VALLEY, NEBRASKA (REUTERS) - As icy, lethal flood waters fed by rains and melting snow recede in Nebraska and Iowa, leaving destroyed homes, drowned cattle and swamped farmland, Midwest states downstream were set on Thursday (March 21) for a relentless surge of the Missouri River.

Flooding from last week's storm has already caused nearly US$1.5 billion (S$2.02 billion) in damage in Nebraska and killed at least four people, with another missing.

"This isn't over," said Mr David Roth, a meteorologist with the National Weather Service's Weather Prediction Centre.

"The river will have several more major crests through next week," he said, not counting the numerous tributaries that feed into the Missouri up and down the Midwest, and will likely inundate communities into next month.

The waters have already swamped a large swathe of Missouri, Nebraska and Iowa along North America's longest river.

A state of emergency has been declared in all or parts of the three Midwestern farm states.

"We're going to get some moderate rain this weekend over eastern Nebraska, maybe a half inch (1.3cm)," Mr Roth said. "It doesn't sound like much, but any more precipitation is bad."

The Missouri River's next major flood crest is forecast to hit St Joseph, Missouri, at 6am on Friday and a day later in Kansas City, Missouri, 90km to the south, according to the US Army Corps of Engineers.

"Where the Missouri meets the Mississippi River in St Charles (Missouri), it has already peaked but it's going to crest again early next week," meteorologist Roth said.

More than 2,400 Nebraska homes and businesses have been destroyed or damaged, with 320km of roads unusable and 11 bridges wiped out, Governor Pete Ricketts said on Wednesday.

Mr Ricketts estimated the floods caused at least US$439 million in damage to public infrastructure and other assets, and US$85 million to privately owned assets. He put flood damage for the state's agricultural sector at nearly US$1 billion.

Offutt Air Force Base near Omaha, which houses the US Strategic Command, remained heavily flooded but operational, officials said on Wednesday.

In Valley, Nebraska, outside Omaha, Mr Pete Smock, 42, worked to clear deep mud surrounding his home and construction business on Wednesday in what will be a long clean-up for the region.

"Devastation is everywhere. I haven't seen anything like this in my lifetime," Mr Smock said.