Re-creation of the mississippi river’s 500-year flood history shows the worst floods are bigger than ever

Science News  April 4, 2018
The world’s longest system of levees and floodways, meant to rein in the mighty Mississippi River, may actually make flooding worse.

Using tree rings and lake sediments, researchers re-created a history of flooding along the lower Mississippi River extending back to the 1500s. This paleoflood record suggests that the past century of river engineering — intended to minimize flood damage to people living along the river’s banks — has instead increased the magnitude of the largest floods by 20 percent, the researchers report April 5 in Nature.

Climate patterns that bring extra rainfall to the region don’t account for the dramatic increase in flood size, the team found. “The obvious culprit is that we have really modified the river itself,” says Samuel Munoz, a geoscientist at Northeastern University in Boston.

Settlers built the first levees on the Mississippi in the early 1800s. After a massive flood displaced hundreds of thousands of people in 1927, the U.S. government built the current system of spillways and levees. The engineering projects profoundly altered the river’s shape and sediment content. But how these changes affected the size of the river’s largest floods has been unclear, in part because water gauges have tracked the river’s flow for just 150 years.