Floods on the mighty Mississippi River are larger and more frequent today than at any time in the past 500 years — in part, a new study suggests, because structures erected to control the river have increased the flood risk.
The research is likely to inflame a long-running debate over how to manage the Mississippi, whose 3,800-kilometre course runs through 10 US states. Some hydrologists say that the river needs more-naturalistic management, to mimic the wandering, dynamic waterway of a century ago. But millions of people live and farm in the historic floodplain and an entire shipping economy is built around the Mississippi1, making that strategy a tall order.
The US Army Corps of Engineers, the government agency that manages the river flow, declined to comment on the study. But Robert Twilley, a coastal-systems ecologist who directs the Louisiana Sea Grant College Program at Louisiana State University in Baton Rouge, says that the study “should be on every desk of every Corps engineer who is designing infrastructure for the Mississippi River”.
To reconstruct the river’s history, Samuel Munoz, a geoscientist at the Woods Hole Oceanographic Institution in Massachusetts, and his colleagues looked at oxbow lakes and oak trees on the lower Mississippi between southern Missouri and Louisiana. Oxbow lakes are coils of river that became detached from the main flow as the Mississippi changed course.
The team examined three lakes that were cut off from the river around 1500, 1722 and 1776. The lake beds preserve layers of coarse material laid down by the high-powered flows of major floods, whose timing the researchers estimated using several methods, including radiocarbon dating of plant fossils. Tree rings from oaks along the river confirmed these flood dates. Flooded oaks grow unusually small and numerous vessels that show up as white rings.
Both dating techniques are relatively new and hadn’t yet been brought to bear on this most famous of American rivers.
The result surprised Munoz. Both the frequency and magnitude of floods on the Mississippi have increased in the past 150 years. Floods were correlated with global weather patterns linked to the El Niño–Southern Oscillation in the Pacific Ocean and the Atlantic Multidecadal Oscillation, both of which influenced when and how much rain entered the system. But these climate cycles couldn’t explain all of the increase.
Munoz and his team suggest that up to three-quarters of the increased flood risk might be attributable to the dams, walls and levees that now confine the river. These structures turned a lazy, meandering behemoth that could slop out excess water along much of its length into a super-charged firehose. Now when the river overtops its banks, the flood is faster, bigger and more powerful than it would be without human intervention, the researchers found.
In essence, the same engineering works that prevent small floods make big floods worse, Munoz says — although he stresses that his team’s conclusions are still preliminary. “It is definitely something that needs to be looked at more,” he says.
Tessa Harden, a hydrologist at the US Geological Survey in Portland, Oregon, who studies ancient floods, says that the study’s conclusions might be preliminary, but the basic trend they describe is useful for making predictions about flood risk. “It highlights the need for longer records,” she says. Federal agencies don’t routinely use palaeoflood data in analyses of flood risk around the country, she adds, but it’s catching on. “Bridges and culverts are designed based on the ‘200-year flood’, and dams are designed for a ‘2,000-year flood’,” Harden says. But if engineers have only 100 years of flood data, the uncertainty about what constitutes a 200-year flood “is pretty outrageous”, she adds.
Munoz and his team have received funding to expand their analysis to cover more of the Mississippi. The researchers hope that the data will be useful to the Army Corps of Engineers and other agencies as they wrestle with how to manage the river, which has been controlled and routed in ways deeply enmeshed in US life and commerce. “At this point, there is no easy solution,” Munoz says. “We’ve built an entire economy around these changes.”
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