The Katrina tab

Despite the hundreds of thousands of words written about Hurricane Katrina, fundamental misunderstandings about Louisiana's vulnerability to hurricanes continue to cloud the policy debate. It is important to clear up those misunderstandings as Congress considers an emergency appropriations bill that should include federal money to complete protection for New Orleans against a "100-year storm," and because it will soon also face multibillion-dollar long-term costs to protect Louisiana from even larger storms.

These costs are a federal responsibility because benefits to the entire nation, including massive engineering projects built in, and providing direct benefit to, states as far away as North Dakota, have in the last 60 years transformed New Orleans from a city reasonably safe from hurricanes to one dangerously vulnerable to them. These projects have had an effect as great as sending saboteurs from 1,500 miles away to dynamite Louisiana's levees.

That analogy may sound like an overstatement, but it may be an understatement. To understand the link between the High Plains and Louisiana, one has to understand the Mississippi River system -- which stretches from New York to Idaho and drains 31 states -- and the sediment load the system carries. This sediment load was so great that it changed the nation's geography. Sixty million years ago, the ocean reached north to Cape Girardeau, Mo., but as the sea level fell, the river dropped enough mud into what geologists call the Mississippi Embayment to create all the land from Cape Girardeau to the sea, a total of 35,000 square miles in seven states.

That land-building process created Louisiana's coast, along with barrier islands that provided a buffer protecting populated areas in Louisiana and part of Mississippi's coast.

Human engineering has reversed that process, causing the loss of roughly 2,000 square miles of land since World War II. If this buffer -- equivalent to the state of Delaware -- had not been destroyed, New Orleans would need little other hurricane protection.

Numerous man-made actions have caused the land loss, but the most important, yet least recognized, may be the decline of sediment in the river. Dams built to provide electricity, irrigation and flood protection in the Upper Midwest and High Plains are largely responsible for the decline; sediment level is now only 30% to 40% of the natural amount. A particular problem has been a series of dams on the upper Missouri River beginning above Bismarck, N.D., and ending above Yankton, S.D. Historically, roughly half of the total sediment load in the Mississippi River came from the upper Missouri, but the dams trapped that sediment upstream. According to the U.S. Geological Survey, since the dams' construction in the 1950s, "the discharge of sediment from the upper Missouri River basin virtually was stopped."