"Every time a storm brings flooding to a large metropolitan area, there are calls to improve the levee systems that are designed to prevent flooding."

"But there's a major problem with doing that. 'We don't know where all of our levees are,' says Samantha Medlock with the Association of State Floodplain Managers.

She says the U.S. Army Corps of Engineers knows about 14,000 miles of levees in this country, because the corps is supposed to be in charge of their upkeep. 'Additionally, the Federal Emergency Management Agency, FEMA, has identified approximately 30 to 34,000 miles of levees,' says Medlock. And there could be thousands of more miles. No one can say for sure.

Why is maintaining an up-to-date levee inventory so difficult? Gerald Galloway, a professor in the department of civil and environmental engineering at the University of Maryland in College Park, says it's because of the way many levees come into being."

Joe Palca reports for NPR November 4, 2012. [2]