River defences worsen flooding

Researchers at Washington University in St Louis have found that, for the same discharge from the Mississippi, the annual floods in the St Louis region have been getting steadily higher since 1860, despite the river being protected by 29 locks and dams north of the city, hundreds of canals, and artificial embankments along its banks, Nature Science reports.

The researchers point to the example of the flood of 1993, which was triggered by about the same discharge as that of 1903, but was about 12 feet higher, causing billions of dollars’ worth of damage in the river basin. Tens of thousands of acres of farmland were inundated and their crops destroyed, roads and bridges were damaged, and thousands of people had to flee their homes.

The researchers, Robert Criss and Everett Shock, say that the flood protection is precisely the problem, with the changes in flood hazards over time being “far more dependent on human activities than on the amount of flood water in the river”.

Criss and Shock compared the rising flood levels of the middle Mississippi, around St Louis, with flood records of the Ohio, the Meramec and the Missouri rivers. The lower Missouri and middle Mississippi have been heavily engineered and constrained by artificial channels and high levees, and both have risen. But on the upper Missouri, above Fort Benton, there are few flood-control measures and no evidence that flood levels have risen significantly for more than a century. The same is true of the Meramec in eastern Missouri, the researchers found, where locals have resisted attempts to engineer this river’s course. The minimally-defended Ohio river in Cincinnati has a similarly unchanged flood record.

“The evidence indicates that levee construction and channelisation of the lower Missouri and the middle Mississippi have greatly magnified their floods,” the researchers said, adding that the effect is “increasing and shows no signs of stopping”. They suggest that the lower Missouri should be allowed to return to its natural braided form rather than being confined within high, narrow banks.