Mighty Mississippi Defies Human Control. Climate Change Makes it Even Mightier.

April 17, 2018

mississippiflood

Conquering nature.

How’s that working out for you?

Nature:

The Mississippi River was once thought to be uncontrollable: no feat of engineering could prevent the river from bursting its banks and sending floodwaters across its natural domain. But since the late nineteenth century, an expanding system of levees, floodways and channel modifications (Fig. 1) has gradually brought the river to heel, largely confining its waters to the main channel and accelerating them downstream. In a paper in Nature, Munoz et al.1 conclude that those same control measures have inadvertently raised the threat of flooding in the lower Mississippi to a level that is unprecedented in the past five centuries.

Munoz and colleagues have worked around that limitation by building their own extended record of flooding on the Mississippi, using evidence of high waters preserved by oxbow lakes and trees. An oxbow lake forms when a meander of a river is cut off to form a free-standing body of water. When such lakes are inundated by floods, they act as natural sediment traps for sand and silt carried in floodwaters. As those particles settle, they create a layer of coarse sediment on the bottom of the lake that is distinct from the clay and fine silt left behind when the lake is not hydrologically connected to the main channel5. Prolonged inundation with floodwater can cause some species of tree — particularly oak — to form wood that has abnormal features. The annual growth rings of such trees in the ‘bottomland’ hardwood forest of the Mississippi floodplain therefore contain a natural record of past floods6. By splicing together sedimentary sequences from lakes in Louisiana and Mississippi with ‘flood-ring’ signatures from living and dead trees in southeastern Missouri, the authors assembled a flood chronology for the lower Mississippi that stretches back to the early sixteenth century.

Together, these natural archives have kept a remarkably faithful account of past floods. The tree rings mark the occurrence of the great floods of 1844 and 1927, as well as l’Année des Grandes Eaux (the Year of the Great Waters) in 1785, which destroyed French–Canadian settlements in Illinois and Missouri. It is more difficult to date individual floods in the lake records because of the lower chronological resolution of that archive, but layers of coarse sediment can be matched to the great floods of 1851, 1927 and 2011, as well as to the flood reported by Spanish conquistadors in 1543. Overall, this new palaeoflood record suggests that, although flood hazards have waxed and waned through time, the Mississippi has risen higher and flooded more frequently in the past century than during any other period in the past 500 years.

The authors propose a provocative explanation for this recent hydrological intensification. Since the start of the twentieth century, records gathered using instruments show that the discharge of the Mississippi has slowly risen and fallen in concert with the surface temperature of the North Atlantic Ocean, which has alternated every two or three decades between warm and cold states7. Munoz and colleagues draw on proxy temperature estimates for the North Atlantic8 to demonstrate that this dependency has held steady since the 1500s. Because the spate of major floods in the past century cannot be explained by the observed temperature behaviour of the North Atlantic, the authors conclude that the trend towards larger and more frequent floods is mostly due to the transformation by humans of the Mississippi River and its basin.

Blaming floods on the infrastructure that was built to guard against them will be controversial. The Mississippi basin has undergone upward trends in precipitation and evapotranspiration — the sum of evaporation and plant transpiration from Earth’s surface — in the past several decades9, so climatic factors other than the influence of the North Atlantic might also have affected the rhythm of the river. And, like the rest of North America, the Mississippi basin has warmed substantially since the end of the Little Ice Age10 (a period of cooling that began in the sixteenth century and ended in the mid-nineteenth century11), so I think it is possible that the long-term trends in hydrology could be the result of climate change, rather than river engineering. Testing these competing explanations will require more palaeoflood work to be performed along the upper Mississippi and its main tributaries, where modifications to the river are less intensive and the climate still dominates the river’s hydrology12.

Washington Post:

The state of Louisiana is proceeding with ambitious plans to redirect the Mississippi River and rebuild some of its rapidly vanishing wetlands — but even this massive intervention may not be enough to save the most threatened lands from fast rising seas, scientists concluded in a study published Wednesday.

The study uses a methodology called “optical dating” to study how the river built an area called the Lafourche subdelta in coastal Louisiana, where the Mississippi dumped loads of sediment as much as 600 years ago, when it changed paths. The technology lets scientists identify the last time that long-buried sand was exposed to sunlight and, therefore, determine the rate at which the river naturally built up land by carrying sediment downstream.

“What we found was that, on average, it produced somewhere between 6 and 8 square kilometers of land per year, and the shoreline migrated seaward by somewhere between 100 and 150 meters per year,” said Torbjorn Tornqvist, a Tulane University geologist who was one of the study’s authors. “Those numbers in themselves I find pretty impressive.”

“But the problem is that if you put that in the context of the rates of wetland loss that we’ve seen over the last century, it doesn’t even come close,” he added.

The study reports that wetland loss at present is more like 45 square kilometers a year, or more than one acre an hour (an acre is close to the size of a football field).

Louisiana’s coastal wetlands are valuable as the home to human communities and also because they help protect New Orleans from hurricanes and sea-level rise. At the same time they’re a major habitat for birds, and they nourish fisheries on which humans rely.

The research was published in Science Advances and led by Tulane’s Elizabeth Chamberlain with colleagues at Tulane and other institutions in the United States, Britain, Austria and the Netherlands.

The scientists took their samples in areas of solid land where you would hardly expect there was once open water or a marsh — but, then, that’s the point. The Mississippi River is a great builder of land as it carries large volumes of sediment and silt downstream.

Some of that sediment is now trapped behind dams along the length of the river, but much still reaches the delta. The problem is that factors that drive wetland loss are simply more powerful — the sinking of the land (subsidence), the intrusion of saltwater as seas rise, the dissolution of wetlands that have been cut into canals to support oil and gas pipes, and more.

And on top of that, sea-level rise is now occurring much faster than it did when the Bayou Lafourche land was built. The current rate is about 3.2 millimeters per year, and it is believed to be accelerating. But when the Lafourche subdelta grew, the sea-level rise rate was 0.6 millimeters per year.

“Lafourche formed during a relatively favorable time when the rate of sea-level rise was about as low as it can get in this region,” Tornqvist said.

The conclusion is that, well — the river just may not be able to keep pace. That’s even though the Louisiana Coastal Protection and Restoration Authority, using funds from the BP settlement, is moving forward with two large sediment “diversions” that within a few years could start channeling huge volumes of river water in new directions, in a bid to protect areas around New Orleans in particular. Many scientists have applauded the plan as a way of harnessing nature’s power to counter land loss.

But the rate at which the diversions may build land may not be enough for many areas, Tornqvist said.

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5 Responses to “Mighty Mississippi Defies Human Control. Climate Change Makes it Even Mightier.”

  1. otter17 Says:

    It is encouraging that some of the projects are down the river are trying to restore wetlands as part of the solution. It does require human infrastructure and settlement to stay back from the river, if they are willing.

  2. otter17 Says:

    Oh, and a tragic death occurred over the weekend, a prominent LGBT activist lawyer committed suicide via immolation. He stated in his last letter that it was an action to protest the lack of efforts to address AGW, particularly in USA politics. I was surprised to see some national coverage of the incident a couple days ago. A sad event if wanted to cover it.

  3. dumboldguy Says:

    More unknown unknowns coming to the surface re: the big picture. Just finished a very informative book that talks much about the Mississippi.

    “The Source: How Rivers Made America and America Remade its Rivers”, Martin Doyle, Norton, 2018.

    A very interesting compilation of geology, history, economics, engineering, politics, and human foibles.

  4. rhymeswithgoalie Says:

    One more recent improvement over river flooding is the use of *passive* weirs along levees rather than *politically-controlled* floodgates or pumps. Whenever a *decision* has to be made about opening a floodgate or deliberately breaching a dam, this opens the government entities to political pressure and post-facto lawsuits. If, instead, the river is high enough to spill over a weir, local residents have in large parts already accepted the natural basis for the disasters they suffer.

    [Note: During the post-Katrina flooding in NOLA, the levees were not overtopped, but *failed* to meet their design specs. Also, the flooding of the Lower 9th was due to an unmoored barge breaching the wall of the Industrial Canal. In other words, the city flooding had a high political component.]

  5. redskylite Says:

    Adding to the research on the chief river of the second-largest drainage system on the North American continent, the good folks at Louisiana State University’s center for river studies have built a 10,800 sq ft replica of the river system, a miniature analogue to help experts work out how best to enact a state plan to fight coastal erosion, becoming increasingly concerning, with wearther extremes and sea level rise.

    Scientists unveil 10,000 sq ft model of Mississippi delta to help save coastline

    https://www.theguardian.com/environment/2018/apr/18/mississippi-delta-model-coastal-erosion-scientists


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