WINFIELD, Missouri - The weakest spot left along the swollen Mississippi River may be the Pin Oak levee, a barrier so tenuous that soil slides down its slope.

Only National Guard soldiers and firefighters in life vests are allowed to stack sandbags, because volunteers and heavy equipment could sink. A single muskrat recently created a geyser of riverwater by digging into the berm.

But the earthen levee is all that's still protecting 100 houses, a city park, several businesses and 3,000 acres of agricultural land in east Winfield, one of the last towns where the upper Mississippi was expected to crest.

For days, emergency management officials in Lincoln County have focused on the 2 1/2-mile-long levee about 45 miles northwest of St. Louis. A storm with thunder and lightning Tuesday was only the latest impediment to the desperate attempts to shore up the Pin Oak.

"This storm is not a good thing," said Jeff Stamper, a structural engineer with the U.S. Army Corps of Engineers. "It pulled everyone off. You can work on a levee in the rain, but not in lightning."

While levees in the nearby communities of Elsberry and Old Monroe held strong, an urgent call went out this week for volunteers to fill up to 50,000 sandbags here.

The Mississippi was expected to finally crest at Winfield sometime late Wednesday, and to flow at its high-water mark - more than 11 feet above flood stage - for several more days. A disturbance as minor as a passing boat could lead to disaster.

"A 2-inch wake could be the difference between saving the levee and catastrophic failure," said Andy Binder, Lincoln County emergency management spokesman.

Overnight and into Tuesday morning, the porous and heavy soil inside the levee created what's called a slide, or a run of soil sinking down the slope of the levee's dry side.

At first light Tuesday, workers used heavy sheet plastic and about 5,000 sandbags to create a 15-by-160 foot "mattress" to add weight and pressure to the weak spot.

"Do we expect more slides? Absolutely," Binder said.

Officials spent nearly six hours choking off the leak caused by a muskrat burrowing in the soft ground early Monday.

Several miles down the river, the Elm Point levee in St. Charles succumbed early Tuesday. But the breach there swamped only a soccer field and a sod farm, and St. Charles Assistant Fire Chief Rich Oney said residents of a nearby mobile home park would likely stay dry.

The flooding from the Elm Point levee break will come close to only two homes, he said, and the residents of both have decided to stay put. There is no threat of flooding along nearby Missouri 370, a major highway running through suburban St. Charles County.

A total of 35 levees have overtopped during the Midwest flooding, and seven of them had been federally designed and constructed, said Ed Hecker, chief of the office of homeland security for the U.S. Army Corps of Engineers. He said the nation's levee system wasn't designed to hold back such extraordinarily high flood waters, particularly in rural communities such as Lincoln County.

"This system pretty much performed as designed," Hecker said.

The river continued to recede Tuesday from the Iowa line down through the lock and dam at Saverton, about 90 miles north of St. Louis. The river had dropped a foot Tuesday morning at Canton following a Sunday crest of 13 feet above flood stage.

"We need to stay vigilant until we're below 24.5 feet - hopefully by the end of this week," said Canton emergency management director Jeff McReynolds. "We're going to be on a yo-yo with the river for a while. It's going to be a long summer."

Rain in the St. Louis area Tuesday wasn't expected to have any effect on the river level. But forecasters are nervous about storms expected to hit northeast Missouri and central Iowa on Wednesday and Thursday, said National Weather Service meteorologist Mark Fuchs.

"It'll keep water in the system. That's for sure," Fuchs said. "That could turn the river around. That could lead to higher crests."

Pending the rains, the weather service said the river wouldn't begin to recede at St. Louis - where there is flooding, but none significant - until Thursday night. Forecasters said the last point on the river to finish cresting would be near Chester, Ill., about 80 miles south of St. Louis, sometime Friday.

Also Tuesday, the governors of Illinois, Indiana, Iowa and Wisconsin asked President Bush to allow the federal government to cover 90 percent of disaster-related costs incurred by state and local governments. The federal government usually covers 75 percent of such costs after the president declares a disaster.

"Our states have suffered recent multiple disasters that have placed enormous stress on state and local governments," said Wisconsin Gov. Jim Doyle in a statement. "Reducing our share of the flooding assistance costs would greatly alleviate the social and economic impact on our families and communities that are suffering."

Associated Press writers Betsy Taylor, Jim Salter and Chris Leonard contributed to this report from St. Louis.