New Orleans' flood defences appeared to withstand Hurricane Isaac, but thousands of people to the north and south of the city had to be evacuated or rescued as the storm lingered over the US Gulf Coast with whistling winds and constant rain. At least two deaths were reported.
The storm flooded neighbourhoods in a rural part of the state and in neighbouring Mississippi. The waters were rising fast, even as Isaac, now a tropical storm, meandered slowly northward on a path toward neighbouring Arkansas.
President Barack Obama declared federal emergencies in Louisiana and Mississippi, according to a statement from the White House, freeing up federal aid for affected areas.
Along the shores of Lake Ponchartrain just north of New Orleans, officials sent scores of vehicles to help evacuate about 3,000 people as rising waters lapped against houses. Floodwaters rose waist-high in some neighbourhoods, and authorities worked 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 exactly seven years after the devastating Hurricane Katrina and passed slightly to the west of New Orleans, where the city's newly fortified levee system, helped by £8.8 billion in federal repairs, easily handled the assault. But low-lying areas outside the city were swamped.
"Hurricane Isaac has reinforced for us once again just how vulnerable these critical areas are," Democratic US Sen Mary Landrieu said.
One person was reported killed in New Orleans, 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.
A man was killed this morning when a tree fell on his truck in Mississippi. Authorities said Isaac was causing heavy rain and strong winds at the time. They did not release the man's name.
In Plaquemines Parish, a sparsely populated area south of the city that is outside the improved 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.
By mid-afternoon yesterday, Isaac had been downgraded to a tropical storm and the Louisiana National Guard ceased rescue operations in Plaquemines Parish, saying it felt confident it had gotten everyone out. There were no serious injuries.
Officials rushed to evacuate more than 100 nursing home residents. In this 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."
By this morning, Isaac's maximum sustained winds had decreased to 45mph and the National Hurricane Centre said it was expected to become a tropical depression by tonight, meaning its top sustained winds would drop below 39mph.
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.
Because Isaac's coiled bands of rain and wind were moving at only 5mph - about the pace of a brisk walk - the threat of storm surges and flooding was expected to linger today.
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 yesterday to rescue at least two dozen people from a neighbourhood flooded in Pearlington.
Back in New Orleans, the storm cancelled remembrance ceremonies for those killed by Katrina.
As hard wind and heavy rain pelted Melba Leggett-Barnes' home in the Lower 9th Ward, an area levelled during Katrina, she felt more secure than she did seven years ago.
"I have a hurricane house this time," said Ms Barnes, who has been living in her newly rebuilt home since 2008.