Officials closed a major interstate highway south of St. Louis, Mo., on Thursday morning amid historic flooding that has already left at least 20 people dead.

Interstate 55 near the town of Arnold was entirely closed early Thursday due to floodwaters from the Meramec River. The Missouri Department of Transportation authorities had already rerouted traffic Wednesday night to allow for sandbagging and pumping.

Other connections between St. Louis and adjacent counties already are closed, including I-44 over the Meramec. The closures forced traffic onto other nearby roads, creating gridlock.

The flooding also prompted evacuations in Missouri and Illinois on Wednesday. As swollen rivers and streams pushed to heights not seen in nearly a quarter-century, officials helped residents get to higher ground amid fears that already dire conditions could worsen as floodwaters began spilling over federal levees protecting communities and farmland.

In Eureka, southwest of St. Louis, firefighters and their boats have been in high demand since Tuesday, accounting for roughly four dozen rescues of people in their homes, businesses or vehicles. Television news footage showed at least one home there drifting in the swollen river Wednesday, when firefighters rescued by boat a man and a dog as floodwaters lapped at the eaves of the house roof on which they'd been trapped for a night.

Missouri flooding

Submerged roads and houses are seen after several days of heavy rain led to flooding, in an aerial view over Union, Mo., on Tuesday. (Kate Munsch/Reuters )

"Our crews are getting dispatched to another rescue now," Scott Barthelmass, a Eureka Fire Protection District spokesman, said mid-afternoon Wednesday as the swollen Meramec River there was cresting. "I think you're seeing people who are desperate or impatient, putting themselves in predicaments."

The U.S. Army Corps of Engineers announced Wednesday that nine levees had been topped by water. Most of those earthen barriers were meant to protect farmland rather than populated areas and another was along now-deserted, manmade Chouteau Island near St. Louis on the Illinois side of the Mississippi River.

Nearly a dozen other levees considered at risk for "possible significant distress," were holding, but people were moving out just in case. Michael Pennise, mayor of the St. Louis suburb of Valley Park, ordered mandatory evacuations for 350 to 400 homes and dozens of businesses in the section of town near the fast-rising Meramec River.

At least 20 deaths over several days in Missouri and Illinois were blamed on flooding, mostly involving vehicles that drove onto swamped roadways, and at least two people were still missing Wednesday.

And search teams went out for a third day in hopes of finding a country music singer from Arkansas who disappeared while duck hunting in a flooded area in northern Oklahoma. The floodwaters there also destroyed a leftover film set used in the 2003 remake of "Where the Red Fern Grows."

A 38-kilometre stretch of Interstate 44 was closed Wednesday at Valley Park southwest of St. Louis due to Meramec River flooding

Missouri Gov. Jay Nixon has activated the National Guard to assist with security in evacuated areas and to help keep road closure sites clear.

Midwest Flooding

A firefighter puts a vehicle into neutral to remove it from floodwater in Fenton, Mo., on Dec. 30, 2015. (J.B. Forbes/St. Louis Post-Dispatch/Associated Press )

Valley Park City Attorney Tim Engelmeyer called the governor's office to send troops to help in the evacuated area of the about 7,000-person town. He was also watching computer projections of the crest — expected to be almost one metre higher than the record of just over 12 metres on Thursday — knowing that an unexpected upgrade could be enough to send water over the levee.

"We're so close," he said. "We're talking about a potential 6-inch difference."

Pennise said the corps is confident the levee, built in 2007, is safe, but ordered evacuations as a precaution.

In the southwestern Missouri tourist destination of Branson, residents of about 150 duplexes and homes had to evacuate Wednesday due to flooding from a manmade lake. But the shopping district along the lake was still open, Fire Chief Ted Martin said, adding, "it has been packed with people, and I don't know where all of them have come from."

Three-day rainfall totals of 22 to almost 28 cm were records in some parts of an area that stretched from southwest to east-central Missouri, said Mark Fuchs, a hydrologist for the National Weather Service in St. Louis. Rainfall totals of that magnitude occur only every 100 to 300 years, according to rainfall frequency data from the National Oceanic and Atmospheric Administration.

On Thursday, the Mississippi River, which runs beside the Gateway Arch and downtown St. Louis, is expected to reach nearly four metres above flood stage there. That would be the second-worst flood on record, behind only the devastating 1993 flood. A flood wall protects the city.

In the historic riverfront city of Alton, Ill., some downtown business owners continued to scramble Wednesday to keep out rising water from the Mississippi.

Most of the damage in the city roughly 24 km north of St. Louis, a stop on the 19th Century Underground Railroad, was confined to high water in some basements. Firefighters and emergency road crews worked to pump out water from flooded storm drains behind a temporary retaining wall reinforced by gravel and sandbags.

The Argosy Alton casino, which shut down on Monday, remained closed. So did the southbound lane of the main highway connecting the city to Missouri.

Alton Mayor Brant Walker said he's "very optimistic that what we've built here will hold" as the Mississippi River is expected to crest at about 11.5 metres on Thursday, about five metres above flood stage.

That confidence was of little consolation to Tim Meeks, who was loading vats of gourmet olive oil from his downtown market onto a trailer headed to higher ground as other employees worked to drain a basement filled with more than two metres of standing water.

"We don't keep anything down there anymore," he said. "All of our product has to go."