The levee failure near St. Charles comes as teams furiously fill sandbags to reinforce other waterlogged embankments guarding towns still waiting for the arrival of the huge river's flood crest.
Forecasters expect the last stretch of the bloated river to crest later this week.
"The spirits are tired, but they are still there and still solid," said Jo Anne Smiley, mayor of Clarksville, where makeshift sandbag levees are keeping the city's small downtown dry. "This is a community that will rise above this."
Smiley toured her town Monday with Gen. Robert Van Antwerp, the U.S. Army Corps of Engineers' chief of engineers. He said he was most concerned about agricultural levees up and down the river.
"I think what they have is holding well," Van Antwerp said. "Now, it's a matter of getting the water off of it."
But in the saturated Elm Point levee at St. Charles, Mo., about 30 miles north of St. Louis, two gaps allowed water to flood hundreds of acres of agricultural land. A mobile home park where about 700 people live near the Elm Point Levee had already been evacuated.
CBS News affiliate station KMOV-TV reports that the levee burst in two spots around 1:30 a.m. on Tuesday. One of those holes is reportedly the size of a football field.
The constant pressure of the river remains the primary concern in Lincoln County, where officials asked for volunteers Monday to help fill 50,000 sandbags to fortify the 2½-mile-long Pin Oak levee, an earthen berm that was so waterlogged that it was like "walking on a waterbed," said county emergency management spokesman Andy Binder.
Federal officials said they couldn't be sure it would survive through the river's crest at Winfield later in the week.
"They have a serious condition on their hands," said Travis Tutka, the Army Corps' chief of dam safety. "This will be quite a test of that levee."
If it breaches, the river will swamp 100 homes in east Winfield, as well as 3,000 acres of farm fields, several businesses and a city ballpark. A muskrat that burrowed a hole in the soft ground released a geyser of water, and officials said it took nearly six hours Monday to choke off the leak.
"There is no guarantee of performance, but we're fighting the good fight," Tutka said.
Only a handful of residents remained in east Winfield on Monday, after emergency workers went door to door urging them to evacuate. Among the holdouts was Sherman Jones, 56, who was all alone in his house except for his dogs, Mugsy and Junior.
"There is no place to go but the high school. I am not going to leave 'til my feet are wet," Jones said. "It's been a rough year, but we'll get through it."
In Foley, north of Winfield, floodwaters late Monday were filling the higher part of town. The east side of Foley was already submerged.
Not far from the Iowa state line, the river was down a few inches at Canton after cresting Sunday at 27 feet - less than a foot short of the record set during the Great Flood of 1993. Jeff McReynolds, the city's emergency management director, said a voluntary evacuation request remained in place in the town of roughly 2,500.
"We were right up there to our nostrils for about 24 hours," McReynolds said. "The concern from our operations center is they (residents) have seen the crest and think the river has come down and want to move back into their homes."
Down river from Quincy, the levee at Hannibal, Mo., was holding the slowly falling river out of the boyhood hometown of Samuel Clemens, who wrote as Mark Twain. Marion County Emergency Management Director John Hark said the city was already planning for its National Tom Sawyer Days, the early July festival celebrating Twain's work.
Hark said that with the river dropping, he could focus on other things that might discourage tourists - such as high gas prices.
"The flood, I think, is easier," he said.