New Orleans is a really interesting city because it's actually below sea level! Sea level is the average height of ocean water. Most land is above sea level, but some parts of land are actually below sea level. If a part of land that is below sea level is no where near the ocean or a river (like Death Valley, California, for example), then it isn't really at a risk for flooding. But New Orleans is special because it's below sea level AND it has a major river that runs through it as it drains to the ocean. Water always flows downhill, so technically rivers that drain to the ocean can't be below sea level (because water can't flow uphill on its way to the ocean!). That means parts of New Orleans are actually below the Mississippi River. So why isn't New Orleans ALWAYS flooded?! Because over hundreds of years, the city has been building levees, which are large walls that run along the river, to keep the river water out of the city. For the most part, those levees stop the river water from flooding the city. But when Hurricane Katrina hit New Orleans, it dropped so much rain that the river rose and spilled over the top of the levees, turning many parts of the city into a lake. Katrina was so destructive because New Orleans is below sea level and the levees that were supposed to hold the Mississippi River out failed.

Much of New Orleans is built below sea level. There are levees - barriers built to keep the sea from rushing in. A powerful storm, which hurricane Katrina was, can b>break those levees, which in turn allowed the ocean to flood large parts of the city.

The reason hurricane Katrina itself was so destructive, unlike any earlier hurricane, is because Katrina happened to be the first hurricane since New Orleans was built to hit the city. Weather experts had known that New Orleans was a hurricane disaster waiting to happen for centuries beforehand - ever since the French governor who built New Orleans in the first place ignored the advice of his chief engineer not to build the city where he built it. When Thomas Jefferson bought New Orleans along with the rest of the Louisiana Purchase that you will read about in history class, the United States inherited the problem that this French governor created.

A lot of New Orleans is below sea level, which is a big problem when there's a big storm! The levees and flood walls around New Orleans were badly built, so they broke open and flooded New Orleans.

And there were so many people who were still in New Orleans, with no way to leave.

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