â¿¿ Failure to control vegetation and invasive animals.

Corps specifications require that levee slopes be kept clear of plants and burrowing critters such as ground squirrels and gophers. The tunnels could weaken the walls by providing pathways for water. Thick vegetation also can conceal cracks, holes and unstable slopes. A 2010 Corps report found parts of a 2.2-mile-long Mississippi River levee in South St. Paul, Minn., dotted with trees, brush, weeds and tree stumps.

â¿¿ Building encroachment.

The Corps requires a 15-foot buffer between levees and man-made structures such as houses, fences and parking lots. But some structures abut levees or rest on top of them.

Part of an 11.5-mile levee built to protect downtown Augusta, Ga., from the Savannah River was incorporated into a park featuring a brick walkway, lighting and landscaping. A townhouse subdivision and access road were built atop the levee, as were sections of a hotel, a church hall and a science museum.

In Toledo, Ohio, some 1,500 homes, patios, stairs and other structures have been placed on the levee that runs along Lake Erie's Maumee Bay. "You name it, it's out there," said Robert Remmers, a Corps levee safety program manager who oversees Toledo's system.

Local officials say that in many cases the Corps allowed such incursions â¿¿ or didn't object to them.

Tom Robertson, a consulting engineer for Augusta, said the Corps signed off on the commercial buildings encroaching on its levee in the 1980s and 1990s, and the Corps knew about the townhouse development and did not formally approve it.

Eric Halpin, Corps special assistant for dam and levee safety, said the agency had sometimes allowed builders to take liberties that wouldn't be permitted now. The Corps doesn't expect local officials to tear down neighborhoods or hotels, but has orders from Congress to tell them about levee problems and risks, he said. In many cases, additional walls or other steps can improve safety.

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