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Dozens of communities coast to coast are facing similar warnings as authorities re-examine the nation's outdated flood-control infrastructure.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.
Congress attempted to address the problem in the 1930s, authorizing the U.S. 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. Otherwise, nearby homeowners could face mandatory flood insurance and developers will have to comply with stricter building codes.
New maps in Washington would lead to the review of projects such as the construction of the Smithsonian's National Museum of African American History and Culture, which would be partly in the new flood zone.
"Regardless of what FEMA does, we recognize that we have to handle the water issue," said Lonnie Bunch, the museum's director.
FEMA recently agreed to rescind new flood maps for Washington after officials pledged to build an improved flood control system by November 2009. But the maps could be reinstated if the city falls short, said D.C. Office of Planning Director Harriet Tregoning.
The district is committing $2.5 million for interim fixes, but more money likely will be needed to implement a permanent solution.
"It's going to be a lot of work in a short amount of time," said Michelle Desiderio, a planner with the National Capital Planning Commission.
Desiderio wrote a report about the threat of flooding in Washington after heavy rains in 2006 damaged several downtown office buildings, closing the Internal Revenue Service headquarters for nearly six months. Though that flooding wasn't caused by a breached levee, it highlighted the vulnerability of the city's low-lying areas.
During six previous floods, officials placed sandbags on 17th Street, which cuts across the National Mall near the Washington Monument. Had there been more serious flooding, plans called for erecting an 8-foot earthen embankment with dirt taken from the grounds of the Washington Monument, said Steve Garbarino, the Corps' project manager for flood protection in the Washington region.
"That's the biggest concern -- the amount of time it would take," Garbarino said.
The promised improvements include a post-and-panel barrier, which would involve placing vertical beams in the road to support metal panels that would slide in to form a wall. Other possibilities include using an inflatable dam or raising the level of 17th Street.
A key challenge for officials is not intruding on the landscape that makes the Mall so iconic for millions of Americans.
"Much of the discussion around levees have been on the engineering side," Desiderio said. "Yet it's incumbent upon us, since it's on the National Mall, for it to be a beautiful system."