A cornfield is flooded from after days of heavy rainfall near Decatur, Ill., on June 5. (Seth Perlman/Associated Press)

Surging floodwaters in the U.S. Midwest headed toward the Mississippi River Monday, leaving in their wake more than a million hectares of washed-out corn and soybean crops and soaring grain prices.

In early overseas trading in Europe, U.S. corn prices reached record levels as news emerged that more than 10 per cent of the corn crop in Iowa has been washed away by the floods.

Soybeans were hit even harder, with 20 per cent of the crop under water so far.

Cold temperatures in May have also stunted the growth of crops in Iowa this year, leaving them more prone to waterlogging and flood damage, farmers said.

"In the lean years, we had beautiful crops but they weren't worth much," corn farmer Dave Timmerman told the New York Times, "Now, with commodity prices sky high, Mother Nature is throwing us these curve balls. I'm 42 years old and these are by far the worst crops I've ever seen."

Corn prices rose 11 per cent last week alone, on news of the floods in Iowa and other producing states.

Farm economists warn that other food prices, for meat, milk and restaurant meals, will also be affected because corn products are central to so much of U.S. food production, as animal feed, sweetener and flour.

"Given the flooding we see today, we're likely to see prices go significantly higher based on the weather," said Chad Hart, an agriculture economist at Iowa State University.

"This is a significant event. We were already having production issues before this occurred, all across the Midwest. This can have gigantic implications," Hart said.

Others states to be hit

Meanwhile, emergency officials in Iowa, Michigan and Illinois were bracing for high water in communities all along the tributaries of the Mississippi River, which is itself approaching flood levels.

The heavy rains that fuelled flooding last week appear to have eased, forecasters say.

Waters were receding Monday in the Iowa cities of Cedar Rapids, Des Moines and Iowa City, leaving behind damaged homes and buildings.

Thousands of people who had been evacuated from their homes were beginning to return to assess damage.

Officials said the cities may have been spared the worst of the flood's ravages because levees had burst upstream, causing havoc in rural areas but dispersing cresting waters over a wider area.

That's also what hit croplands the hardest, according to Iowa's agriculture department.

The state's governor, Chet Culver, has warned that billions of dollars will be needed to repair damage from the flood and compensate farmers who've lost their crops just a month after planting.

Almost all of the state's 99 counties have been affected, Culver said.

Four people have died in flood-related incidents and 12 were killed by tornadoes that swept through the state earlier this month.