Forecasters were nervous about storms expected to hit northeast Missouri and central Iowa on Wednesday and Thursday, said National Weather Service meteorologist Mark Fuchs.
"It'll keep water in the system. That's for sure," Fuchs said. "That could turn the river around. That could lead to higher crests."
Some roads were flooded in the northern and western parts of Missouri. Six homes were evacuated in Linn County, about 100 miles from the Mississippi River, emergency director Jamie Stallo said.
Stallo said no injuries were reported, and the floodwaters were beginning to recede Wednesday morning.
Flooding in the American Midwest in the last couple of weeks has killed 24 people, driven tens of thousands from their homes and caused billions of dollars in damage.
On Tuesday, the governors of Illinois, Indiana, Iowa and Wisconsin asked President George W. Bush to allow the federal government to cover 90 percent of disaster-related costs incurred by state and local governments. The federal government usually covers 75 percent of such costs after the president declares a disaster.
"Our states have suffered recent multiple disasters that have placed enormous stress on state and local governments," said Wisconsin Gov. Jim Doyle in a statement. "Reducing our share of the flooding assistance costs would greatly alleviate the social and economic impact on our families and communities that are suffering."
Illinois got good news Tuesday when Bush declared 13 counties federal disaster areas. Gov. Rod Blagojevich has declared 24 counties along the Mississippi, in southeastern and northern Illinois, state disaster areas.
Meanwhile, an earthen levee was all that was still protecting 100 houses, a city park, several businesses and 3,000 acres of agricultural land in east Winfield, one of the last towns in eastern Missouri where the upper Mississippi was expected to crest.
Only National Guard soldiers and firefighters in life vests were allowed to stack sandbags, because volunteers and heavy equipment could sink. A single muskrat recently created a geyser of river water by digging into the berm.
For days, emergency management officials in Lincoln County have focused on the levee about 45 miles northwest of St. Louis. A storm with thunder and lightning Tuesday was only the latest impediment to the desperate attempts to shore up the Pin Oak levee.
"This storm is not a good thing," said Jeff Stamper, a structural engineer with the U.S. Army Corps of Engineers. "It pulled everyone off. You can work on a levee in the rain, but not in lightning."
The Mississippi was expected to finally crest at Winfield sometime late Wednesday, and to flow at its high-water mark - more than 11 feet above flood stage - for several more days. A disturbance as minor as a passing boat could lead to disaster.
A total of 35 levees have overtopped during the Midwest flooding, and seven of them had been federally designed and constructed, said Ed Hecker, chief of the office of homeland security for the U.S. Army Corps of Engineers. He said the nation's levee system wasn't designed to hold back such extraordinarily high flood waters, particularly in rural communities.
"This system pretty much performed as designed," Hecker said.
The river continued to recede Tuesday from the Iowa line down through the lock and dam at Saverton, about 90 miles north of St. Louis. The river had dropped a foot Tuesday morning at Canton following a Sunday crest of 13 feet above flood stage.
Pending further rains, the weather service said the river wouldn't begin to recede at St. Louis - where there is flooding, but none significant - until Thursday night. Forecasters said the last point on the river to finish cresting would be near Chester, Illinois, about 80 miles south of St. Louis, sometime Friday.