Rivers in straitjackets
As a youngster I went on a school journey to the USA, and visited the headquarters of the Tennessee Valley Authority, where I was impressed by a vast concrete model of the entire river system, complete with little obstructions on its bed to simulate turbulence and water speed in different places.
The TVA was set up in 1933 to correct the river’s unruly flooding behaviour, as well as to generate electricity. It inherited one dam on the Tennessee river and from 1936 to 1942 built another seven there and 20 more on its tributaries, with a final major dam being completed in 1967. This barrage of dams is claimed to provide flood protection to more than 2.4 million hectares of land, while reducing the frequency of flooding on another 1.6 million, but the evidence isn’t wholly convincing.
The Tennessee river flooded seriously enough to be recorded in local histories on average once every three years and 10 months between 1808 and 1932, just before the TVA was created, and once every two years and 10 months between 1933 and 2003. Thus the frequency of flooding seems at best unaffected by the dam-building programme in the Tennessee basin in the 1930s and 1940s.
But the Tennessee river is only a small part of the great Mississippi system, which is fed by other big rivers and drains 41% of the continental USA. Dams, locks, canals and artificial banks were built by the US Army Corps of Engineers throughout the system during the 20th century, aiming to enhance navigation and control floods. In the 1950s, government scientists decided that the lower Mississippi was trying to join the Atchafalaya river as an easier path to the sea.
This would have isolated New Orleans from the main river flow. The Old River Control Structure put a stop to this escape, but the force of the Mississippi denied its way was so strong that it had to be supplemented by another flow control station, built in 1986. As things turned out, New Orleans might have been better off on a side channel of the Mississippi, rather than right on the main river, when Hurricane Katrina came calling in 2005.
Meanwhile, the innumerable flood control works that had been built further upstream were having a profound effect on the river’s behaviour. To stop water getting into its floodplain, artificial levées up to 15 metres high had been constructed on at least 10,000 km of the Mississippi’s banks and those of its tributaries. The river had also been straightened by cutting through meanders, and for 1,750 km it flowed through artificial channels.
In 1992, a federal government inter-agency study of floodplain management concluded that 60 years of building flood control structures in the Mississippi basin hadn’t had any real effect in reducing deaths and property damage.
The very next summer, after most of the river’s catchment had received up to 200% more rain than normal, water speeded downstream along the straightened and restricted channel, and hit the city of St Louis where the Mississippi and Missouri rivers meet. The river brushed aside the levées that hemmed it in, and 487 counties in Illinois, Iowa, Kansas, Minnesota, Missouri, North and South Dakota, Nebraska and Wisconsin became flood disaster areas in a matter of hours.
Read on: Ground water.