Iowa's natural grassy wetlands have been replaced by highly efficient industrial agriculture, a machine that churns out more corn and soybeans than any other US state. However this change has also compromised the ecosystem's ability to absorb large volumes of rain.

Several areas of the state have received more that three times normal rainfall since the beginning of the month.

"Pre-settlement, most of Iowa was under water, a shallow wetland type of system. That landscape has been altered for production purposes so the hydrology of the area has changed radically in the last century-and-a-half," said Kevin Baskins, spokesman with the Iowa Department of Natural Resources.

Farms have installed underground drainage systems and rerouted streams and creeks to protect crops, speeding the rate of water run-off to rivers, he said.

The shallow root systems of corn and soybeans planted annually throughout much of the state do little to slow the speed of water run-off, prompting calls for more farmland to be converted back to native grasslands.

Baskins said: "With civilization, there come trade-offs. There are cases like this when you realize that the river is much more powerful than we humans are and there are some places that we have to give back to nature."

CLIMATE CHANGE LINK?

Concerns about global climate change have also popped up after record snowfall in parts of the Midwest were followed by abnormally heavy spring rains.

Many are calling this year's flood a 500-year-flood, meaning there is only a 0.2 percent chance every year for flooding of such magnitude, although the last such flood was just 15 years ago.

Iowa State meteorologist Elwynn Taylor said a cyclical weather pattern has soaked the region for the past 30 years and drier weather will eventually return.

Others, like Iowa State University agricultural meteorology professor Gene Takle, disagree.

"There has been in the last 30 years a tendency toward more heavy rainfall events in the central US We have a past trend and our models, based on increased greenhouse gasses produced by humans, indicate that the trend will continue."

But he cautioned: "Was this particular event linked to global warming? No, I can't say that with any certainty."

Still, some argue the flood was more a case of bad luck.

"We had 15 inches of rain in a week. That's a whole lot of water. We're looking for someone to pin this on, but floods are a natural phenomenon." (Additional reporting by Andrew Stern, editing by Alan Elsner and Peter Bohan)