The worst of the flooding that has ravaged the midwestern United States is nearly over, but it will be weeks before the murky water recedes in many areas, the National Weather service warned Monday.

Tens of thousands of people were evacuated from their homes in Iowa, Wisconsin, Illinois, Missouri and Indiana due to the heavy rains and deadly storms which swept through the region in recent weeks.

Early estimates place the damage in the billions as roads were washed out, rail and barge traffic shut down and millions of acres of crops were swamped.

Scores of levees collapsed or were overtopped by the rushing waters which swallowed entire towns.

The extreme weather which began May 25 and included a series of deadly tornadoes claimed the lives of 22 people, 17 of whom were in Iowa.

More than 11 million people in nine midwestern states were affected by the flooding and extreme weather, the Federal Emergency Management Agency (FEMA) said.

"The worst of it is likely over ... for the bulk of the people," said Steve Buan, a hydrologist with the National Weather Service.

All the tributary rivers of the Mississippi above St. Louis, Missouri have now crested, Buan said, but it will take three more days before the towns down river know whether their levies will hold.

The river is expected to rise by about another six inches (15 centimeters) in areas up to 150 miles (240 kilometers) downstream.

"A lot of these rivers won't go back below flood stage until mid-July," Buan told AFP. "It's going to take a long time to dry out."