NEW ORLEANS — 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.