. At one time the Gulf itself reached to Cape Girardeau, then sea level fell.

  Over thousands of years the river and its tributaries have poured 1,280 cubic miles of sediment—the equivalent of 1,280 separate mountains of earth, each one a mile high, a mile wide, and a mile long—into this declivity. Aided by the falling sea level, this sediment filled in the embayment and made land. Throughout the Mississippi’s alluvial valley, this sedimentary deposit has an average thickness of 132 feet; in some areas the deposits reach down 350 feet. Its weight is great enough that some geologists believe its downward pressure pushed up surrounding land, creating hills.

  There were two basic, and to some extent contradictory, approaches that engineers historically embraced to protect this valley from floods: levees or outlets. Levees confined the Mississippi; outlets released it. Levees represented man’s power over nature; outlets represented man’s accommodation to nature. Which approach was the right one depended largely upon the answer to the question of what caused the river to carry more sediment, and what caused it to deposit sediment it already carried.

  A LEVEE IS NOTHING MORE than earth mounded into a hill to contain water. Babylonians leveed the Euphrates. Rome leveed the Tiber and Po. By 1700 the Danube, the Rhône, the Rhine, the Volga, and other European rivers had levees, while Holland made the most extensive use of them (a levee and a dike are the same thing).

  The Mississippi creates natural levees. When the river overflows, it deposits the heaviest sediment first, thus building up the land closest to the
river. Generally, these natural levees extend for half a mile to a mile from the riverbank. “Bottomlands” farther away are lower and often marsh and swamp. New Orleans was founded on a natural levee, and its French Quarter is the highest ground in the region. By 1726, artificial levees with a height ranging from four to six feet also protected the city.

  But levee building never stopped; levees were extended above and below New Orleans, then to the opposite bank. Those levees increased the pressure on old ones. The reason is simple: when the river was leveed on only one bank, in flood it simply overflowed the opposite bank. But with both banks leveed, the river could not spread out. Therefore, it rose up. Thus the levees, by holding the water in, forced the river higher. In turn, men tried to contain the flood height by building levees still higher. By 1812, levees in Louisiana began just below New Orleans and extended 155 miles north on the east bank of the river and 180 miles on the west bank. By 1858, levees on the two sides of the river totaled well over 1,000 miles.

  In some stretches the levee rose to a height of 38 feet. These heights changed the equations of force along the river. Without levees, even a great flood—a great “high water”—meant only a gradual and gentle rising and spreading of water. But if a levee towering as high as a four-story building gave way, the river could explode upon the land with the power and suddenness of a dam bursting.

  From the first, some critics argued that building the levees higher simply increased the dangers should a crevasse, or levee break, occur, and insisted that a means to lower flood heights be used in conjunction with levees. There were three main ways to lower the flood level. One was to build reservoirs on tributaries to withhold water from the Mississippi during floods. A second was to cut a line through the sharp S curves of the river; these cutoffs would move the water in a shorter and straighter line, increase its slope, and hence its speed (a book arguing for cutoffs would later be titled Speeding Floods to the Sea). A third way was to let water escape from the river through outlets. All three proposals had detractors, but outlets had the most—because it also had the most advocates.

  As early as 1816, proposals were made to create artificial outlets, also called spillways or waste weirs, on the east bank of the Mississippi near New Orleans. One proposal called for a spillway above the city to drain Mississippi floodwater into Lake Pontchartrain, while another called for one below the city to drain into Lake Borgne. Both “lakes” are really more akin to saltwater bays and empty into the sea, and at the proposed sites the river flowed within five miles of them.

  Simple logic drove the argument for outlets. Removing water from the river would lower flood levels, proponents of the scheme insisted, just as removing the plug in a bathtub lowered the water level there.

  Critics of outlets who instead insisted upon levees, and levees only—it soon became known as the “levees-only” position—generally subscribed to an engineering theory developed from observations of the Po made by the seventeenth-century Italian engineer Guglielmini. Guglielmini argued that alluvial rivers, like the Mississippi, always carried the maximum amount of sediment possible, and that the faster the current, the more sediment the river had to carry. His hypothesis further argued that increasing the volume of water in the river also increased the velocity of the current, thus compelling the river to pick up more sediment. The main source for this sediment had to be the riverbed, so confining the river and increasing the current forced a scouring and deepening of the bottom. In effect, adherents of this theory argued, levees would transform the river into a machine that dredges its own bottom, thus allowing it to carry more water without overflowing.

  Levees-only advocates argued that outlets, by allowing water to escape from the river, were counterproductive since they removed volume from the river, lowered the slope, and caused the current velocity to slow. This not only prevented the current from scouring out the bottom, but actually caused the deposit of sediment—thus raising the bottom and in turn the flood height. According to the levees-only theory, using outlets was like taking water out of a bathtub, then dumping so much gravel into it that the tub ended up holding less water. The levees-only hypothesis argued that outlets, rather than lowering the flood height, would actually raise it.

  In an 1850 report to the Louisiana legislature, a professor of engineering endorsed the hypothesis: “Concentration of force increases the abrasive power…. Levees confine and concentrate the waters, concentrate and increase the force, therefore increase the abrasion, therefore the capacity of the channel…. Outlets diffuse the waters, reduce the abrasive force, and therefore reduce the capacity of the channel.”

  Strict adherents of Guglielmini’s theory even called for closing natural outlets to force even more water into the main channel of the Mississippi, claiming the increase in volume would also increase its scouring effect.

  In fact there was no doubt that levees did increase current velocity, which in turn did increase the scouring out of the channel. But the question was, how much? Floods might carry twenty times the low-water volume of the river. Could levees increase scour enough to accommodate that much water?

  As Humphreys observed soon after arriving in New Orleans: “The public mind here is bewildered by the contradictory opinions given by the Engineers in the state as to what ought and ought not to be done. One says cut-offs is the only means of protecting the country. Another says cut-offs will ruin the country, make levees only…. A third says make outlets. Each one quotes opinions of foreign engineers and partial facts and pretended facts respecting the Mississippi to support his views. No wonder the legislature does nothing.” Ellet and Humphreys—rather, Ellet or Humphreys, whoever won their contest—would decide the issue.

  AT THE TIME, few would have bet against Charles Ellet in any competition. But the survey was not the work of his life, nor did he intend to spend long at it. He had already developed his ideas studying the Ohio, and, even with his wife and children beside him, he disliked New Orleans. In March 1851, not long before he returned north to write his report, he told his mother: “We have been to see Jenny Lind [who was managed by P. T. Barnum] and I must admit we paid a full price for that music…. I have pretty near come to the conclusion that instead of controlling these floods I would do service to the work to sweep away…New Orleans with all its boardinghouses, grog shops, and music to boot.”

  Humphreys had arrived in Louisiana at the same time as Ellet. He came alone, without his family. He never saw Jenny Lind. He worked. While Ellet was preparing to leave, Humphreys was writing a colleague: “I cannot understand how any man can be willing to assume charge of a work without making it his business to know everything about it from A to Izzard…. Having got to work I am ready to go into it up to the armpits.”

  The next few months would be the truest of Humphreys’ life. He proceeded deliberately, exploring every issue in exquisite detail, compiling mountains of data, rejecting anything that threatened the integrity of his findings. He protected the survey’s integrity at all times, for example resisting pressure to hire one assistant who was “a most active partisan of levees only, to the exclusion of outlets, and his mind is biased. He could not perceive the force of any factor or argument on the other side.”

  For the moment, Humphreys believed truth would make his reputation. He asked himself such questions as “What is the reason that the Po—and the Mississippi—do not carry gravel to their mouths when their velocities in floods are more than sufficient, according to the books? Answer? Make a profile of the bottom and see.” He literally chewed on the problem, tasting mud dredged from the bottom, 150 feet deep, as if it had some mystery to impart, noting, “The clay itself has a somewhat gritty feel between the teeth and a peculiar taste.”

  He also chose two outstanding deputies: Caleb Forshey and Lieutenant G. K. Warren. Forshey was a professor of mathematics and engineering and a leading expert on the river; Warren, later a prominent explorer, had just graduated from West Point and had declined an offer of a mathematics professors
hip there to work on the survey. Humphreys gave each of them detailed instructions; the three each took charge of a work party and proceeded independently, hundreds of miles apart, recording rigorous measurements and observations.

  It was hard work, physical work, being constantly out on the river. Humphreys was precise, dressed always in full uniform. On the water there was no relief from the sun. Spring was hot. Summer was hotter. The heat drove him nearly mad. But the work exhilarated! How it must have felt to stand on the bank of the Mississippi in the middle of the nineteenth century, to push one’s way through a wild and thick jungle of cane, vines, and willow, to hear the animal sounds mixed with the rush of water, to see water a mile wide, boiling, dark, and angry, two hundred and more feet deep, to watch it thunder and roll south at a speed so great a boat with six men at oars could not move upstream. How godlike it must have felt to a man who intended to find a way to command it.