Defence worsens floodingRiver engineering is doing more harm than good.
28 September 2001
|Billions of pounds worth of damage: the St Louis flood of 1993.|
Flood-management measures may be increasing the risk of the Mississippi and other rivers bursting their banks, new research suggests1.
Robert Criss and Everett Shock of Washington University find that, for the same total amount of water flowing down the Mississippi - its discharge - the annual floods in the St Louis region have been getting steadily higher since 1860.
The flood of 1993, for example, 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.
This kind of thing wasn't meant to happen any more. The Mississippi is supposed to be protected by 29 locks and dams north of St Louis, hundreds of canals, and artificial embankments (levees) along its banks.
That protection, say Criss and Shock, is precisely the problem. The changes in flood hazards over time "are far more dependent on human activities than on the amount of flood water in the river", they point out.
The duo compared the rising flood levels of the middle Mississippi - the St Louis region - with flood records of the Ohio, the Meramec and the Missouri rivers. This last joins the Mississippi upstream of St Louis.
The lower Missouri and middle Mississippi have been heavily engineered, constrained by artificial channels and high levees. Both have risen.
Certain management practices should be reconsidered
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, 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," say Criss and Shock, "indicates that levee construction and channelization of the lower Missouri and the middle Mississippi have greatly magnified their floods." The researchers add that "this effect is increasing and shows no signs of stopping".
"Certain [flood] management practices should be reconsidered," say the researchers. They suggest that the lower Missouri should be allowed to return to its natural braided form rather than being confined within high, narrow banks.