At least three deaths have been blamed on the flooding since Sunday.
In southeast Missouri, authorities said a man in Bollinger County drowned when his pickup truck was swept away by floodwaters Monday. He had managed to climb out of the truck but got tangled among limbs and other debris.
To the west, in Iron County, another man died Sunday when high water swept him from a tree that he had climbed to escape the flood, said Sheriff Allen Mathes.
At St. Louis, the Mississippi River was forecast to reach 9 feet above flood stage by Wednesday, and water already was creeping up the riverfront steps that lead to the Gateway Arch.
"If you've got interests along the river, you better pay attention," said Dale Bechtold, a National Weather Service forecaster. "We're going to have problems throughout the week."
In Ohio, one man drowned Sunday night after his raft went over a small dam on the swollen Chagrin River near Willoughby, and his companion was still missing Monday.
"I saw them go over and get thrown out of the raft," Arney Price said. "There were a lot of logs, some bigger than I am, just beating on them."
Rain fell at more than an inch an hour in parts of Indiana early Monday and flood warnings were posted for streams including the White River, which was already above flood stage on the north side of Indianapolis.
In Ellettsville, Ind., Jack's Defeat Creek flooded the police and fire department offices.
"We have 10 to 12 inches of water standing in the police department," Detective Marshal Tony Bowlen said. "This matches about the worst flooding we've had."
Four west-central Indiana counties declared a state of emergency, and the Weather Service said flooding in some areas could be the worst since major flooding in late December 1990.
In central and southern Illinois, flood alerts were posted, roads remained closed and volunteers stacked sandbags to stave off swollen rivers Monday. Although storms had tapered off, problems remained because the ground was saturated by three weeks of periodic rainfall, officials said.
The Sangamon River was expected to crest at 30.5 feet Monday at Riverton and Thursday at Petersburg, the second-highest level on record for both locations, said Ed Holicky, a meteorologist in the Weather Service in Lincoln.