Hundreds of people were being evacuated from their homes in Missouri after a levee was breached Monday night.
Officials in St. Charles County near St. Louis activated warning sirens after the levee broke, a statement from the county said.
Earlier Monday, a bridge connecting West Alton, Missouri, and Alton, Illinois, was shutdown after a temporary flood barricade gave way.
Forecasters say there could be major flooding Tuesday. The Mississippi River at St. Louis was 10.1 feet above flood stage Monday night, according to the U.S. Geological Survey website.
Flood warnings and advisories were in effect from counties in northern Illinois down to Louisiana.
As authorities dealt with rising water in Missouri and Illinois, officials in Oklahoma wrapped up another day of looking for people who remained missing after last week's tornadoes and heavy rain.
The fate of the five missing people remains uncertain, officials said.
Searchers found the body of an 8-year-old girl Monday afternoon, Oklahoma City Deputy Fire Chief Marc Woodard said. She drowned in the Deep Fork River.
At least 18 people died in the storm, authorities said Monday. Among them were four people who died Saturday in separate drowning incidents, according to officials.
Also killed were seven members of an extended family from Guatemala -- including four children -- who died when they were swept away by rains after hiding from the approaching weather in a storm drain. One woman remains missing.
The flash flooding swept some bodies up to 5 miles downstream, Woodard said.
The family grew panicked when they saw reports of tornadoes and sought shelter in the storm drain, a 7-foot drop from their property, which, in the end, was not hit by high winds.
But suddenly the water in the drain rose quickly. The high, powerful water hurled them through the dark concrete tunnel.
The brother of one of the women who died said, "I have an emptiness in my heart because I've lost my sister."
Family and friends searched a muddy creek Monday looking for an eighth body.
Other deaths came in wake of storm
In eastern Oklahoma County, a 65-year-old man on his way to work was killed early Saturday when he drove off a washed out bridge and drowned in a creek.
Three others who died were storm chasers: Timothy Samaras, his son Paul and Carl Young.
The damage wasn't confined to Oklahoma. In Missouri, a twister left more than 10 miles of significant damage "that caused dozens and dozens of houses to be literally blown up," Gov. Jay Nixon told CNN affiliate KSDK.
No one was killed in that tornado, but three people drowned in Missouri, Nixon said.
And in Arkansas, flooding killed at least four people: a sheriff's deputy, a wildlife officer and two women they were trying to save from a deluged home.
'Memories just tossed about'
But it was Oklahoma City and its surrounding areas that sustained most of the damage.
The storm system mowed down power lines and uprooted trees, flipped big rigs on their sides and ripped off part of the terminal roof at Oklahoma City's Will Rogers World Airport, where some 1,500 area residents had taken shelter in a tunnel.
The twisters tore open brick houses like cartons, sucking out their contents and tossing them out onto lawns.