Story highlights Mississippi River at major flood level near St. Louis

18 dead, according to authorities in Oklahoma

Five people are still missing, officials said

Three people drowned in Missouri, and four people were killed by flooding in Arkansas

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.

Photos: Severe weather hits Midwest again Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Crews haul away a mangled truck on Sunday, June 2, that storm chasers Tim Samaras, his son Paul Samaras and Carl Young were using to track the tornado that hit El Reno, Oklahoma. The three men, who had devoted their lives to hunting powerful storms died in the middle of the chase. Seventeen tornadoes were reported in the Oklahoma City and St. Louis areas on Friday, May 31. Hide Caption 1 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Dustin Horn, second from left, helps his parents, JoAnn and Fred Horn, salvage items from their home on June 2, two days after a tornado ripped through El Reno, Oklahoma. Hide Caption 2 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Mark Reynolds sifts through debris to salvage personal items in El Reno on Saturday, June 1. Hide Caption 3 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Downed trees sit in the parking lot of the Canadian Valley Technology Center in El Reno on June 1. Hide Caption 4 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – An overturned airplane sits amid rubble at the Canadian Valley Technology Center in El Reno on June 1. Hide Caption 5 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Bill Thesing, who was injured in his home when a tornado hit, returns from the hospital to to salvage items from his home with the help of his friend Tammy Hicks on Saturday, June 1, in El Reno, Oklahoma. Hide Caption 6 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Yvonne Merritt stands among the things she was able to save from her home in El Reno on June 1. Hide Caption 7 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Mikie Hooper collects her belongings from her RV, which was destroyed by a tornado in El Reno, Oklahoma, on June 1. Hide Caption 8 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – High winds left sheet metal wrapped around the branches of this tree along Route 66 in El Reno on June 1. Hide Caption 9 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Two pickup trucks are pictured stuck in high water along North West 23rd Street in El Reno on June 1. Heavy rain has caused flooding in the area. Hide Caption 10 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – The tornado severed the pole supporting these power lines, leaving the remnants dangling near El Reno on June 1. Hide Caption 11 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Two men walk by a damaged OKC-West Livestock Market building near El Reno on June 1. Hide Caption 12 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – An American flag flies above the destroyed remains of a mobile home in El Reno on June 1. Hide Caption 13 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – The tail section of a plane juts out of a crumbling building at a technology school in El Reno on June 1. Hide Caption 14 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Kim Vanaken, left, consoles her sister Angela Coble, center, along with Amber Kelley while looking at what is left of Coble's house in El Reno on Saturday, June 1. Hide Caption 15 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – A man searches through the rubble of a home in El Reno on June 1. A large part of Moore, Oklahoma, was without power, as were parts of El Reno and Union City. Hide Caption 16 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Destroyed trees and cars stand along a road at the Canadian Valley Technology Center in El Reno on June 1. Hide Caption 17 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – A tornado forms from a severe thunderstorm near Meridian outside of Oklahoma City on Friday, May 31. Hide Caption 18 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Shredded trees and debris are scattered along Interstate 40 near El Reno, Oklahoma, on May 31. Hide Caption 19 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – A semi tractor-trailer damaged by a tornado lies along I-40 just east of El Reno on May 31. Hide Caption 20 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Oklahoma Highway Patrol Troopers inspect an overturned vehicle sitting alongside I-40 near El Reno on May 31. Hide Caption 21 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Storm clouds that produced a tornado roll through near El Reno on May 31. Hide Caption 22 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Lightning from a tornadic thunderstorm strikes in Cushing, Oklahoma, on May 31. Hide Caption 23 of 24 Photos: Severe weather hits Midwest again Severe weather hits Midwest again – Tornado debris hangs from a billboard sign, which was destroyed along Interstate-40 near El Reno on May 31. Hide Caption 24 of 24

JUST WATCHED Family hides from tornado, dies in drain Replay More Videos ... MUST WATCH Family hides from tornado, dies in drain 02:38

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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.

"It's a sombering thing to think about life and to see all your memories just tossed about," Kris Merritt said as he surveyed the damage at his parents' house. "Everything from your childhood on up."

Though Friday's tornadoes were not as strong as the EF-5 twister that killed 24 people on May 20, fear drove some people to flee in cars, ignoring warnings not to drive.

Some drove on the wrong side of the highway. Interstates turned into parking lots.

'We were overwhelmed'

Once the tornadoes passed, Oklahomans faced a new threat: floods.

Eight to 11 inches of rain hosed Oklahoma City, stranding motorists and hitting apartments in low-lying areas of town hard.

"We saw flooding in areas that we don't see flooding," said police Lt. Jay Barnett. "We were overwhelmed."

The National Weather Service said there was a possibility of severe storms in central Oklahoma on Tuesday night that could bring large hail, damaging wind and localized flooding.