Vast stretches of the Midwest remained threatened by what the National Weather Service described late Monday as "major to historic" flooding, even as officials and residents were still grappling with a recent deluge that left three people dead in Iowa and Nebraska.

The flooding occurred after downpours and snow melt swelled rivers and creeks across the region last week.

Flood warnings and advisories remained in effect across the Plains, the Mississippi Valley, and parts of the Ohio Valley region on Monday, the weather agency said.

Speaking to reporters Monday, Iowa Gov. Kim Reynolds called the floods "catastrophic" and "unbelievable."

"It was heart-wrenching to see the breadth of the flood," she said, recalling a flight over the Missouri River earlier.

Forty-two of Iowa's counties had declared emergencies, she said. Two-thirds of the town of Hamburg, just east of the Missouri river, was "lost," she said.