The I-510 bridge goes over the Intracostal Waterway with the Mississippi River Gulf Outlet. Alex Brandon/AP

The most important thing that people need to understand about the New Orleans area is the interplay between geology and engineering, and their unintended consequences. This involves two issues: how the city became vulnerable and rising sea level.

Nature did not make New Orleans vulnerable to hurricanes. Engineers did. The sea once reached north to Cape Girardeau, Missouri. But the Mississippi River deposited enough sediment into the ocean to create 34,000 square miles of land — three quarters the area of Texas— from there to the sea.

Engineers cut the natural amount of sediment in the Mississippi River by 60-70%. They did this to protect lives and property in the entire Mississippi Valley, which stretches from New York to Idaho, from North Carolina to New Mexico. Keeping river banks on the Arkansas, Missouri, and other rivers from collapsing into the river stops sediment from getting into the Mississippi. Then, engineers built jetties two and a half miles out into the Gulf of Mexico, which carry the remaining sediment into deep water. Engineers did this to keep the mouth of the river open to shipping. The Mississippi River makes Pittsburgh, Tulsa, Kansas City, Omaha, and many other cities into ports with direct access to the ocean, but they can reach the sea only through New Orleans. But those actions deprived coastal Louisiana and some of Mississippi of the sediment that created it. Another, final engineering blow came from the oil industry, which dredged 10,000 miles of canals and pipelines through the marsh, destroying it with salt water intrusion.

The result: 2,100 square miles of land and barrier islands have melted into the sea. Since every mile of land over which a hurricane travels absorbs one foot of storm surge, that land loss has enormously increased the danger to populated areas. Although much of that loss cannot be regained, enough sediment remains in the river to rebuild land in key spots and make New Orleans much safer. Better levees are of course also needed. Rebuilding this land also counteracts rising sea level. The reason: the coastal marsh is alive. It's dynamic. It will fight for life. It will, within limits, adjust to a rising sea level.

Protecting people from floods and improving local economies as far away as Montana and Pennsylvania actually makes life more dangerous in Louisiana. The nation as a whole is getting most of the benefits of all this engineering, while Louisiana and part of coastal Mississippi pay 100% of the price. Nothing demonstrates that as well as New Orleans East, the lower Ninth Ward, and most of St. Bernard Parish, where 175,000 people were flooded by three man-made shipping canals that create almost no jobs there but carry barge traffic from Houston to Florida, or ocean shipping from the entire river valley to and from the rest of the world.. Think about that when you think about New Orleans.

John M. Barry is author of Rising Tide and The Great Influenza and serves as secretary of the Southeast Louisiana Flood Protection Authority East, which oversees six levee districts in the New Orleans metropolitan area.

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