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In the Alexander County seat of Cairo, where the Mississippi and Ohio rivers meet, the Ohio is expected to crest Sunday at 56.5 feet, more than 3 feet above major flooding stage.
Water has already gone over the top of one levee, prompting people to evacuate areas nearby, the office of Illinois Gov. Bruce Rauner said.
A second levee has "good potential" of overtopping, Patty Thompson with the Illinois Emergency Management Agency said.
Sheriff's deputies went door-to door recommending people evacuate from the towns of East Cape Girardeau and McClure.
College student Megan Gilpin told CNN affiliate WSIL
that she and her parents were taking no chances.
"They just told me pack up my clothes and if I have any pictures I want just to make sure that I have them packed and ready to go in case," she said.
In McClure, Randy Seyer helped his sister pack up her home.
"I've lived here for my whole life and that wall has never caused any trouble," he said. "Don't think it will. This is just a precaution."
Rauner activated members of the Illinois National Guard to help local authorities in the south part of the state.
Interstates reopen in Missouri
A couple of hours south in St. Louis, the swollen Mississippi River crested, as flood warnings still covered areas where 9.3 million people live in 17 states.
The Meramec, which meets the Mississippi near Arnold, crested at 47.2 feet Thursday
, about 9 feet above what is considered a major flood stage in the community of about 20,000. Sandbags saved some homes there, but about 10 were flooded, with one home getting about 7 feet of water, CNN affiliate KMOV reported.
But by Friday afternoon, conditions eased in the St. Louis area.
All portions of I-44 and Interstate 55 in the St. Louis area had reopened. Parts of those highways, including a 24-mile stretch of I-44, were closed because of flooding earlier this week.
St. Louis County's chief executive lifted a state of emergency Friday, CNN affiliate KTVI reported
. Residents of Valley Park who evacuated will be allowed back into some sections of the city, KTVI said
A week ago, bad weather spread across the country, starting with a spate of tornadoes. By the weekend, the Midwest was flooded. The clouds have long cleared out, and no more rain is expected in the Mississippi River basin until late next week. But runoff has swelled rivers, and in areas south of St. Louis, they have yet to crest.
Water has submerged neighborhoods, schools and shopping centers, and carried off whole houses. The storms have killed 15 people in Missouri, officials have said.
In Illinois, the death toll grew to nine with the discovery of an 18-year-old's body, according to Christian County Coroner Amy Calvert Winans. Search teams traced pings from a cell phone to locate a pickup truck in which the teen was believed to have been riding, the sheriff's office said. A second teenager last seen in the truck is missing.
Many of those who died drove into high, rushing water and were carried away in their cars.
Beer company distributes water
On Thursday, the flooding breached a St. Louis-area wastewater treatment plant near the Meramec River -- the second such breach there in a week -- sending untreated waste into the river. Missouri American Water spokeswoman Ann Dettmer said the water in homes and businesses in the area still is safe to use.
"We are seeing higher levels of bacteria in the river water ... but we're managing it," she told CNN.
Other plants are treating the river water, she said. "We are meeting state and federal standards. They don't have to worry about their drinking water."
Anheuser-Busch has provided or 84,672 cans of drinking water for use by St. Louis-area and Oklahoma residents affected by flooding and storms, the brewery announced
River crest records
At its peak, the Mississippi should be at its highest level ever, Missouri Gov. Jay Nixon has said, beating the highest level of the great flood of 1993, the benchmark for flood catastrophes in the region.
As the runoff from the deluges that hit around Christmas continues gathering in rivers that empty into the Mississippi River, downstream, gauges are predicting flooding in areas farther south as deep torrents roll that way -- in Arkansas, Tennessee, Mississippi and Louisiana, in early January.
Communities along the Mississippi River in southern Illinois and southern Missouri are expected to see the river rise to record levels into early next week.
Hundreds of miles to the south, in Baton Rouge, Louisiana, the river is expected to crest above flood stage on January 19.