Even as Hurricane Katrina devastated New Orleans, the Mississippi River's levees held up when those in other parts of the city did not.

But as Tropical Storm Barry, which could become Hurricane Barry, threatened New Orleans with torrential rains that will test the city's flood defenses this weekend, the height of the city's river levees was the U.S. Army Corps of Engineers' greatest concern, spokesman Ricky Boyett said Thursday.

A hurricane warning was in effect along the Louisiana coast, with forecasters predicting landfall as a possible category 1 hurricane by early Saturday.

The danger to New Orleans — bound by the Mississippi River on its south side, Lake Pontchartrain on its north side and tributaries leading into the nearby Gulf of Mexico on the east — is threefold: storm surges from the sea, rain from the sky and water from the rising river if the levees fail.

While the Corps wasn't expecting the already-swollen river to spill over into the city, the threat from Barry was real with a storm that was forecast to dump 10 to 20 inches (25 to 50 centimeters) of rain on New Orleans through Sunday, with isolated areas getting 25 inches (64 centimeters).

The river was expected to crest at about 19 feet (5.8 meters) on Saturday in New Orleans, where the levees protecting it from the water range from about 20 to 25 feet (6 to 7.5 meters) in height, said Jeff Graschel, a hydrologist with the National Weather Service.

The river's levees haven't been overtopped in New Orleans since the early 1920s. but state officials warned that a change in the storm's direction or intensity could change that.

National Guard troops and rescue crews were stationed around the state with boats and high-water vehicles. Helicopters were also on standby, and supplies including drinking water and blankets were ready for distribution, the Guard said.

President Donald Trump on Thursday night declared a federal declaration of emergency for Louisiana, authorizing the Department of Homeland Security and Federal Emergency Management Agency to coordinate all disaster relief efforts.

As of early Friday, Barry was about 80 miles (130 kilometers) south of the mouth of the Mississippi, with winds around 50 mph (80 kph). Tracking forecasts showed the brunt of the storm blowing into the Louisiana delta west of New Orleans on a path that could continue toward Chicago, swelling the Mississippi River basin with water that must eventually flow south again.

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