The river that drains much of the flood-soaked United States is still running higher than normal, menacing New Orleans in multiple ways just as the hurricane season intensifies.

For months now, a massive volume of water has been pushing against the levees keeping a city mostly below sea level from being inundated. The Mississippi River ran past New Orleans at more than 11 feet (3.4 meters) above sea level for more than 200 days, dropping below that height only Monday.

“The big threat is water getting through or underneath,” said Nicholas Pinter, an expert on river dynamics and flood risks who’s studied levee breaches across the nation. “The longer the duration, the greater the threat.”

Locals walked up levees from Baton Rouge to New Orleans to see the river for themselves as Tropical Storm Barry briefly menaced Louisiana last month, but the real damage runs underneath, experts say: All that rushing floodwater can scour levees along their foundations, causing damage in places that can’t easily be seen.

“That ultimately could undermine the levee as well and cause a breech or a failure,” said Cassandra Rutherford, assistant professor of geotechnical engineering at Iowa State University.