The hurricane hit land just east of Grand Isle, Louisiana, at the town of Buras at 1110 GMT on Monday 29 August.
Most of Grand Isle's 1,200 residents had left after warnings by officials and police. Buildings were badly damaged, but no deaths were reported.
The storm generated winds of 233 km/h as it headed north towards New Orleans.
The area hit by hurricane force winds (119 km/h or faster) spread out for nearly 200 km from the centre of the storm.
Hundreds of thousands of people left the city, they had been told to expect a direct hit from the storm.
In fact Katrina swerved to the east, so New Orleans escaped the very worst of the winds. The city was still hit by 160km/h winds and an enormous amount of rain fell.
Flood walls protecting the low-lying city were punctured in at least three places.
About 80% of the city was reckoned to be under flood water up to 6m deep.
Damage was widespread and conditions got pretty desperate as there was no clean water for the tens of thousands of people still in the city to drink.
Power and telephone links failed and there was widespread looting.
It may be a matter of months before the residents can return.
The storm hit Gulfport with 217 km/h winds, and boats were driven into buildings along the coast.
In nearby Bay St Louis, a wall of water ,called a storm surge, nearly 7m high came ashore. It travelled up to 6 km inland.
Thousands of people used the Superdome sports stadium for shelter
Gulfport's fire chief said that three quarters of the buildings in the city suffered roof damage, "if they have a roof left at all," he added.
Biloxi, a town of 50,000 people, was hit by a 9-metre surge of water.
Thirty people were killed when the wall of water smashed into an apartment block.
It's possible the flooding destroyed
90% of buildings along the coast at Biloxi and Gulfport, leaving great piles of debris.
Residents returned to find nothing left of their homes.
An oil platform that was being repaired in Mobile Bay was washed off of its moorings and pushed under a bridge.
Waves full of mud and fine sand hit the eastern shore of Mobile Bay and flooded mansions worth millions of dollars.