Volunteers worked on Friday to fortify levees strained by the worst US Midwest flooding in 15 years as the swollen Mississippi River moved toward an expected weekend crest after causing huge losses.

"It's a beautiful river, but it can turn very vicious and ugly in a hurry," said John Hark, emergency management director for the city of Hannibal and Marion County, Missouri.

"Until that river goes back within its banks where it belongs, I'll take no chances."    

Hannibal, boyhood home of famed American author Mark Twain, is protected by an earth levee and floodwall, and it is not considered at risk. But the river has breached or overtopped 23 levees so far this week, with 25 more seen at risk before its expected crest near St. Louis, Missouri, on Sunday.

North of Hannibal, Butch Harlan and his son Derrick manned a pumping station and tended to an earth levee in the Fabius Drainage District, fighting "boils" -- holes where water under pressure beneath levees bubbled up through the ground.

"We have to stop the boils otherwise they'll undermine the levee and the river will come rushing in," Butch Harlan said. "It's not water coming over the top that's the problem, it's what's happening beneath our feet."    

Days without rain have allowed rivers and creeks to recede in Iowa, Illinois, Wisconsin and Indiana, revealing the scope of the multibillion-dollar flood disaster. Scattered rainstorms were forecast, but nothing like the deluges that dumped a foot (0.3 metre) of rain on parts of the region this month.

Thousands of acres of prime farmland in the heart of the world's largest grain exporter have been inundated. Corn prices edged lower on Thursday on profit-taking after record highs this week, but US cattle and hog prices set records.

The cost of the flooding across the US corn belt will be felt by consumers worldwide in terms of higher food prices, and in business losses yet to be totaled.

Bridges and highways have been swamped, factories shut down, water and power utilities damaged, and the earnings of railroads, farmers, and myriad other businesses disrupted.

President George W Bush visited flooded Iowa cities on Thursday as his administration promised funding from a disaster relief fund.

"I know a lot of farmers and cattlemen are hurting right now," Bush said at an emergency center in Cedar Rapids, Iowa, among the cities hit hardest by this month's flooding.

Federal Emergency Management Agency administrator David Paulison said the $4 billion now in FEMA's Disaster Relief Fund should be "more than enough" to provide federal aid. Ultimately, the cost of the disaster may rival that of 1993 Midwest floods that caused more than $20 billion in damage and 48 deaths. Twenty-four deaths have been blamed on flooding and violent storms since late May as rivers overflowed their banks. Another 40,000 people have been forced from their homes. - Reuters