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 northwards 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 on Thursday 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.