Across America, earthen flood levees protect big cities and small towns, wealthy suburbs and rich farmland. But the Army Corps of Engineers, the federal agency that oversees levees, lacks an inventory of thousands of them and has no idea of their condition, the corps' chief levee expert told The Associated Press.

The uncertainty, amid an unusually wet spring that has already caused significant flooding across many states, is creating worry even within the corps.

"We have to get our arms around this issue and understand how many levees there are in the country, who's watching over them, what populations and properties are behind them," Eric Halpin, the corps' special assistant for dam and levee safety, said in an interview last month. "What is the risk posed to the public?"

Critics are troubled that the government doesn't know the answer.

Robert Bea, a University of California at Berkeley levee expert, said many levees are old, with rusting infrastructure and built to protect against relatively common floods _ not the big ones like the Great Flood of 1993, when 1,100 levees were broken or had water spill over their tops.

"Once they do get an inventory," Bea said, "I think we're not going to like what we find."

Residents along the Mississippi River have been fighting floods with levees since the 19th century. After a devastating 1927 flood, Congress got involved, approving construction of levees and reservoirs along the Mississippi and Missouri river basins.

Today, about 2,000 levees are either operated by the corps or by local entities in partnership with the corps, generally protecting major population areas such as St. Louis and New Orleans.

Thousands of others _ no one is sure how many _ are privately owned, operated and maintained. The majority of those are "farm" levees keeping water out of fields, but some protect populated areas, industries and businesses.

For example, flooding in March breached private levees near the southeastern Missouri towns of Dutchtown and Poplar Bluff.

In 2006, prompted in part by the devastation wrought by Hurricane Katrina in New Orleans the year before, Congress provided funding for the corps to inventory the levees it maintains or helps fund. That initial inventory is complete, Halpin said.

Some of what was found was troubling. For example, corps levees in Missouri and Illinois that are supposed to protect against a 500-year flood fall short of even 100-year protection, said Col. Lewis Setliff III, commander of the corps district in St. Louis. Getting those nine levees up to standard would cost an estimated $200 million.

Last year, Congress passed the National Levee Safety Act, which for the first time directed the corps to inventory all private levees. But so far, Congress hasn't provided funding and won't likely do so until 2009 at the earliest.

Still, the project is long overdue, said Susan Gilson, executive director of the Washington-based National Association of Flood & Stormwater Management Agencies.

"No. 1, we have to identify all the levees," Gilson said. "We need to identify where there are problems with the levees. Then the next stage will be repairs."

Flooding in March killed nearly two dozen people and damaged or destroyed thousands of homes across a swath of Midwestern states. With the ground saturated and rivers still running high, some worry that more flooding is on the way.

Just across the Mississippi River from St. Louis is the Wood River levee in Illinois, which protects a ConocoPhillips refinery. Flooding there could spell an environmental and economic disaster.

Water seeped through the levee in 1993, but it held. Levee district commissioner Leroy Emerick worries that the next big test might not go as well.

Residents of the tony St. Louis suburb of Chesterfield, Mo., already know what happens if the Monarch Levee breaks.

It happened in 1993, sending the Missouri River surging into the region known as the Chesterfield Valley. Within hours, muddy water reached the rooftop at the popular Annie Gunn's restaurant _ seven miles from the river.

In those days, Annie Gunn's was among a few businesses in the valley. Today, the area is home to dozens of big box stores, shopping centers and high-end restaurants.

The development came after the Monarch levee was rebuilt to protect against a 500-year flood, meaning an area has a 1-in-500 chance of being flooded to a certain level in any given year. But David Human, a lawyer for the Monarch district, said there are still small sections of the levee that fall short.

"By fall, we expect 98 percent of the levee system will be at the 500-year level of protection. But guess what? That's not 100 percent," Human said.

Flooding in March nearly wiped out tiny Dutchtown, a community of 99 residents in southeast Missouri. Several waterways _ the Castor and Whitewater rivers and Hubble Creek _ flow into what's known as the diversion channel there. Torrential rain caused a quick rise in water that tore through a small, private levee.

Weeks after the flood, residents are still ripping out water-soaked carpet and ruined furniture, cleaning debris from their yards, and power-washing mud caked from cars and siding.

"It was so much water at one time, and the levee couldn't handle it," resident Robert Reed, 72, said.

Halpin knows that another major flood would be more than many levees could handle.

"It's not a question of if it will happen. It's a question of when and where it will happen," he said. "There are a lot of vulnerable spots in this country."

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AP Southern Illinois correspondent Jim Suhr contributed to this report.