New Orleans Mayor Ray Nagin says Hurricane Katrina probably killed hundreds and "most likely, thousands" of residents. "We know there is a significant number of dead bodies in the water," and others dead on roofs and in attics, Nagin told the AP. Over 20,000 refugees, mostly in the Superdome sports stadium, are to be taken by 500 buses to the Astrodome in Houston 328 miles away, officials now say. It may be weeks before people are allowed to return, they said.

Eight US Navy vessels, including the hospital ship Comfort, and emergency personnel from around the country are headed to the Gulf coast. The Federal Emergency Management Administration (FEMA) sent 39 medical teams, 18 urban search teams and over 1,700 trucks carrying supplies. Homeland Security Department chief Michael Chertoff said "the department has the lead responsibility to coordinate relief efforts." New Orleans' Louis Armstrong International Airport is operating with only one runway for flights connected to the relief effort. The Gulfport-Biloxi Regional Airport in Mississippi is also closed. (Bloomberg, Aug. 31)

One Mississippi county alone said its death toll was at least 100, and officials are "very, very worried that this is going to go a lot higher," said Joe Spraggins, civil defense director for Harrison County, home to Biloxi and Gulfport. The flooding in New Orleans grew worse by the minute, prompting the evacuation of hotels and hospitals and an audacious plan to drop huge sandbags from helicopters to close up one of the breached levees. At the same time, looting broke out in some neighborhoods, the sweltering city of 480,000 had no drinkable water, and the electricity could be out for weeks. (WGNO, Aug. 30)

For generations, many in New Orleans have feared the worst. The city lies in a wide, shallow bowl on delicate marshlands well below sea level, with the Mississippi River running through it. It sits below a lake more than twice its size, while to the south and east are the Gulf of Mexico, one of the earth's worst hurricane zones. City planners designed and built a complex system of flood defenses after the Mississippi burst its banks in devastating fashion in 1927.

New Orleans' extensive levee system was built to withstand a "category three" storm. When Katrina appeared on the weather maps it was a category five—the strongest Atlantic hurricane in a generation.

As water levels rose in Lake Pontchartrain to the city's north, and the Mississippi to the south, two levees built to hold back high waters buckled under the strain. The lake spilled into the heart of the city. Experts agree that a "catastrophic" failure of the levee system has been narrowly avoided. A frontal assault by a category four or five hurricane would break or destroy many more levees.

Yet if the levees saved the city from utter ruin, they may have contributed to its partial destruction. Years of flood-control engineering have tamed the Mississippi. Without regular river floods to feed the swampy delta with precious silt and nutrients, vast swathes of Louisiana's coastal wetlands have disappeared in the past 75 years. The sprawling coastal wetlands can bear the brunt of a hurricane better than the concrete of a modern city. The US Geological Survey calls the wetlands a "natural buffer" in a high-risk area. Plans to stop further erosion have run aground in Congress.

With fewer wetlands to absorb the rains, Katrina swelled the Mississippi and filled Lake Pontchartrain. In the oldest part of New Orleans, the French quarter, the river runs along the crest of a ridge. When the pressure was too much to take, two of the levees broke, pouring water into the city. (BBC, Aug. 31)

See our last post on Katrina.