Rivers in Midwest brought flood warnings for over 12 million people on December 30 as scores of buildings were submerged after days of intense rain in which 24 people have died.
Two rivers west of St. Louis crested at historic levels, flooding local towns, disabling sewer plants and forcing the evacuation of hundreds of residents.
Major rivers including the Mississippi are expected to reach record highs as flood waters rush toward the Gulf of Mexico, the National Weather Service said.
The flooding has closed many roads and parts of I-44, a major artery running from west Texas to St. Louis. It poses a threat to livestock and crops in farm areas stretching from Illinois to Louisiana.
Jay Nixon spoke with President Barack Obama on December 30 and received a pledge of federal support.
About 300 people in Valley Park, Missouri, west of St. Louis, were evacuated in case a levee is breached on the Meramec River, said Chief Rick Wilken of the Valley Park Fire District.
At least 24 people have died, mostly from driving into flooded areas, in Missouri, Illinois, Arkansas and Oklahoma after days of downpours with as much as 12 inches of rain.
In Eureka, Missouri, along the Meramec River, Mayor Kevin Coffey said a man was rescued from atop the cab of his pick-up truck after spending the night in a parking lot to watch over his gun shop business.
Historic floods on the Mississippi in 1993, 1995 and 2011 occurred during warm weather, after snow melts in the north.
While the rains have stopped for now, freezing weather is setting in.
Agriculture experts said water standing more than a week could kill the soft red winter wheat crop. Export premiums for corn and soybeans were at their highest levels in weeks because of stalled barge traffic on swollen rivers.