The Mississippi river is predicted to rise above the record level set in 1993
People along the Mississippi river in the US state of Iowa are preparing for flooding after thousands were forced from their homes in neighbouring areas.
With record water levels predicted in the coming days, the authorities have been reinforcing levees with sandbags to stop the river bursting its banks.
Earlier, two more deaths were reported, bringing the regional total to seven.
The flooding has also ruined crops of corn, the price of which hit a record
high of $8 (£4) a bushel on Monday.
Iowa is the biggest corn producer in the US and the floods have devastated large areas of farmland at a time of rising food prices.
Hundreds of National Guard troops and volunteers worked throughout Monday to reinforce levees along the Mississippi in low-lying parts of Iowa as officials predicted the river would rise above the record level set in 1993.
Sandbags were used in Burlington, a key rail hub, to protect it from the river. In Keokuk, efforts were also made to protect the city's water supply.
"It's likely that we will see major and serious flooding in the south-east", Iowa Governor Chet Culver said.
"We are taking precautionary steps, we are evacuating where necessary, but that is going to be the next round here."
More than 38,000 residents in 26 communities throughout the state had been evacuated by Monday morning.
Some 25,000 were from Cedar Rapids, one of the worst affected areas where 438 streets were submerged in the past four days. The cost of the damage in the city alone has been estimated at $737m (£380m).
Miles of sandbag barriers were unable
to contain the Iowa river at Oakville
In western Illinois, National Guard troops hoped to fill about 500,000 sandbags to fortify levees along a 15-mile stretch of the Mississippi near the city of Quincy.
Flood waters have meanwhile began to recede in western Michigan after the Iowa river peaked earlier and lower than expected, possibly because of a number of earlier levee breaches downstream, the National Weather Service said.
Emergency officials have advised people not to drink or swim in any remaining water, saying it might be contaminated by sewage, farm chemicals, refuse or dead animals.
Earlier, the American Red Cross said its disaster relief fund had been completely spent and that it was being forced to borrow money to help flood victims.
President George W Bush will visit the region on Thursday to inspect some of the flood damage.
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