CEDAR RAPIDS, Iowa (CNN) -- Flood water could spill over about two dozen levees along the Mississippi River in Iowa and Missouri this week unless people top the levees with enough sandbags, a spokesman for the U.S. Army Corps of Engineers said Monday.
That assessment raises the prospect of further floods in Iowa as well as in Missouri after floodwaters killed five people, displaced 38,000 others and damaged $1 billion worth of crops in Iowa alone.
If National Weather Service forecasts are accurate, rain will raise the Mississippi River this week to a point where water will spill over 26 or 27 levees between Davenport, Iowa, and St. Louis, Missouri, about 200 miles away, said Ron Fournier, a spokesman for the U.S. Army Corps of Engineers.
Communities at high risk of major flooding include Quincy, Illinois, and Hannibal, Missouri, according to NWS modeling. Moderate flooding is possible in Alton, Illinois, and St. Louis, Missouri.
Floods of a few inches to a few feet deep could seep into communities behind the levees, Fournier said, but people can prevent possible flooding by topping the levees with enough sandbags, he said.
Several major rivers that feed into the Mississippi have overflowed their banks, including the Cedar, Des Moines and Iowa rivers.
"We're looking at all-time flood records in terms of levels," Gov. Chet Culver told reporters Monday evening. "The challenges remain, but we are more determined than ever to win the fight."
Authorities worried about towns along the Mississippi River as floodwaters in eastern Iowa began to drain toward the river.
In Washington, Sen. Tom Harkin of Iowa said the flooding has devastated his state's corn crop and may have inflicted up to $1 billion in damage to Iowa's agricultural sector. Floods prompted farm-equipment manufacturer John Deere to idle two plants in Waterloo, Iowa, he said.
"Across eastern Iowa, the flooding rivers have washed out railroad lines; Mississippi barge traffic has come to a halt; and closed major roadways," said Harkin, the Democratic chairman of the Senate Agriculture Committee. "Thousands of Iowa businesses, large and small, have been impacted."
Harkin said Iowans "are a resilient and resourceful people," but will need "generous federal assistance" to recover.
"The destruction is so vast that it is simply beyond the capacity of local governments and the Iowa state government to handle it by themselves," he said.
The Federal Emergency Management Agency has set up six disaster recovery centers in Iowa and has provided nearly $4 million in assistance, state and federal officials reported. So far, 24 counties are under federal disaster declarations, making residents eligible for individual aid, Lt. Gov. Patty Judge reported.
In Cedar Rapids, Iowa, the Cedar River inundated more than 400 city blocks. Authorities told thousands of evacuated residents that it was too dangerous for them to return home -- even though many had waited in line Sunday to receive permission to go home to gather belongings. Fire officials said it could be some time before they are allowed to return.
"The good news is there has not been a fatality here in Cedar Rapids, and we want to keep it that way," said Gene Meyer, Iowa's public safety commissioner.
Meanwhile, the death of one woman whose body was found in her car Monday was determined unrelated to the floods, said Courtney Greene, a spokeswoman for the governor's office and the state Emergency Operations Center.
The woman was found dead near New London, south of Iowa City, after her stopped car was hit by a National Guard bus that was transporting soldiers and airmen to flood duty. Greene said a preliminary autopsy showed she did not die from crash injuries, but no definitive cause of death had been determined.
In addition to floods causing problems, tornadoes in Iowa have killed 12 people recently, Greene said.