Tourists visit the World War II memorial and the Lincoln Monument, background, in April 2007 on the National Mall in Washington. To the left and right of the reflecting pool are levees meant to hold back water in case of flooding. Built on swamp land with almost no natural barriers against high water, the District of Columbia is under the constant threat of flooding.

Washington ? Strolling beside the Reflecting Pool with the Lincoln Memorial in the distance, it’s easy to overlook a gentle rise in the landscape a few yards to the north.

The small berm is part of an inconspicuous levee system designed to protect world-famous museums, the National Archives and federal office buildings from flooding.

But the nearly 70-year-old levee is at risk of failing during a major storm – a catastrophe that could swamp portions of downtown in up to 10 feet of water and cause $200 million in damages, according to federal officials.

Dozens of communities coast to coast are facing similar warnings as authorities re-examine the nation’s outdated flood-control infrastructure.

“We have built a series of structures and walked away from them historically,” said Leonard Shabman, a water resources expert with the think tank Resources for the Future. “If you’ve got potholes in the road, people go out and fix them; that’s not the case with levees.”

In Washington, the problem has received heightened urgency given the threat to important real estate in a flood zone that begins near the base of the Washington Monument and stretches to a neighborhood south of the Capitol. The area includes the White House visitor center, National Gallery of Art, the Smithsonian’s National Air and Space Museum and Justice Department headquarters.

Built on reclaimed swamp land with few natural barriers against high water, Washington always has been vulnerable to flooding. And experts say the threat is worsening.

The sea level has risen about 2 feet since the city was founded more than 200 years ago at the confluence of the Anacostia and Potomac rivers, said James Titus, a sea-level expert at the Environmental Protection Agency. In addition, urban development has increased rainwater runoff, and global warming is believed to have fueled more potent storms.

Congress attempted to address the problem in the 1930s, authorizing the Army Corps of Engineers to build a levee between the Lincoln Memorial and Washington Monument with dirt removed for the Reflecting Pool. But gaps were left where several roads slice through the barrier.

For years, using sandbags to plug the holes during the threat of high water was considered acceptable. But the Corps toughened its standards after Hurricane Katrina breached New Orleans’ levees in 2005.

In 2006, the Corps cited deficiencies with 122 of the 2,000 levees in a federal rehabilitation program, pointing out problems such as erosion and the movement of floodwalls. Three of the levees were in Washington, including the Potomac Park levee near the Reflecting Pool.

Since that report, Congress has authorized the Corps to inventory all private levees to better understand the nation’s flood risks. Officials say the exact number of those levees is unknown.

Federal officials estimate it will cost billions to make sure levees are strong enough to withstand a 100-year flood.