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.
Missouri Governor Jay Nixon has formally lifted the state of emergency that he had declared in Ferguson last month.
Jay Nixon had declared state of emergency on August 19, following demonstrations against the killing of black teenager Michael Brown by white policeman Darren Wilson.
Michael Brown, 18, was shot dead after being stopped for jaywalking.
Jay Nixon said people were “getting back to their normal routines” and the order was no longer needed.
“This progress is a testament to the efforts of community and faith leaders, working alongside state and local law enforcement officers, to bring peace to the streets of Ferguson and much-needed stability to its citizens,” the governor said in a statement.
Missouri Governor Jay Nixon has formally lifted the state of emergency that he had declared in Ferguson last month (photo Reuters)
Michael Brown was killed on August 9 after white police officer Darren Wilson stopped him and a friend for walking in the middle of the street.
Eye-witnesses have said Michael Brown raised his hands in a gesture of surrender before he was shot multiple times by Darren Wilson.
Police accounts have disputed this, saying there was a struggle between Michael Brown and Darren Wilson which led to the shooting.
Many local residents saw the crime as an example of white police brutality.
Ferguson’s police force is more than 90% white, while its population is more than 60% black.
Protestors say police officer Darren Wilson, who has been place on administrative leave, has not been adequately punished.
A St Louis County grand jury has begun hearing evidence about the killing and will decide whether or not to charge Darren Wilson with a crime.
The US Justice Department has opened its own investigation into the shooting of Michael Brown.
Last week, the Ferguson police began using body cameras to record interactions with the public.
Missouri Governor Jay Nixon has criticized Ferguson police for releasing a surveillance video which apparently shows shooting victim Michael Brown stealing from a convenience store.
Jay Nixon said the release of CCTV footage nearly a week after Officer Darren Wilson shot black teenager Michael Brown in Ferguson, St Louis, “appeared to cast aspersions” on the dead man.
“It made emotions raw,” Jay Nixon told ABC on Sunday.
Attorney General Eric Holder has ordered a federal post-mortem on Michael Brown’s body.
Riot police used tear gas and smoke bombs to disperse protesters overnight as they defied a curfew in Ferguson. The curfew is being extended for a second day, beginning at midnight on August 17 and running for five hours.
The killing of Michael Brown by a white policeman in a street on 9 August has inflamed racial tensions in the largely black suburb.
It caused dismay across the US while images of riot police deployed in an American city suburb have made headlines worldwide.
The video “appeared to cast aspersions on a young man that was gunned down in the street”, said Governor Jay Nixon.
On Saturday, Jay Nixon declared the curfew after looters targeted stores in Ferguson overnight.
Governor Jay Nixon has criticized Ferguson police for releasing a surveillance video which apparently shows shooting victim Michael Brown stealing from a convenience store
Seven arrests were made as police used smoke bombs and tear gas to disperse a crowd of about 150 in Ferguson, after being called to the scene of a disturbance at a restaurant after the curfew started.
A police car reportedly came under fire and a person was hurt in an apparently unrelated shooting in the same area.
As police were nearing the restaurant, a man with a handgun went into the street but ran away, AP news agency reports.
Someone also shot at a police car but it was not clear if it had been hit.
In the same area, a person was shot and critically wounded and police were searching for the attacker.
Demonstrators complained that the curfew would make matters worse.
Jayson Ross, who was leading the protesters toward the police before tear gas was fired, was quoted as saying by AP: “They got guns. We got guns. We are ready.”
Local politician Antonio French, who was with the protesters, tweeted: “Too many young men talking about they’re ready to die tonight.”
The US justice department is investigating the shooting of Michael Brown, with 40 FBI agents dispatched to Ferguson to gather information.
The justice department said on Sunday that Eric Holder had ordered a federal post-mortem examination at the request of Michael Brown’s family, in addition to a Missouri state autopsy.
A preliminary autopsy by the St Louis County Medical Examiner’s office on the day after Michael Brown’s death found he had been shot, police said, without disclosing how many times.
Earlier, the lawyer for Michael Brown’s family, Anthony Gray, announced that it had hired one of America’s most famous forensic pathologists, Michael Baden, to conduct an independent examination.
Michael Baden, 80, has worked on investigations into the deaths of such public figures as President John F. Kennedy, the punk star Sid Vicious and the family of the last Russian emperor, Tsar Nicholas II, according to a profile on Fox News, where he works as a TV host.
Darren Wilson, a six-year police veteran with no previous complaints against him, has been on paid administrative leave since the shooting.
Darren Wilson had reportedly stopped Michael Brown for walking in the street, disrupting traffic.