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.
Water rose to the rooftops of homes and businesses in Missouri, where Governor Jay Nixon called the flooding “historic and dangerous”.
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.
A 5-mile section of the Mississippi River near St Louis, Missouri, has been closed to vessels as rising water levels caused “hazardous conditions”, the US Coast Guard announced.
Storms and tornadoes have lashed the region in recent days, swelling rivers and causing flash flooding.
Missouri Governor Jay Nixon said 13 people in the state had died in the floods.
Jay Nixon said the National Guard had been called in to help local authorities.
Aerial footage showed water from the Mississippi River engulfing buildings in the evacuated town of West Alton, north of St Louis, on December 29.
In the town of Union, about 50 miles west of St Louis, buildings were partly submerged by severe flooding from the Missouri, Meramec and Bourbeuse rivers.
Coast Guard spokesman Capt Martin Malloy said the high water levels and fast currents had led them to close the section of the Mississippi near St Louis – a busy route for commercial shipping.
River levels are forecast to peak on December 31 and Gov. Jay Nixon warned that the situation could get worse before it gets better.
Jay Nixon said the National Guard would provide security in evacuated areas and direct traffic away from closed roads.
“These citizen soldiers will provide much-needed support to state and local first responders, many of whom have spent the last several days working around the clock responding to record rainfall and flooding,” the governor said in a statement.
Jay Nixon added that three new flood-related deaths had been discovered on December 29, raising the death toll in the state since the storms began over the weekend to 13.
Many of the victims have been trapped in vehicles swept off flooded roads.
South-west of St Louis, a section of Interstate 44 was closed by flooding near the town of Rolla while part of Interstate 70 was also closed in the neighboring state of Illinois.
Many other smaller roads were also closed across the two states, where flood warnings were in effect.
Floods also inundated a wastewater treatment plant south of St Louis on December 28, causing sewage to flow directly into rivers and streams.
The flooding in Missouri and southern Illinois began over the weekend after as much as 10ins of rain fell in some areas in a matter of hours.
It came after severe storms over the Christmas holiday claimed at least 49 lives across the South and Midwest.
Parts of eastern Oklahoma, Arkansas and Illinois are still subject to flood warnings.