Hurricane Isaac sidestepped New Orleans on Wednesday, sending the worst of its howling wind and heavy rain into a cluster of rural fishing villages that had few defences against the slow-moving storm that could bring days of unending rain.

Isaac arrived exactly 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.

The city's biggest problems seemed to be downed power lines, scattered tree limbs and minor flooding. Just one person was reported killed, compared with 1,800 deaths from Katrina in Louisiana and Mississippi. And police reported few problems with looting. Mayor Mitch Landrieu ordered a dusk-to-dawn curfew just to be sure.

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 27-kilometre levee and put so much pressure on it that authorities planned to intentionally puncture the floodwall to relieve the strain.

"I'm getting text messages from all over asking for help," said Joshua Brockhaus, an electrician who was rescuing neighbours in his boat. "I'm dropping my dogs off, and I'm going back out there."

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A man crosses New Orleans' Canal Street early Wednesday in the wind and rain from Hurricane Isaac. (David J. Phillip/Associated Press)

By midafternoon, Isaac had been downgraded to a tropical storm. The Louisiana National Guard wrapped up rescue operations in Plaquemines Parish, saying they felt confident they had got everyone out and there were no serious injuries, but would stay in the area over the coming days to help, National Guard spokesman Capt. Lance Cagnolatti said.

Slow-moving storm

Isaac's maximum sustained winds had decreased to 100 km/h by Wednesday evening. Even at its strongest, Isaac 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 10 km/h — about the pace of a brisk walk — the threat of storm surges and flooding was expected to last into a second night as the immense comma-shaped system crawled across Louisiana.

Plaquemines Parish ordered a mandatory evacuation for the west bank of the Mississippi below Belle Chasse because of worries about a storm surge. The order affected about 3,000 people, including a nursing home with 112 residents. In Jefferson Parish, the sheriff ordered a dusk-to-dawn curfew.

West of New Orleans in St. John the Baptist Parish, flooding from Isaac forced 1,500 people to evacuate. And Gov. Bobby Jindal's office said thousands in the area needed to evacuate. Rising water closed off all main thoroughfares into the parish, and in many areas, water lapped up against houses and left cars stranded.

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

Back in New Orleans, the storm cancelled 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.

Army Corps of Engineers spokeswoman Rachel Rodi said the flood-control measures were working "as intended" during Isaac.

Wall of water

Isaac came ashore late Tuesday as a Category 1 hurricane, with 130 km/h winds near the mouth of the Mississippi River. It drove a wall of water more than three metres high inland.

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 500 millimetres of rain in some areas. But every system is different.

Slashing rain and wind gusts up to 100 mph buffeted New Orleans skyscrapers.

In the French Quarter near Bourbon Street, Jimmy Maiuri was shooting video from outside his second-floor apartment. Maiuri, who fled from Katrina at the last minute, stayed behind this time with no regrets. He was amazed at the storm's timing.

"It's definitely not one to take lightly, but it's not Katrina," he said. "No one is going to forget Aug. 29, forever — not here at least."

Forecasters expected Isaac to move 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.