The Mississippi river has burst through one levee in the US state of Illinois and threatens to break many more throughout the American Midwest.
Three feet of water flooded the area near the town of Gulfport, closing a bridge and cutting commuters off.
Millions of sandbags are now being stacked at the 27 levees which the government says are most vulnerable.
Seven people have so far died in the flooding, which in some areas is the worst in a decade.
"We were very, very disappointed this levee broke today," said emergency official Patti Thompson.
The Mississippi was "a very powerful river and it can be hard to harness", she added.
Help promised
Thousands have been forced from their homes in the flooding, which has so far caused $1.5bn damage in the state of Iowa - a figure set to rise as the floods move downstream.
The 27 targeted levees, reinforced by National Guard troops and volunteers, are those predicted to overflow if bad weather continues, as forecast. Officials predict that the river could rise above the record level set in 1993.
President George W Bush promised help to the affected areas.
"I fully understand people are upset when they lose their home. A person's home is their most valued possession," he said.
"We want to work with the state and local folks to have a clear strategy to help people."
Mr Bush will visit the region on Thursday to inspect some of the flood damage.
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