WINFIELD — A thunderstorm Tuesday interrupted for only an hour the intense effort to save the area's last levee that hasn't been overwhelmed by the flooding Mississippi River.

Floodwater lapped at sandbags weary National Guard members spent days stacking by hand atop the sodden two-mile Pin Oak levee. Of eight main levees in Lincoln County, it was the only one that hadn't been overtopped or breached, said Andy Binder, spokesman for the county's emergency management service.

But the levee, too squishy to support heavy equipment, remained in such danger of failure that guardsmen and firefighters wore life vests on the sandbagging line.

Levees damaged in this month's flood were built by levee districts organized years ago to protect mainly farmland from floods. Few of the northeast Missouri levees, most decades old, were built to withstand more than floods that come every 15 to 20 years.

After this flood recedes, the Army Corps of Engineers will inspect the levees, which may be eligible for federal money to make repairs. Corps officials said most levees have performed well, considering their age and the strain they are under now.

For example, Binder said, the ground next to the Pin Oak levee was so soggy "it's like walking on a waterbed."

Any levee changes, such as building them higher, would require state permission, a corps spokesman said. Estimating costs of repairs or possible improvements cannot be made until after the river recedes, officials said.

In the St. Louis region, 89 federally recognized levees and flood protection systems have been built along the Mississippi and Missouri rivers and their major tributaries.

Metro East levees built to protect the Mississippi's floodplain have held, officials said. Flooded areas of Grafton and Alton are unprotected by levees. In Madison, Monroe and St. Clair counties, five levee systems protect 156,000 people and 4,000 businesses. "They're performing very well," said Alan Dooley, a corps spokesman.

None has failed or been overtopped and he said no serious problems were anticipated. Dooley said the flooding on the Mississippi downstream from the Missouri River was "roughly a 10-year event." He said the predicted 37.5-foot crest expected at St. Louis today would be only the 15th or 16th highest on record, far below the record 49.6-foot crest on Aug. 1, 1993. The Mississippi's channel widens below the Missouri, Dooley said.

Metro-East Sanitary District levees protect a 92-square-mile area in Madison and St. Clair counties, including the city of East St. Louis.

Bob Shipley, the district's executive director, said planned improvements to the region's decades-old levees are much needed but added he was confident they will hold during the current flooding.

"They're good levees," he said. "They're strong levees."

He said the multimillion-dollar renovation would add numerous relief wells to help ease concerns about levee failures caused by underseepage.

Levees can fail in a variety of ways. On Tuesday in Lincoln County, National Guard members worked to shore up a 160-foot-wide landslide on the Pin Oak levee's dry side. Guardsmen piled dirt and sandbags against the weakened area to keep it from sliding more. Authorities stockpiled sandbags in anticipation of more slides until the river drops substantially next week.

Upstream in Clarksville, the river crested Monday night at 36.7 feet, nearly 12 feet above flood stage and slightly below the 1993 record of 37.5 feet. The water slowly receded Tuesday as officials kept close watch on sandbag walls. "We're holding our own right now and we are holding our breath," said Linda Blanke, a volunteer.

As the Mississippi fell slowly in some areas, flood levels continued to rise slightly in Winfield and Foley, a Lincoln County town largely under water. The mayor of Foley said earlier this week he may ask the town's approximately 175 residents to consider relocating.

If the Pin Oak levee fails or is overtopped, the Mississippi could flood nearly 1,300 additional acres of farmland and about 100 Winfield residences. Many of the potentially affected residents had already evacuated.

For yet another day, workers struggled to contain Pin Oak leaks of floodwater driven under or through the levee. The water bubbles up in what are called sand boils. Floodwater forced through muskrat holes added to workers' frustration.

A sudden midday thunderstorm forced suspension of work by more than 100 volunteers filling sandbags in the parking lot at Winfield High School. Lightning endangered National Guard members toiling at the levee.

"We're getting our butts kicked with this weather and the sand boils," said National Guard Sgt. Brian Gardner.

The sudden storm had no major effect on the Mississippi but added a bit to the Cuivre River at Old Monroe. As a result, volunteers filled sandbags on a playground's volleyball court and stacked them next to the Old Highway 79 bridge. The town remained dry.

Also Tuesday, President George W. Bush declared Illinois a "major disaster" area and ordered aid for 13 counties affected by severe storms and flooding.

The counties are Adams, Calhoun, Clark, Coles, Crawford, Cumberland, Hancock, Henderson, Jasper, Lawrence, Mercer, Pike, and Rock Island.

The Associated Press and Jessica Bock of the Post-Dispatch contributed to this report.

tbryant@post-dispatch.com | 636-255-7212