Officials said about 60 percent of New Orleans was still under water, but that was down from a peak of about 80 percent. Most of the gain came because the Army Corps of Engineers began opening gaps in the city's levees after the water level in surrounding bodies of water fell. The holes ensured that the levees — designed to keep water out of the below sea-level city — would not hold it in.

Four of the approximately 40 pumping stations in the New Orleans area were running Tuesday at least at partial capacity, officials said, but fitfully; a fifth giant one, at the 17th Street Canal, site of a major levee breach, started up but had to be stopped again because the pumps sucked in debris. Officials said it would take 24 days to pump the water from an eastern section of New Orleans and 80 days to clear the flooding from Chalmette, the nearby seat of St. Bernard Parish.

Evacuees continued to flood back into Jefferson Parish to check on their homes, overwhelming roads and bridges. Interstate 10, which connects Baton Rouge and New Orleans, was backed up for about five miles.

Louisiana officials offered a first glimpse at the environmental wreckage. The state secretary of environmental quality, Michael D. McDaniel, said that wildlife habitats along hundreds of miles of coastline had been destroyed and that the hurricane had exacerbated the slow coastal erosion that has already made the Gulf Coast more vulnerable to hurricanes and typhoons.

McDaniel said there was no alternative to pumping billions of gallons of brackish water back into Lake Pontchartrain, but he said it was too early to determine the harmfulness of the toxins and pollutants that were being slowly sifted out of New Orleans.

"I know there's been a lot of discussion about 'toxic soup' and 'witch's brew,"' he said. "I've seen no data to date that backs up that kind of statement. We do know and would expect that there are a lot of bacteriological contaminants in the water."

In New Orleans four major fires had broken out by Tuesday morning, gas leaks were numerous and mosquitoes had begun to fly after swarming deceased hurricane victims, Nagin said.

"I don't want make any statement that suggests I'm giving up on New Orleans," Nagin said at a news conference. "But it's a very volatile situation in the city right now. There's lots of oil on the water and there's gas leaks where it's bubbling up, and there's fire on top of that. If those two unite, God bless us. I don't know what's going to happen."

He said in an interview that a new evacuation order would eliminate exemptions that have allowed people to continue to stay in hotels and hospitals. Essentially, the city would be closed to everyone but law enforcement, military and public safety and health officials while it is drained of water and utilities are restored. The 82nd Airborne closed the Hyatt Hotel afternoon to civilians Tuesday afternoon.