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Scientists say the Mississippi is flooding more than it has in 500 years — and we caused it

A swing set sits submerged in floodwater from the Mississippi River, April 21, 2013, in Clarksville, Mo. (Jeff Roberson/AP)

A new scientific study stoked a long-standing controversy Wednesday with the claim that human interventions, in the form of levees and other engineered structures, have made the Lower Mississippi River more likely to flood, as it has to damaging effect several times in the past decade.

The research centers on the Mississippi below the Ohio River confluence, in Cairo, Ill. It uses river sediment records and tree rings from once-drowned oaks to infer past flow patterns and floods, and finds that the past 150 years stand out from the past 500 in terms of the volume and frequency of recent flooding events.

“For the first time, we’re able to get a picture of what flooding on the Mississippi has looked like over the last 500 years,” said Samuel Muñoz, the study’s lead author and a geoscientist with the Woods Hole Oceanographic Institution and Northeastern University.

Taming the mighty Mississippi

Before that, records of past Mississippi flood events were based on written records, and those weren’t adequate to construct a record 500 years long, Muñoz said.

He completed the research with scientists from Woods Hole and several other U.S. and British universities. The work was published in the journal Nature.

“How often the river has flooded, and also how big those floods were, both of those are much bigger, in the last century-and-a-half, than in the previous four centuries,” Muñoz said.

More specifically, the study finds that the risk of the proverbial “100 year flood” has increased by 20 percent in the past 500 years, and most of the cause is the engineering of the river.

Since the great flood of 1927, the Mississippi River has been “channelized” by the U.S. Army Corps of Engineers, which was  directed by Congress to undertake the project.

The river has been made straighter to aid navigation, and it has been lined with levees and revetments that control the flow of water. Before that, the river oscillated and meandered, but now it is more confined to a straight path.

The Corps says that $100 billion in damages has been avoided as a result since 1928. The agency did not immediately respond to a request for comment regarding the new research.

For more than 100 years, some experts have argued that channelizing the river is a bad idea because it concentrates the water in one place, also increasing its flow speed.

“It’s like if you clog up your arteries with a bunch of cholesterol, and get atherosclerosis, your blood pressure is going to go up,” said Robert Criss, a geoscientist at Washington University in St. Louis. “If you clog up the river with levees and navigation structures, you’re going to make floods worse. And it isn’t a surprise.” Criss was not involved with the research.

What may raise questions, though, is the study’s inferred causal link between river engineering and floods over a long period and, particularly, in the most recent years. After all, there are also climatic factors that affect how much water the Mississippi receives, and those might also be changing.

The study finds that the Mississippi’s flows are controlled by natural climate events such as El Niño, which shape rainfall, yet says that engineering is a far bigger factor than modern human-caused climate change in increasing flood risks.

Not everyone agrees. “I think it is possible that the long-term trends in hydrology could be the result of climate change, rather than river engineering,” wrote Scott St. George, a geographer at the University of Minnesota, in a Nature commentary published on the study.

Reactions were varied to the new research.

“This study provides an example of how well-intended human actions, in this case the construction of flood-protection systems, produces adverse effects, including higher floods,” said Torbjörn Törnqvist, a geoscientist at Tulane University in New Orleans.

But Criss, who said he agrees with the research’s underlying conclusion, had critiques of the study itself.

“It is an important problem; to their credit they have tried to use multiple indexes,” Criss said. “Multiple ways to measure discharge.” But he said he is not sure how “robust” the study’s approach is.

Criss contends that the idea that the Mississippi is being corralled into a narrower channel and that that increases flood risk is kind of “old news.”

“It’s an old story of messing with natural systems, and we’ve made a monster out of it at this point,” he said. “And we pay the price every few years.”

But Criss and Muñoz would probably agree that the river could benefit by being a bit more liberated from its constraints, at least in some cases.

“This kind of work supports efforts to basically give nature a little bit more room, to give the river more room, and basically do things like restoring wetlands, and maybe even move levees back further, so the river has more room to move around,” Muñoz said.