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The swollen Mississippi River was pushing downstream at 10 times the speed of the Niagara Falls on Saturday, threatening more floods in rural southern Missouri and Illinois.

Another levee along its course succumbed Saturday in Illinois, bringing to at least 12 the number of levee failures. In Arnold, Missouri, an estimated 150 homes were underwater.

President Barack Obama signed a federal emergency declaration for Missouri on Saturday that allows for federal aid and Federal Emergency Management Agency assets to be funneled to the state. Missouri Gov. Jay Nixon had signed an emergency declaration requesting federal assistance to remove debris earlier Saturday.

"Before you can rebuild, you've got to remove the debris," Nixon said from the waterlogged town of Eureka. He added that thousands of homes and hundreds of businesses had taken in rubble from the floodwaters.

"When you see a historic flood, we are committed to a historic response," he said.

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The flood, fueled by more than 10 inches of rain over a three-day period that began last weekend, is blamed for at least 24 deaths, 15 of which were in Missouri.

A flooded-out football field is seen at the Jefferson County Youth Association on Jan. 2, 2016 in Arnold, Missouri. Michael B. Thomas / Getty Images

The worst of the dangerous, deadly winter flood has been in the St. Louis area, leaving residents of several communities to assess damage, clean up and figure out how to bounce back — or in some cases, where to live.

On Friday, searchers found the body of a teenager in central Illinois: Devan R. Everett, 18, who had been missing since Monday when he and another teen disappeared while driving a pickup truck.

The search continued for the other teen, as well as two men in Missouri and a country music singer in Oklahoma. Craig Strickland, the lead singer of the Arkansas-based country rock band Backroad Anthem, disappeared during storms Sunday while duck hunting.

The surge in water from the flooding was expected to hit cities farther south, such as Memphis, Tennessee, during next week. The National Weather Service issued a flood advisory for the Cumberland River at Dover, Tennessee, through Monday evening.

Meanwhile, the lower Mississippi could see flooding into the middle of January as the floodwaters move downstream, according to the NWS.

Traffic moves freely in both directions along Interstate 44 at Highway 141 after floodwaters from the Meramec River receded in St. Louis on Jan. 1, 2016. David Carson / St. Louis Post-Dispatch via AP

“Our biggest concern is looking out for those who haven’t evacuated,” U.S. Coast Guard officer Nicholas Litchfield told NBC News.

Meanwhile, other areas experienced some relief. Receding waters enabled the Missouri Department of Transportation to reopen all of Interstate 55, which had been closed Wednesday to allow crews to place sandbags and pumps because it was in danger of being overtaken by the Meramec River.

A spokesperson told the AP that typically, 76,000 vehicles pass through the area on a daily basis.

Interstate 44, which had also been closed for two days over a 24-mile stretch, was also drying up, and reopened later Friday. A state of emergency for St. Louis County was lifted Friday.

Nixon called the flood a "devastating force."

"I've just never seen anything this high," he said.