Communities along the Mississippi River and other Midwestern waterways are vigilantly eyeing, and in some cases hastily fortifying, makeshift levees to hold back floodwaters that meteorologists say could worsen or be prolonged by looming storms.
25 mm of rain was expected to fall from Oklahoma to Michigan through Tuesday, a new drenching that led the National Weather Service to heighten the forecast crest of some stretches of rivers while blunting the progress of other waterways' slow retreat.
Mark Fuchs, a National Weather Service hydrologist, said the latest dousing could be especially troubling for communities along the Illinois River, which he said is headed for record crests.
"Along the Illinois, any increase is going to be cause for alarm, adding to their uncertainty and, in some cases, misery," he said late Monday afternoon.
Last week's downpours brought on sudden flooding throughout the Midwest, and high water is blamed for at least three deaths.
Authorities in LaSalle, Ill., spent Monday searching for a woman whose van was spotted days earlier near a bridge, and a 12-year-old boy was in critical condition after being pulled from a river near Leadwood, Mo., about 100 km south of St. Louis.
The additional rain isn't welcome news in Clarksville, Mo., about 110 km north of St. Louis.
Days after bused-in prison inmates worked shoulder to shoulder with the National Guard and local volunteers to build a makeshift floodwall of sand and gravel, the barrier showed signs of strain Monday.
Crews scrambled to patch trouble spots and build a second sandbag wall to catch any water weaseling through.
In Grafton, Ill., some 65 km northeast of St. Louis, Mayor Tom Thompson said small community was holding its own against the Mississippi that by early Monday afternoon was 10 feet above flood stage.
Waters lapped against some downtown buildings, forcing shops such as Hawg Pit BBQ to clear out and detours to be put up around town — one key intersection was under 200 mm of water.
"If it gets another foot (higher), it's going to become another issue," Thompson said.
Many businesses "are kinda watching and holding their breath. Some things are going to really be close to the wire."
Elsewhere, smaller rivers caused big problems.
In Grand Rapids, Mich., the Grand River hit a record 21.85 feet, driving hundreds of people from their homes and flooding parts of downtown.
Spots south of St. Louis aren't expected to crest until late this week, and significant flooding is possible in places like Ste. Genevieve, Mo., Cape Girardeau, Mo., and Cairo, Ill.
Further downriver, flood warnings have been issued for Kentucky and Tennessee.
With files from CNN