Isaac, a vast storm that tore through southeastern Louisiana on the seventh anniversary of Hurricane Katrina, weakened from a hurricane to a tropical storm after pushing floodwaters over a levee in marshy Plaquemines Parish and forcing the dramatic rescue of dozens of stranded residents.

Many of the rescues were conducted Wednesday by local residents, who scrambled to help their neighbors in private boats while high winds blocked government crews from crossing the Mississippi River. The citizen rescuers were being hailed as heroes in remote Plaquemines, a region known for its shrimp boats and citrus orchards that was hit hard by Katrina in 2005.

Even as ferocious winds were ripping through the area, parish officials took the unusual step of ordering the evacuation of 3,000 people on the west bank because of concerns about possible storm surges topping a levee there. Evacuation orders are usually issued before storms, and it’s unclear how many — if any — of the residents who were ordered to leave braved the difficult conditions outside and fled. Louisiana National Guard troops also were dispatched to the west bank late Wednesday to move more than 100 residents from a nursing home to a more secure shelter.

Isaac, which had maximum sustained winds of about 80 mph, knocked out power to more than 600,000 homes and businesses across the state, shredded roofs in St. Bernard Parish — one of the hardest-hit areas during Katrina — and toppled trees and a smattering of streetlights in New Orleans. But Isaac appeared likely to spare New Orleans from the devastation wrought by the much more powerful Katrina. Still, armed National Guard troops patrolled the city’s streets, and Mayor Mitch Landrieu imposed a dusk-to-dawn curfew, warning that looters would face mandatory three-year sentences.

“If you loot, you get an orange suit,” Landrieu said.

Isaac, which made landfall Tuesday evening near the mouth of the Mississippi, moved sluggishly Wednesday on a wobbly northwestern route, at times becoming almost stationary and dumping large amounts of rain in isolated areas before moving on. It was progressing so slowly that Louisiana Gov. Bobby Jindal, who skipped the Republican National Convention in Tampa to oversee the state’s response, said the storm could remain in the state as long as Friday morning, and utility officials were warning that it could take days to restore power. The storm is forecast to continue to weaken as it moves through Louisiana and enters Arkansas and the Midwest, where it is expected to provide some relief for drought-hit fields.

In New Orleans, the intricate network of levees, pumps and floodgates built since Katrina was “performing the way it was designed to,” said Rene Poche, a spokesman for the U.S. Army Corps of Engineers. Huge floodgates were clamped shut to prevent storm surges from entering the city, particularly on its vulnerable eastern side.

A community hit hard

The storm’s most dramatic impact was felt in the tiny community of Braithwaite, which sits in a low-slung section of Plaquemines, 20 miles southeast of New Orleans and outside the massive ring of federally maintained levees constructed or upgraded with $14.5 billion allocated by Congress after Katrina. Isaac’s Category 1 winds pushed a 12-foot surge of water over a small, locally maintained levee that was partially under construction, parish officials said. As many as 800 homes in the parish may have been damaged, according to a preliminary estimate announced by Jindal.

“It’s worse than Katrina in that area,” Plaquemines Parish council member Robert Griffin said in an interview.

To rescue dozens of neighbors, residents launched boats into floodwaters that reached as high as 10 feet. Griffin, who represents the flooded area, said residents had been warned for days to leave. He estimated that 1,900 of the area’s approximately 2,000 residents evacuated. The 100 or so who remained did so at great personal risk, he said. “That’s been puzzling me all day,” Griffin said. “Why did they stay?”

Jindal and Plaquemines Parish President Billy Nungesser said late Wednesday that officials plan to punch a hole in a portion of a levee in Plaquemines to release floodwaters. In previous storms, the parish has been able to drain floodwaters in three days by cutting holes in a levee, while it could take two weeks to do so by pumping out the water, Nungesser said.

The overtopped levee has been the subject of furious debate, said Griffin and another council member, Kirk Lepine. Parish officials have been pressing to strengthen and raise the levee, but Griffin said they have grown frustrated with what he described as a slow permitting process overseen by the Army Corps of Engineers, which has generally been praised for expediting permits for flood projects in New Orleans.

“That’s the federal government — they’re slow about everything,” he said, characterizing the delays as “filibustering” by federal officials.

Lengthy response effort

On a call Wednesday afternoon, National Hurricane Center Director Rick Knabb cautioned that even as Isaac once again becomes a tropical storm and leaves New Orleans, plenty of danger lies ahead, including flash floods and tornadoes.

“For some folks in the path of this, the event — in terms of the weather and the impacts — hasn’t even begun,” Knabb said. “We have to remember that it’s not just a coastal event or even a coastal region event. . . . There’s some heavy rain on the way, and it’s going to last quite a while,” he said. “This is not to be taken lightly anywhere in Louisiana, Mississippi and Arkansas.”

The federal Bureau of Safety and Environmental Enforcement reported Wednesday that oil and gas industry workers had been evacuated from 505 production platforms and 50 rigs in the Gulf of Mexico ahead of Isaac’s arrival. Production platforms are the structures located offshore from which oil and natural gas are produced. Unlike drilling rigs, which typically move from location to location, production facilities remain in one place.

The agency said that after Isaac has passed, officials will inspect the facilities for damage. Only after such an infrastructure check is completed will authorities give companies a green light to resume production, the bureau said.

In Washington, Federal Emergency Management Agency officials moved to 12-hour shifts as part of the agency’s elevated operations status, instituted because of the storm. Working in a windowless conference room ringed by flat-screen television screens, officials briefed FEMA Administrator Craig Fugate as he rode into Louisiana from Mississippi. After hearing about the evacuation of more than 700 nursing home residents, he urged his colleagues to prepare for a lengthy response effort.

“We need to be as fast as we can, but it’s going to be a frustrating day, I’m afraid, because the storm’s going to be moving slowly,” he said. “Conditions will not improve, and it’s going to be a slow response.”

Isaac was powerful enough to hurl a large concrete bench and its concrete base into the street along the New Orleans lakefront, a scenic spot where generations of residents have gone to gaze at the enormous Lake Pontchartrain. But the lakefront neighborhood, which filled with putrid floodwaters during Katrina, was spared.

In the Lower Ninth Ward — a poor, largely African American neighborhood where hundreds of homes were obliterated by floodwaters that poured through a levee breach during Katrina — the newly reinforced levee held strong. Irvin Walker’s newly built house shook in the wind “like it was about to fall,” he said. But it didn’t.

Dennis reported from Washington. Ed O’Keefe in Washington contributed to this report.