Case study: The Mississippi floods of 1993, USA
What happened?
In the summer of 1993, the Mississippi River in the USA burst its banks.
 150 levees (embankments) collapsed under the pressure of water.
 Dams burst and bridges were closed. By mid July 100 tributaries had flooded and the Mississippi
spread across the flood plain for 10 - 25 kilometres.
What were the effects?
 Nine states were affected. Floodwater stretched from Memphis in the south to Minneapolis in the north
covering 23 million acres. 48 people were killed.
 26.5 million sandbags were used.
 Almost 70,000 people were evacuated from their homes.
 Final damage costs were estimated at $10 billion. Over 25% of this was crop losses.
 Some areas never recovered. The town of Valmeyer, Illinois, was abandoned after the floods and
rebuilt on higher ground.
 The river was closed to traffic for two months - 15% of the USA's freight uses the Mississippi.
Climatic conditions leading up to the flood.
From April and all through the summer, record heavy rain fell across the north part of the Mississippi
drainage basin. This was partly caused by high pressure system around Bermuda. Thunderstorms brought
torrential rain in June and July adding to the rising waters. Rainfall in these two months was the highest
since 1895.
Taming the Mississippi
The Mississippi is a vital transport route running from north to south in the centre of the USA. Engineers
have had to work to maintain a channel of water deep enough for river traffic and to control the river's
regular floods, protecting settlements and farmland throughout the river basin. Engineers have altered the
Mississippi in the following ways:
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'Wing dykes' have been built to slow down the river on one side whilst speeding it up on the other,
creating a deeper channel for navigation.
Sections of the river have been straightened by cutting through meanders. The river is now 150
miles shorter.
1600 kms of levees (embankments) have been built to prevent the river spilling over the flood
plain.
Many dams and reservoirs have been built on tributaries to hold back and store water.
Much of the river has been lined with concrete slabs.
Engineers caused these floods
Despite all these control methods the flood of 1993 was the worst ever. Engineers called it a 'one in a
hundred' flood caused by exceptionally heavy rainfall, Many environmentalists argue that far from
controlling the Mississippi, the engineers' efforts have made things worse by:
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shortening the river, causing it to flow more quickly and increasing erosion.
restricting the flow of water inside levees, speeding up the flow and increasing the pressure on the
levees and the likelihood of flooding. In 1993 a large number of levees broke or were too low to
stop flooding.
altering the natural flow of the river to such an extent that the 1993 floods were the worst ever,
even though there was less water in the river than during previous major floods.
increasing the energy in the river, because there is less sediment being moved around.