The Midwest region has also been hard hit by a series of tornadoes including one which killed four teenage boy scouts on Wednesday night when they were camping in western Iowa.
Forty eight people were also injured when the tornado ripped though the camp near Little Sioux.
One of the worst flooded areas is Cedar Rapids, Iowa's second largest city, where more than 100 city blocks were inundated after the Cedar River broke its bank.
'Shut down'
Officials say that about 8,000 people have had to be evacuated and rescuers have been forced to use boats to get to residents trapped by the rising waters.
In Cedar Rapids more than 100 city blocks have been flooded
"It hits everything. Colleges are shut down, stores, it's devastating," Lisa Fox, vice-president of the Iowa Association of Business and Industry, told Reuters news agency.
"Cedar Rapids is completely shut down. It's going to be a long-term recovery."
Wednesday and Thursday's storms brought 13cm (5in) of rain to western and central parts of Iowa.
"We are seeing a historic hydrological event taking place with unprecedented river levels occurring," Brian Pierce, a meteorologist with the National Weather Service in Davenport, was quoted by the Associated Press news agency as saying.
"We're in uncharted territory - this is an event beyond what anybody could even imagine."
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