NEW ORLEANS — Isaac’s whistling winds lashed this city and the storm dumped nearly a foot of rain on its darkened and desolate streets, but the system of levee pumps, walls and gates appeared to withstand one of the stiffest challenges yet. To the north and south, though, people had to be evacuated or rescued as Isaac lingered over Louisiana.

The rain fell almost constantly for more than a day, flooding neighborhoods in a rural part of the state and as far away as Mississippi. Officials had to respond quickly because the waters were rising fast — even as Isaac meandered slowly northward Thursday on a path toward Arkansas.

President Barack Obama declared federal emergencies in Louisiana and Mississippi late Wednesday, according to a statement from the White House. The disaster declarations free up federal aid for affected areas.

Along the shores of Lake Ponchartrain, officials sent scores of buses and dozens of high-water vehicles to help evacuate about 3,000 people as rising waters lapped against houses and left cars stranded. Floodwaters rose waist-high in some neighborhoods, and the Louisiana National Guard was working with sheriff’s deputies to rescue people stranded in their homes.

The floodwaters “were shockingly fast-rising, from what I understand from talking to people,” Lt. Gov. Jay Dardenne said. “It caught everybody by surprise.”

Isaac arrived seven years after Hurricane Katrina and passed slightly to the west of New Orleans, where the city’s fortified levee system easily handled the assault.

“Unfortunately, that’s not been the case for low-lying areas outside the federal system, in particular lower Jefferson and Plaquemines parishes,” said Louisiana Democratic US. Sen. Mary Landrieu. “Hurricane Isaac has reinforced for us once again just how vulnerable these critical areas are. We must re-engage the Corps of Engineers on this.”

Jerry Larpenter, sheriff in nearby Terrebonne Parish, said he thinks the storm’s impact took many by surprise.

“I think a lot of people were caught with their pants down,” he said. “This storm was never predicted right since it entered the Gulf. It was supposed to go to Florida, Panama City, Biloxi, New Orleans. We hope it loses its punch once it comes in all the way.”

New Orleans’ biggest problems seemed to be downed power lines, scattered tree limbs and minor flooding. One person was reported killed, compared with 1,800 deaths from Katrina in Louisiana and Mississippi. And police reported few problems with looting. New Orleans Mayor Mitch Landrieu ordered a dusk-to-dawn curfew.

But in Plaquemines Parish, a sparsely populated area south of the city that is outside the federal levee system, dozens of people were stranded in flooded coastal areas and had to be rescued. The storm pushed water over an 18-mile levee and put so much pressure on it that authorities planned to intentionally puncture the floodwall to relieve the strain.

Officials rushed to evacuate more than 100 nursing home residents from Plaquemines Parish, an area with a reputation for weathering storms and perhaps the hardest hit by Isaac. In this hardscrabble, mostly rural parish, even the sick and elderly are hardened storm veterans.

“I don’t think we had to evacuate to begin with,” said Romaine Dahl, 59, as he sat in a wheelchair. “The weather was a hell of a lot worse last night than it is now. And I got an idea that after all this is said and done they’re going to say everything is over with, go on back home.”

Other residents in the Riverbend Nursing and Rehabilitation Center were loaded into ambulances and taken to a nearby naval station. Residents had their names and birth dates attached to their shirts.

Josephine King, 84, handled the move well, waiting in a wheelchair. “I’m feeling good,” she said.

By midafternoon Wednesday, Isaac had been downgraded to a tropical storm. The Louisiana National Guard ceased rescue operations in Plaquemines Parish, saying it felt confident it had gotten everyone out. There were no serious injuries. National Guard spokesman Capt. Lance Cagnolatti said guardsmen would stay in the area over the coming days to help.

By early Thursday, Isaac’s maximum sustained winds had decreased to 50 mph. Even at its strongest, the storm was far weaker than Hurricane Katrina, which crippled New Orleans in 2005. Because Isaac’s coiled bands of rain and wind were moving at only 5 mph — about the pace of a brisk walk — the threat of storm surges and flooding was expected to linger Thursday as the immense comma-shaped system crawled across Louisiana.

The storm knocked out power to as many as 700,000 people, stripped branches off trees and flattened fields of sugar cane so completely that they looked as if a tank had driven over them.

In coastal Mississippi, officials used small motorboats Wednesday to rescue at least two dozen people from a neighborhood Isaac flooded in Pearlington. In addition, the National Weather Service said there were reports of at least three possible tornadoes touching down in coastal counties. No injuries were reported.

None of the reports had been confirmed. Until the weather clears, there is no way for survey teams to assess the area to determine whether damage was done by tornadoes or straight-line winds, said NWS Meteorologist Shawn O’Neil.

Back in New Orleans, the storm canceled remembrance ceremonies for those killed by Katrina. Since that catastrophe, the city’s levee system has been bolstered by $14 billion in federal repairs and improvements. The bigger, stronger levees were tested for the first time by Hurricane Gustav in 2008.

Isaac came ashore late Tuesday as a Category 1 hurricane, with 80 mph winds near the mouth of the Mississippi River. It drove a wall of water nearly 11 feet high inland.

In Vermilion Parish, a 36-year-old man died after falling 18 feet from a tree while helping friends move a vehicle ahead of the storm. Deputies did not know why he climbed the tree.

The storm stalled for several hours before resuming a slow trek inland, and forecasters said that was in keeping with its erratic history. The slow motion over land means Isaac could be a major soaker, dumping up to 20 inches of rain in some areas. New Orleans reported at about 10 inches in some places as rain continued to fall late Wednesday.

As hard wind and heavy rain pelted Melba Leggett-Barnes’ home in the Lower 9th Ward, an area leveled during Katrina, she felt more secure than she did seven years ago.

“I have a hurricane house this time,” said Barnes, who has been living in her newly rebuilt home since 2008. She and her husband, Baxter Barnes, were among the first to get a home through Brad Pitt’s Make It Right program.

Her yellow house with a large porch and iron trellis was taking a beating but holding strong.

“I don’t have power, but I’m all right,” said Barnes, a cafeteria worker for the New Orleans school system.

Forecasters expected Isaac to move farther inland over the next several days, dumping rain on drought-stricken states across the nation’s midsection before finally breaking up over the weekend. The storm was expected to weaken to a tropical depression Thursday, according to the U.S. National Hurricane Center in Miami.