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The End of Deltaic Birth?
The Old River Control Structure, a series of steel gates mounted in the Mississippi River levee 170 miles upstream from New Orleans, very nearly failed in 1973. The map of America almost changed. If the structure had failed, the lower end of the Mississippi River would have swung to the west, following the course of the Atchafalaya River. Rather than flowing past Baton Rouge and New Orleans to discharge through the Bird's Foot Delta into deep Gulf of Mexico water, a distance of 315 miles, the longest river in North America would have discharged into the shallows of Atchafalaya Bay, only 142 miles away, down a steeper gradient to sea level. If the Old River Control Structure had failed, colossal flows would have moved down the Atchafalaya River, creating ten-story-deep scour holes that would have migrated along the riverbed, undermining bridge pilings. The Interstate 10 bridge would have collapsed. The U.S. Highway 190 bridge would have collapsed. Scouring would have exposed gas and oil pipelines buffed beneath the bed of the Atchafalaya River, and, unsupported, they would have shivered in the flow and then, after some time, ruptured from the continuous stress. Gas supplies as far away as Massachusetts and Rhode Island would have been interrupted. Floodwaters would have isolated and then overcome Houma, Raceland, and Thibodaux. Morgan City would have been drowned, too, and then buried—drowned by rising floodwaters and buried by silt as floodwaters receded. Rotting carcasses of drowned wildlife and livestock would have been snagged in tree branches andbeached on high ground when water levels dropped.
After the flood, return to the status quo would not have been possible. With the Old River Control Structure washed out, most of the flow of the Mississippi River would have continued down the Atchafalaya River. At the Old River Control Structure, the Mississippi River's bed is below sea level, and, if the structure had failed, the reduced flow in the Mississippi River downstream from the structure would have been overcome by the pressure of saltwater from the Gulf of Mexico. A salt wedge would have extended upstream from the Gulf, underneath the lighter freshwater, contaminating water supplies needed for drinking and industry, first in Port Sulphur and then in New Orleans itself. Authorities would have had to truck in and ration drinking water. As salinities increased, power plants using river water for cooling would have had to shut down or face plumbing failure from saltwater corrosion. If municipalities continued to supply water for bathing and laundry, residential plumbing would corrode. Water heaters would be especially susceptible. Freshwater needed to process sugarcane no longer would have been available. The Mississippi River channel just south of the Old River Control Structure would have silted in, cutting off barge traffic, isolating the nation's seventh and fourth busiest port cities, New Orleans and Baton Rouge, from the barge routes north to Memphis, Saint Louis, and Minneapolis. The outlet of the Atchafalaya River, which allows ship traffic to move in and out of Morgan City, would have silted in. Oyster and shrimp fisheries would have crashed from changes in water quality.
But the Old River Control Structure did not fail. Throughout the winter of 1972 and spring of 1973, the Old River Control Structure regulated the flow of repeated flood pulses tumbling down the Mississippi River, releasing some of the water into the Atchafalaya River, behind the structure, and thereby lowering the Mississippi River downstream from the structure, keeping flood waters from over-topping levees, protecting New Orleans and Baton Rouge from high water.
The structure had been designed for events as bad and worse than the flood of 1973. What caused the near failure? For the conditions existing when the structure was designed, it was more than adequate. But the rivers—both the Mississippi and the Atchafalaya—had changed since the structure was designed. The Mississippi River below the structure, toward New Orleans and the Gulf of Mexico, did not have the capacity that it had twenty years earlier. New levees contained its flow, held it in, pinched it. Changes to the watershed and the river itself changed the behavior of flood peaks. In 1940, a flow of 850,000 cubic feet per second resulted in a river height of just over 39 feet at Saint Louis, but the same flow in 1973 resulted in a river height of 43 feet. The 30 percent of the Mississippi River's water that was routinely released through the Old River Control Structure during periods of normal flow had deepened the Atchafalaya River. Water moved more quickly down the Atchafalaya than it had when the structure was designed. In 1973, at flood stage, the Old River Control Structure was only marginally adequate. Scouring in front of the Low Sill Structure—part of the Old River Control Structure—dug downward, exposing 50 feet of the steel pilings that supported the structure. A whirlpool formed. A 67-foot-tall concrete wing wall designed to guide flow through the structure's openings collapsed. The Old River Control Structure almost lost control.
Now, 50 rural two-lane highway miles north of Baton Rouge, Louisiana, 40 rural two-lane highway miles south of Natchez, Mississippi, surrounded by cotton and grazing land interspersed with swamp forest, I stand next to the repaired Low Sill Structure. It is late in the afternoon in January 1999. The stifling heat and soggy air of Louisiana summer are months away. The sky is deep blue and cloudless. The sun is shining. A light breeze blows.
Some distance away, at the upstream end of the canal that leads to the Low Sill Structure, the Mississippi River goes about its business, well below flood stage. During the year, if this is an average year, 145 cubic miles of water will flow past, enough water to fill more than twenty swimming pools for every man, woman, and child in the United States. In the same year, the river will carry 210 million tons of sediment to the Gulf of Mexico, an amount more than ten times the combined weight of every man, woman, and child in the United States. From the Low Sill Structure, it would be difficult to get down to the water, to touch the Mississippi River. The banks of the canal connecting the structure and the Mississippi River are steep and protected by fences. Piled stone, slick with mud and algae, supplements fences, stretching along the canal's edge, a fortified border between land and water.
In front of me but some distance away, beyond the massive pile of stone that replaced the 67-foot-tall collapsed wing wall, the water stain of higher river levels marks the tree trunks of a cypress forest. The trees are naked with winter leaflessness, and from here the water stain is a straight line several feet above the swollen bases of the trees, a subtle color change, little more than a shadow against gray bark. On the structure itself, a fading stain on the concrete, a scant 6 feet from the top of the structure, marks the 1997 high water. Behind me, across the road, water discharges through three partly open gates in the Low Sill Structure, dropping from the level of the Mississippi River to that of the Atchafalaya River. In all, there are eleven gates, each one 44 feet wide, each in its own concrete bay. After the water drops through the structure, it splashes up 12 feet against the structure's walls and forms standing waves 6 feet tall in the middle of the channel. It is flowing toward the Atchafalaya River, but not with the calm regularity of a slow southern stream. The surface is chaotic, a mixing bowl, white water foam contrasting against chocolate brown whirlpools and eddies. At the top of the structure a sign with white letters against a red background says "Danger Turbulent Water Keep 175 Feet Back." There is no punctuation and no threat of fines for violators. The water itself negates the need for punctuation and fines.
Back in my car, I drive across the structure, over the turbulence, a few minutes south. I stop at the Old River Control Structure office. The office is next to the Old River Auxiliary Control Structure, built after the near-disaster of 1973 to provide an alternative path of flow into the Atchafalaya, an extra safety valve for an overzealous Mississippi River. I want to talk to the workers, the men who control the structure that controls the Mississippi River. The man I find in the office is perhaps thirty-five years old, with a southern accent, but not the accent of Louisiana. His waistline strains the fabric of too tight jeans, a sign that the battle to control the Mississippi River is fought from behind a desk. I tell him that I am interested in the flood of 1973.
"I'm from South Carolina," he tells me. "And I was just a boy in 1973. But I can tell you anything you want to know about the structure."
He has an uneasy smile that exposes beautifully straight white teeth and transforms itself into a nervous drawn-out clucking chuckle when I ask questions that he cannot answer. When I ask him how much concrete is in the structure, he clucks. When I ask him how they replaced the collapsed wing wall, he clucks again. When I ask him the thickness of the steel floodgates, he supplements his clucking with a fidgety dance, moving his weight from one foot to the other several times. He turns his lack of knowledge into a joke. "You stumped me again," he says, smiling, clucking, dancing. "I feel like I'm failing an exam." But when I ask him if he knows anything about the impact of the Old River Control Structure on Louisiana's wetlands, he ends our conversation. "Our office of Counsel," he tells me, "has instructed us not to answer any questions about wetlands." He is still clucking, still dancing, but now he is very serious. He hands me a black-and-white brochure called "Atchafalaya Outlet Mississippi River and Tributaries," published in 1981. Then he places one hand on my shoulder and walks me to the door. He tells me that I am welcome to look around on my own. Our conversation is over.
I drive farther south and park near the locks. There is a boat ramp in the parking area, and at the edge of the boat ramp I can touch the water. From a distance, at the Low Sill Structure, the water had the color of chocolate, but close up the color is more like creamed coffee. Suspended sediment in the water flows in loops and swirls. I let the water lap around the soles of my boots.
Next to the boat ramp, a man wearing a red ball cap low over his eyes sits in a van with his window down. Other than his van and my car, the parking lot next to the boat ramp is empty. He is a crew member on a tugboat, waiting for his boat. He will leave the van with the man he is replacing. The tug is coming down from the Black River, which joins with the Little River to become the Red River, which flows south to become the Atchafalaya River below the Old River Control Structure. Boats shuffle back and forth between the Mississippi River and the Atchafalaya-Red through the lock. The man does not know how long it takes to pass through the lock, or where his tug is going after it meets him. He is just crew, he tells me. His accent is from somewhere far to the north.
The lock exits into a canal before it joins the short stretch of water known as Old River, just below the Old River Control Structure, on the Atchafalaya River side of the structure. From there, Old River flows into the Atchafalaya-Red. In the fourteenth century, the Red flowed parallel to the Mississippi, but the two rivers did not join. By the seventeenth century, a bend in the Mississippi—a meandering loop of current later called Turnbull's Bend—intercepted the Red, and the Red became a tributary of the Mississippi. Just downstream, the Atchafalaya acted as a distributary stream, sucking Mississippi River water away from the lower end of Turnbull's Bend. In 1831, the steamboat captain Henry Shreve cut a channel through the base of the peninsula that formed Turnbull's Bend, providing a shortcut known as Shreve's Cutoff. Shreve's Cutoff, being shorter and therefore steeper than Turnbull's Bend, captured the main flow of the Mississippi River. The upper half of Turnbull's Bend, abandoned by the Mississippi River, silted in, but the midsection remained open, flushed by the Red River's flow into the Atchafalaya River. The trailing end of the lower half of Turnbull's Bend still joined the Atchafalaya and Mississippi Rivers; and, depending on water heights in the Mississippi, Red, and Atchafalaya Rivers, water could flow in either direction through this connection. The connection became known as Old River in reference to the fact that, prior to Shreeve, it had in fact been the River—the Mississippi River.
But there is more to the shifting of the Mississippi River than minor changes like the swing of Turnbull's Bend, a swing that captured the Red River. The main channel of the Mississippi River has swung back and forth for thousands of years, like a great serpent gradually snaking across the land. Three thousand years ago, the lower end of the Mississippi River flowed farther north than its current path, closer to what would become Biloxi, Mississippi. As the river spilled into the Gulf of Mexico and spread across the shallows, the current velocity slowed, and sediment carried in the water dropped out, accumulating on the floor of the Gulf of Mexico to form the Saint Bernard Delta. Sand dropped out first, and lighter sediments dropped out as the water spread farther into the Gulf. Where sediment accumulation created exposed flats during times of low water, marsh plants took hold. The marsh plants contributed to the process of land building. The current velocity, already slowed as it encountered the Gulf of Mexico and spread over a wider area, was slowed even further as it ran through stems and leaves, and more sediment dropped out of the water column. Roots seized the sediments and held them in place. And as the plants grew, converting carbon dioxide and water to plant matter, they added more material to the growing land, in the form of roots and decaying leaves. The roots pushed elevations higher from below, and decaying leaves and stems built onto the elevation from above.
But even as the sediment accumulated, it sank under its own weight. In the parlance of coastal Louisiana, it subsided. Weight from newly deposited sediments bore down on the older deposits, slowly squeezing the grains more closely together. It was a race, a contest between subsidence, which lowered the elevation, and the combination of sediment deposition and plant growth, which raised the elevation. As long as the Mississippi River's sediment load and plant growth outpaced subsidence, the Saint Bernard Delta continued to grow. But as the growing delta extended outward into the Gulf of Mexico, lengthening the distance river water had to travel before it reached sea level, it became less efficient than alternative pathways; the Mississippi River could find a shorter, steeper path to sea level. A few centuries after the birth of Christ, around the time of the fall of the Han Dynasty, the Mississippi River abandoned the Saint Bernard Delta, swinging well to the southwest, running past points on the landscape corresponding to today's Houma and Port Fourchon, to form the Lafourche Delta. In the abandoned Saint Bernard Delta, subsidence outpaced deposition. Plant growth alone could not produce enough material to keep up with subsidence; without the Mississippi River's sediment, the Saint Bernard Delta began to sink. As the marsh elevations decreased, periods of prolonged flooding increased. The marsh plants, with only a limited tolerance to continuous flooding, died. They no longer produced leaves and roots that could offset the rate of subsidence; and, as dead roots disappeared, decomposing back to the carbon dioxide and water from which they had come, their ability to stabilize sediments also disappeared. Erosion added to subsidence. The Saint Bernard Delta began to disappear at a faster rate.
While the Saint Bernard Delta began to disappear, the Lafourche Delta began to grow. But near the end of the Middle Ages, as Thomas Aquinas wrote the Summa Theologiae and Genghis Khan organized cavalry assaults, the Mississippi River shifted again, this time to its present course—a long time ago for a historian, but no more than the bat of a geologist's eyelash. The Bird's Foot Delta was born. As it formed, modern civilization grew up around the Mississippi River. Henry Shreve dug Shreve's Cutoff through the Turnbull's Bend peninsula. Plantation owners and government agencies built levee systems to manage the river, to protect a growing population from flooding, and to improve navigation. As the approach to flood control became more sophisticated, people realized that what appeared to be a permanent fixture in the landscape was in fact not permanent at all. The river's course could change, not just by a few miles, but by more than a hundred miles. By the 1940s, members of the Mississippi River Commission were openly discussing the possibility of the Atchafalaya River capturing the Mississippi. By May 1951, a study undertaken by the Mississippi River Commission concluded that the Atchafalaya—Old River channel would capture the Mississippi River before 1980 unless a regulating structure was installed. Seven short months later, a meeting of scientists and engineers convened; their job was to predict the effect of a regulating structure on the Mississippi and Atchafalaya Rivers. One of these scientists was Professor Hans Einstein, son of Albert Einstein. Hans Einstein's reputation as a hydraulic engineer was built around theories of sediment transport in rivers. He argued that success of a regulating structure would require a means of guiding sediment into the Atchafalaya, to prevent ongoing deepening of the Atchafalaya. It had to be more than just a water control structure. Discussions wandered toward detailed points of science and arguments about theory, and the group had to be reminded of the practical issues at hand. At a follow-up meeting in 1952, in the presence of open acknowledgment that precise predictions about the fate of the Mississippi River were not possible, support for a structure at Old River grew.
Congress authorized construction of the Old River Control Project in 1955. Seven years and $67 million later, the structure was completed. With the structure in place, the cycle that built the Bird's Foot Delta would not end in the same way that it had ended for the Saint Bernard Delta and the Lafourche Delta. But still, the cycle would end. The Bird's Foot Delta now extended far out into the Gulf of Mexico, out to the edge of the continental shelf Mississippi River sediment that had built the Bird's Foot Delta in the shallows of the Gulf of Mexico, on the edge of the continental shelf, was now discharging into deep water, tumbling down the continental slope. The Mississippi River was spewing its sediment load into the abyss. No new marshes were being built. The Old River Control Project had saved New Orleans and Baton Rouge, but it had broken the cycle of deltaic death and birth.
At the locks, a man wearing a hard hat and dark blue coveralls pulls up in a white government utility vehicle. He is in his fifties. He has come down to the locks during his afternoon break to savor the weather. His brother-in-law has a new boat, he tells me, with swivel seats and an eighty-horsepower Mercury motor; and the man and his brother-in-law are thinking of taking it out that afternoon, after his shift. We talk about the Old River Control Structure. The man has been part of the structure's maintenance crew for most of his adult life. I tell him about my conversation at the office, about the moratorium on discussions about wetlands.
"I don't know why he told you we can't talk about wetlands," the man says. "Some of these young guys working for the government, they're so worried about rules and regulations that they start to imagine new ones. We tall to people about wetlands all the time." For several minutes we talk about the river and its effect on Louisiana's wetlands. His job occasionally has him guiding school groups, and he has heard all about wetlands before. He offers to show me around. I follow him to the Auxiliary Control Structure and then walk behind him onto the structure itself. Stringy cobwebs cross the walkway, and the man uses his hard hat to sweep the webs away as we walk. "They build new webs every night," he tells me.
He was here in 1973. He felt the Low Sill Structure trembling. He felt the fear of near failure. He helped dump stone where the wing wall had collapsed. He worked here throughout the construction of the Auxiliary Control Structure. "Since then," he tells me, "We've had water higher than the '73 flood." He pats the handrail on the edge of the Auxiliary Control Structure, then gives a sweeping gesture that takes in the structure as a whole. "With the new structure," he says, "we've never had a hint of a problem."
Excerpted from Saving Louisiana? by Bill Streever. Copyright © 2001 by Bill Streever. Excerpted by permission. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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|Gaining Ground in the Atchafalaya||46|
|Watching the Marsh Sink||64|
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