Two levees burst Tuesday, flooding the city of New Orleans in the aftermath of Hurricane Katrina, which had already leveled much of the Gulf Coast from Louisiana to Alabama in one of the nation's worst natural disasters.

The flooding showed that the damage from the historic hurricane that hit early Monday with 145-mph winds was only just beginning. Rescuers in boats pushed aside the dead floating in the brown, churning waves to reach survivors trapped on rooftops as authorities urged residents to flee.

While Katrina flooded the bowl that is New Orleans, its winds and 25-foot storm surge killed an estimated 110 people in Mississippi. An oil platform, torn from its moorings in the Gulf, beached near Dauphin Island, Ala. The devastation stretched across three states, with the hurricane shredding waterfront hotels, toppling concrete bridges and injuring countless people.

Communication was sporadic or nonexistent. Nearly 3 million people were without electricity and drinking water. Interstates across Lake Pontchartrain were battered, buckled and broken, and most other roads also were impassable more than a day after Katrina had passed by as a Category 4 hurricane -- one of the strongest ever to hit the continental United States.

Although Katrina's winds had fallen to 35 mph Tuesday, downgrading the storm to a tropical depression, forecasters predicted it could drop as much as eight inches of rain in Tennessee and the Ohio Valley as it headed north.

Oil prices rose above $70 a barrel for the second day in a row in markets nervous about the future of the Gulf's refineries and oil rigs. Seven offshore drilling platforms lost their moorings off the Louisiana coast, and oil companies were sending tugs to corral them before they smashed into production platforms, said Nelson Robinson, a weather consultant for the companies.

In New Orleans, floodwaters breached the city's storied levees to flood entire districts in a swamp of dirty water. "We probably have 80 percent of our city" submerged, New Orleans Mayor Ray Nagin told television station WWL early Tuesday. "Within some sections of our city, the water is as deep as 20 feet."

As floodwaters rose, emergency generators began to drown and fail throughout the city. Tulane University Hospital officials told CNN that they had lost a generator around midday, and floodwaters were causing the hospital to evacuate patients in helicopters that landed on the hospital parking garage.

Several feet of water surrounded the relief center at the Louisiana Superdome, squelching emergency lighting. Already without air conditioning, and with overflowing toilets and leaks in the roof, the 65,000-capacity downtown stadium became a festering hellhole for 10,000 people who had taken refuge there before Katrina's arrival Monday.

Even as the floodwaters rose, looters roamed the city, sacking department stores and grocery stores and floating their spoils away in plastic garbage cans, watched unmolested in many cases by patrolling police and National Guardsmen.

Nagin told the Associated Press that rescuers in boats were "just pushing [corpses] aside" to get at survivors clinging to the roofs of houses.

Still, Gov. Kathleen Babineaux Blanco (D) left no doubt that her state's ordeal had only just begun. "The devastation is greater than our worst fears," Blanco said early Tuesday. "It's just totally overwhelming." And though Louisiana had not counted its dead, she added, "we know many lives have been lost."

In Mississippi, residents and officials in Biloxi painted a terrifying picture of a 25-foot storm surge Monday that swept in from the Mississippi Sound and leapt over a seawall to engulf and eventually destroy low-rise waterfront apartments.

"We had to swim for our lives," said Landon Williams, 19, who stroked from building to building in the Quiet Water Beach apartments with his 55-year-old grandmother as the surge rose higher and higher, until he finally reached high ground and safety. Tuesday his apartment building was a flattened pile of lumber and debris.

By late Tuesday, Harrison County, which includes Biloxi and Gulfport, another hard-hit Mississippi city, reported more than 100 dead. "We are very, very worried that the figures will go much higher. The death toll rises each time we go out," county civil defense director Joe Spraggins said. An additional 10 deaths were reported in neighboring Jackson County.

In Mobile, Ala., Katrina's storm surge left behind downtown floodwaters up to 15 feet deep. Winds snapped off tree branches and downed power lines. Mobile County officials kept schools and most businesses closed and imposed a dusk-to-dawn curfew Tuesday night, urging residents who had evacuated the city to stay away for at least a few more days.

Louisiana's Blanco said at a news conference that authorities were working on a plan to evacuate the residents who remained in New Orleans despite Nagin's order Sunday to evacuate the city.

Residents had been counting themselves somewhat lucky Monday, because while Katrina had packed a heavy wallop, it had lost strength and veered eastward toward the Mississippi border at the last moment, dealing the city a not-quite-direct hit.

But by Monday evening, the congratulations had turned to alarm as water from two breaches in New Orleans's levees -- one of them more than 200 feet long -- began to cascade into the city, much of which lies below sea level.

Wet streets suddenly became flooded. The storied Canal Street lived up to its name, and dirty water began to swirl about the Superdome.

"It's not a very comfortable situation" in the Superdome, Blanco said, who later added that the stadium would be evacuated within two days. "You can imagine: There's no power. It's hot. It's difficult to get food to them. I saw people walking in about knee-deep water as they were trying to get into the Superdome from the ground floor."

Michael D. Brown, director of the Federal Emergency Management Agency, said the U.S. Army Corps of Engineers had scouted the levees and was planning to begin shortly to close the breaches, a job that "they feel they can accomplish in relatively short order."

Brown said the Corps wanted to make sure that "the plan they come up with will hold," and suggested later that they were contemplating the possibility of filling shipping containers -- the truck-trailer-size cargo holders that freighters carry on deck -- with sand and dropping them into the levees from helicopters.

As the water rose, rescue boats fanned out across New Orleans, grabbing people from upper windows and the roofs of the city's flooded neighborhoods and delivering them exhausted and thirsty to holding areas. Bright sunshine, absent most of Monday, raised the temperature to 90 degrees, further worrying rescue officials.

Looting began on Canal Street in the morning, as people carrying plastic garbage pails waded through waist-deep water to break into department stores. In drier areas, looters raced into smashed stores and pharmacies, and by nightfall the pillage was widespread.

In many cases the looting took place in plain view of police and National Guardsmen charged with controlling it. At one Canal Street pharmacy just outside the French Quarter, the AP reported, two police officers stood guard as workers loaded large laundry bins full of medications, snack foods and bottled water.

"This is for the sick," Officer Jeff Jacob said. "We can commandeer whatever we see fit, whatever is necessary to maintain law."

Then another officer, D.J. Butler, told the crowd that police would soon leave. "I'm not saying you're welcome to it. This is the situation we're in. We have to make the best of it," he said.

In Biloxi, Katrina had wrought perhaps its most serious devastation, veering eastward as it approached the coast and pushing the leading edge of its storm surge through the Mississippi Sound toward a cataclysmic impact with the seawall protecting the city of 55,000.

"It looked like the tsunami" that hit South Asia late last year, said municipal government spokesman Vincent Creel, who watched the surge from the upper floors of City Hall. "Our city is decimated. It looks like a bomb hit it."

Biloxi was without electricity, water and sewage service, Creel said. Police, some of them working steadily since the weekend, were overwhelmed. "One dispatcher was talking about how many calls she was getting and how she couldn't do anything," Creel said. "She started crying as she was telling the story.

"We reached a point where there wasn't anything else we could do," he added. "Everyone was warned there might be 175-mph winds and a 28-foot surge, but we could not fathom those numbers -- you're going to be looking at hundreds of deaths along the Gulf coast."

Creel suggested that much of the suffering may have fallen to relatively new arrivals who had come to Biloxi in the past 15 years to work on the floating casinos moored to a peninsula poking into the Sound.

But the casinos were reduced to wreckage Tuesday or simply gone, as was much of the neighborhood next to it. "It appears the entire peninsula was submerged," Creel said. "Nobody imagined that."

Gugliotta reported from Washington. Staff writers Christopher Lee in Mobile, Peter Slevin and Sylvia Moreno in Biloxi, Sam Coates in New Orleans and Ann Scott Tyson in Washington contributed to this report.

Water covers roads and surrounds homes in New Orleans a day after Hurricane Katrina made landfall. The Superdome is in the background. Orleans Parish prisoners are held on a highway overpass, amid roadways flooded by Hurricane Katrina. The flooding showed that the damage from the historic hurricane that hit early Monday was only just beginning.