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.

The results could be catastrophic and could swamp portions of downtown in up to 10 feet of water and cause $200 million in damages, federal officials said.

Dozens of communities coast to coast are facing similar warnings as authorities reexamine the nation's outdated flood-control infrastructure.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."

The problem has heightened urgency in Washington, 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, the National Gallery of Art, the Smithsonian's National Air and Space Museum, and Justice Department headquarters.

Built on reclaimed swamp, 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.