Dr. Jeff Masters ·

Above: Aerial view of the four structures of the Mississippi River Old River Control Structure, looking downstream to the south. Water flows from the Mississippi River through the four structures, to the Atchafalaya River (right). Image credit: U.S. Army Corps of Engineers.

America has an Achilles' heel. It lies on a quiet, unpopulated stretch of the Mississippi River in Louisiana, 45 miles upstream from Baton Rouge. Rising up from the flat, wooded west flood plain of the Mississippi River are four massive concrete and steel structures that would make a pharaoh envious: the U.S. Army Corps of Engineers’ greatest work, the two billion-dollar Old River Control Structure (ORCS). The ORCS saw its second highest flood on record in March 2019, and flood levels have risen again this week to their fifth highest level on record. While the structure is built to handle the unusual stress this year's floods have subjected it to, there is reason for concern for its long-term survival, since failure of the Old RIver Control Structure would be a catastrophe with global impact.

This first part of a 3-part series will study the history and importance of this critical structure, and how it almost failed in 1973. Part II, scheduled to run on Monday, is titled, Escalating Flood Heights Puts Mississippi River’s Old River Control Structure at Increasing Threat of Failure. Part III is titled, If the Old River Control Structure Fails: A Catastrophe With Global Impact, and will run later next week.

Figure 1. Louisiana as seen by the MODIS instrument on March 21, 2019, showing the location of the Mississippi and Atchafalaya rivers, plus the Old River Control Structure, which diverts 30% of the flow of the Mississippi into the Atchafalaya. Floods of the two rivers were creating large sediment plumes in the Gulf of Mexico. Image credit NASA.

Chaining the Mississippi to its current channel

This marvel of modern civil engineering has, for fifty-five years, done what many thought impossible—impose man's will on the Mississippi River. Mark Twain, who captained a Mississippi river boat for many years, wrote in his book Life on the Mississippi, "ten thousand river commissions, with the mines of the world at their back, cannot tame that lawless stream, cannot curb it or define it, cannot say to it 'Go here,' or Go there, and make it obey; cannot save a shore which it has sentenced; cannot bar its path with an obstruction which it will not tear down, dance over, and laugh at." The great river wants to carve a new path to the Gulf of Mexico; only the Old River Control Structure keeps it at bay.

Failure of the Old River Control Structure and the resulting jump of the Mississippi to a new path to the Gulf would be a severe blow to America's economy, robbing New Orleans, Baton Rouge, and the critical industrial corridor between them of the fresh water needed to live and do business. Since a huge portion of our imports and exports ship along the Mississippi River, a closure would cost $295 million per day, said Gary LaGrange, executive director of the Port of New Orleans, during the great flood of 2011. An extended closure of the Lower Mississippi to shipping might cost tens of billions. Since barges on the Mississippi carry 60% of U.S. grain to market, a long closure of the river to barge traffic could cause a significant spike in global food prices, potentially resulting in political upheaval like the “Arab Spring” unrest in 2011, and the specter of famine in vulnerable food-insecure nations of the Third World.

Figure 2. Two views of the Mississippi River. Left: the meander paths of the Mississippi over time, as published in "Geological Investigation of the Alluvial Valley of the Lower Mississippi River" (Harold N. Fisk, 1944, available online). Right: The Army Corps of Engineers' view of Mississippi River, with peak flow rates in cubic feet per second for a maximum 1-in-500-year "Project Flood”. The Corps has straightened the river’s path by cutting off meander bends, and has built multiple flood control structures capable of diverting a portion of the river's flow.

A shorter and steeper path to the Gulf

The mighty Mississippi River keeps on rollin' along its final 300 miles to the Gulf of Mexico south of New Orleans—but unwillingly. There is a more attractive way to the Gulf—150 miles shorter, and more than twice as steep. This path lies down the Atchafalaya River, which connects to the Mississippi at a point 45 miles north-northwest of Baton Rouge, 300 river miles from where the river empties into the Gulf of Mexico southeast of New Orleans.

Each year, the path down the Atchafalaya grows more inviting. As the massive amounts of sediment the Mississippi carries—scoured from fully 41% of the continental U.S. land area, plus a portion of southern Canada—reach the Gulf of Mexico, the river's path grows longer. This forces it to dump large amounts of sediment hundreds of miles upstream, in order to build its bed higher and maintain the flow rates needed to flush such huge amounts of sediment to the sea. Thus, the difference in elevation between the bed of the Mississippi and the Atchafalaya—currently about 17 - 19 feet at typical flow rates of the rivers—grows ever steeper, and the path to the Gulf down the Atchafalaya more appealing.

The two highest floods on record at the Old River Control Structure occurred in 2011 and 2019, and the 2018 flood ranked as the fourth highest on record. Floods like these further increase the slope, as flood waters scour out the bed of the Atchafalaya. Without the Old River Control Structure, the Mississippi River would have carved a new path to the Gulf in the 1970s, leaving Baton Rouge and New Orleans stranded on a salt water estuary, bereft of their main source of fresh water to supply their people and industry.

History of Old River

The Mississippi River has been carving a path to the ocean since time immemorial, always seeking the shortest and steepest route possible. Approximately once every 1000 years, the river jumps out of its banks and carves a new path to the sea. The Mississippi has been flowing along its current course past New Orleans since about 1000 A.D., but beginning in the 1800s, the Mississippi began slowly shifting more and more of its flow down the Atchafalaya River, along the path it used to take to the Gulf about 3000 years ago.

This diversion accelerated in 1831, when steamboat captain Henry Miller Shreve used his steam-powered snag boat Heliopolis to dredge a new channel for the Mississippi River. Shreve cut off a huge meander bend which shortened navigation of the Mississippi River by 18 miles and shifted the main channel 6 miles to the east. The two arms of the old severed meander created what is now called Old River, which connects the Mississippi to the Atchafalaya. The clearing of an ancient 40-mile-long log jam on the Atchafalaya in the 1840s, to allow navigation on the river for the first time, further increased the flow of water coming down the river from the Mississippi.

Figure 3. October 20, 1937: German-Swiss-US mathematical physicist Albert Einstein (1879 - 1955) (left) being greeted by his son Hans Albert on arrival at New York. In the 1950s, the younger Einstein helped with the design of the Old River Control Structure. Image credit: Keystone/Getty Images.

Building the Old River Control Structure

By the early 1950s, the Atchafalaya had captured over 20% of the Mississippi River’s flow, and the Army Corps of Engineers grew concerned that the Mississippi might change course as early as 1968. One of the experts that they called in to help study the problem was one of the world’s leading authorities on river sediment transport: UC-Berkeley hydraulic engineer Dr. Hans Albert Einstein, son of the famous Albert Einstein. With Hans Albert Einstein’s help, the Corps drew up the design for the Old River Control Structure, and construction began in the late 1950s. The structure resembled a dam with gates to control the amount of water escaping from the Mississippi to the Atchafalaya. This "Low Sill Structure", completed in 1963, consisted of a 566-foot-long dam with 11 gates, each 44 feet wide, that could be raised or lowered to control the amount of flow leaving the Mississippi. A companion "Overbank Structure" was built on dry land next to the Low Sill Structure, in order to control extreme water flows during major floods. The total cost of the two structures: about $560 million (2019 dollars).

Figure 4. In 1987, after the newly-built Auxiliary Structure began operating, the Army Corps drained the channel to the Low Sill Structure and repaired some of the damages incurred in the 1973 flood. Above, we see the outflow side of the structure. The Low Sill Structure was designed to withstand a 37-foot difference in water levels (“head”) between the higher Mississippi River and the lower Atchafalaya River. Due to permanent damages to the structure during the 1973 flood, engineers determined that a safe differential in water surfaces on either side of the structure should be no more than 22 feet. Image credit: USACE.

The flood of 1973: Old River Control Structure almost fails

For the first ten years after completion of the Old River Control Structure, no major floods tested it, leading the Army Corps to declare, "We harnessed it, straightened it, regularized it, shackled it." But the structure underwent a severe battering during the flood of 1973, one that left it permanently damaged. During the fall and winter of 1972, exceptionally heavy rains fell throughout the Tennessee and Lower Mississippi Valley, saturating the soil and raising the level of the river. Then in early March, prodigious rains hit the watersheds of the Missouri and Upper Mississippi, and the Corps knew they might be facing a Project Flood—the maximum flood theoretically possible. The situation worsened in April, when an usually heavy snowpack in the Rockies melted suddenly, sending a massive pulse of flood waters down the Missouri and Arkansas rivers. When more heavy rains hit the Mississippi Valley in late March, the Mississippi River rose to dangerously high levels, forcing the opening of the Bonnet Carré Spillway for the first time in 23 years.

During this period, the Low Sill Structure was operated at more than 50 percent of a Project Flood flow—half a million cubic feet per second. The Army Corps began preparations to open the nearby Morganza Floodway to take pressure off of the Old River Control Structure, and promised to give five days of warning before opening the floodway. But as it turned out, it would have to be opened with very little warning.

Figure 5. The Old River Control Structure’s Low Sill Structure as seen in April 1973. Turbulence from a major flood caused the 67-foot long southern wing wall on the intake channel leading to the Mississippi River to collapse on April 14, and a large eddy can be seen where the wall used to be. The eddy helped scour out a football field-sized hole up to 50 feet deep that undermined 7 of the structure’s 11 gates and nearly caused its failure. A ramp leading to the eddy was built, and an emergency stone replacement dike was built. Image credit: USACE.

James Barnett Jr.’s fantastically detailed 2017 book Beyond Control: The Mississippi River's New Channel to the Gulf of Mexico tells the story of how on April 14, 1973, the Old River Control Structure foreman walked out on the Low Sill Structure for an inspection, and witnessed the collapse of a 67-foot-long wall along the south side of the intake channel, facing the Mississippi. The six Niagara Falls' worth of water pouring through the channel pulverized the wall’s fragments and rammed them through the structure. A giant whirlpool replaced the missing wall and began attacking the structure, scouring away at the 90-foot deep steel pilings that supported it. The foreman reported that you could drive out onto the highway that crossed the structure, open your car door, and the vibration from the water hammering through the structure would close the door for you.

A camera was lowered down a hole drilled down through the center of the structure. The camera showed fish where there should have been solid earth. The river had begun scouring out a football field-sized hole over 50 feet deep underneath seven of the low sill's eleven gate bays. The scour hole came within 150 feet of a second, even-larger scour hole which had developed on the downstream side of the gates. Had the two scour holes merged, the entire structure would have been undermined and failed.

The head of the Army Corps’ flood fight efforts, Major General Charles C. Noble, gave the order to open the Morganza Floodway two days later to relieve pressure on the Low Sill Structure. The governor of Louisiana telephoned and asked if he had the authority to order that the Morganza Floodway not be opened. General Noble told him no. When complaints arose that he was not giving the promised five days' notice, he replied that the river didn't give him five days' notice.

The Corps brought in construction barges that continuously dumped boulders into the scour holes to keep Low Sill Structure from collapsing. In the end, the Low Sill Structure was saved but was permanently damaged, and can now only handle 60% of the flow it once could. So, the Corps was forced to build two new structures to stabilize the situation: the Auxiliary Structure (cost: $480 million 2019 dollars) and the Sidney A. Murray Junior Hydroelectric Plant, costing $1 billion (2019 dollars). We now have a much safer situation than in 1973—but as we will see in Part II, ever-higher flood crests are threatening to put an end to this reprieve.

Related works

Part 2 of this series: Escalating Floods Putting Mississippi River's Old River Control Structure at Risk

Part 3 of this series: If the Old River Control Structure Fails: A Catastrophe WIth Global Impact

Beyond Control: The Mississippi River's New Channel to the Gulf of Mexico, James Barnett Jr.’s detailed 2017 book.

John McPhee's 1987 essay, The Control of Nature.

Divine providence: the 2011 flood in the Mississippi River and Tributaries Project, by Army Corps historian Charles Camillo.