CAHOKIA, Ill.  Even as many found cheer that floodwaters along the Mississippi River here would fall well short of what had been predicted, residents and the authorities in this town discovered a stark reminder of what might lie ahead: a sand boil on the aged levee that protects the town, a telltale sign that the swollen river had begun eroding the structure from beneath.

“I didn’t even know what a sand boil was,” Mayor Frank Bergman said as he surveyed the waterlogged levee, referring to the mix of sand and water that bubbles up from the ground. “But it’s clearly time to upgrade our levees and keep our system safe.”

The flooding that has claimed 24 lives and forced thousands to evacuate across six states continued Sunday, as the Mississippi crested in the northern Missouri town of Canton. Just upriver from St. Louis, Winfield, Mo., and Grafton, Ill., continued battling rising waters. Forecasters from the National Weather Service predicted that floodwaters would begin receding in the St. Louis area later this week.

As the river at nearby St. Louis leveled out at around 12 feet below the record set during the 1993 flood and about two feet below what had been predicted for the weekend, investigators had identified and contained several more boils on the levee, and the marshland that sits just behind it had crept to within 100 yards of the village hall.