BETTY ANN BOWSER, NewsHour Correspondent: Take an airboat ride through the wetlands south of New Orleans and you might think you're moving through an unchanging wilderness.
The air is thick with egrets and blue herons; alligators swim through the water or sun themselves on mud banks. But the pristine look is deceptive.
Before Katrina, Louisiana was already losing 25 to 35 square miles of these wetlands a year, the equivalent of a football field every half hour. Coastal scientist Charles Villarrubia says, in the past, wetlands slowed down the force of hurricanes.
CHARLES VILLARRUBIA, Louisiana Department of Natural Resources: When they start hitting land, they start getting friction, they start to slow down a little bit and weaken. In fact,, years ago, even a few years ago, my parents never even thought of evacuating from New Orleans, but now they do, because of the loss of wetlands. The storms are coming in stronger than they had previous.
BETTY ANN BOWSER: The change in wetlands pre- and post-Katrina shows up most dramatically in satellite photos. Villarrubia says what is now open water was green when he visited before the storm.
CHARLES VILLARRUBIA: We were standing just right across the canal here in freshwater vegetation over our heads. And further down south, I've got pictures of beautiful brackish marsh with freshwater marsh coming in on the sides of the canals, just like what's supposed to happen.
BETTY ANN BOWSER: But the erosion goes back generations. Robert Twilley of Louisiana State University has plotted the loss on his computer.
So all those green areas, that's what the wetlands looked like in 1839?
ROBERT TWILLEY, Louisiana State University: It is an extensive, extensive area of landscape in 1839. And what you notice in this landscape is that it's pretty solid land. And what happens when you move in time is that you change the spatial configuration of the landscape. And then, when you get to 1993, you can really see water that is now closer to communities.
BETTY ANN BOWSER: In past ages, the Mississippi and other rivers carried sediment that sometimes overflowed its banks and helped build up the land, counteracting a natural tendency to sink. Plants took root and grew, creating and restoring the wetlands naturally.
ROBERT TWILLEY: For 5,000 years, there were hurricanes. For 5,000 years, there were floods, there was sea level rise and there was subsidence. So, you know, there are forces of nature that wetlands have been able to survive. The one different ingredient in our landscape in the last 300 years is humans.
BETTY ANN BOWSER: The biggest human intervention was generations of levee building. After the catastrophic 1927 Mississippi River flood, the federal government subsidized an almost continuous series of levees on the lower river. The flooding was controlled, but sediment that once built up the wetlands flowed out to the Gulf of Mexico.
The wetlands also suffered because of the extensive canal system built to help oil companies move their products from offshore platforms to market; those allowed in saltwater, killing thousands of acres of freshwater wetlands.
ROBERT TWILLEY: This wetland complex is so vulnerable to human settlement that it has actually led to removing the coastal processes that stabilize this landscape. And that degradation has increased the risk to every coastal community, because it's not that the people in 300 years have been moving closer to the sea; the sea is moving closer to the people.
BETTY ANN BOWSER: Randy Hanchey, the number-two man at the Louisiana Department of Natural Resources, gave us an aerial tour of the wetlands that remain.
RANDY HANCHEY, Louisiana Department of Natural Resources: I don't know that the Gulf is going to show up on the doorstep of New Orleans in 50 years, but you're going to have shallow, open water all the way to the New Orleans metropolitan area if we don't do something about restoring some of these wetlands.
BETTY ANN BOWSER: The state has been trying to restore some of the thousands of acres of wetlands. One of their projects is 15 miles south of New Orleans.
Here, steel gates allow thousands of tons of fresh water to flow from the Mississippi River under the levee and into the open wetlands beyond. The fresh water supports healthy plants that create more land. The process works the way it did before levees were built, but in a more controlled way, without the floods.
But the state estimates that a serious program to restore wetlands would require many more water diversions than those already built. There would also have to be more projects to rebuild barrier islands in the Gulf, like East Timbalier Island.
RANDY HANCHEY: This island was broken through in several places; it was very narrow.
BETTY ANN BOWSER: Hanchey says, at East Timbalier, the state rebuilt dunes and marshes and replanted lost grass. Once restored, the island acted like a speed bump to slow down incoming storms. But a major wetlands restoration would cost big bucks, which is why New Orleans Mayor Ray Nagin was in Congress recently asking for additional funds.
RAY NAGIN, Mayor of New Orleans: For every mile of wetlands, it allows for us to subside a storm surge by one foot. And since the coastal erosion problem in Louisiana is so severe right now, we probably lost 100 years worth of coastline, as it relates to this storm.
BETTY ANN BOWSER: Eight years ago, the state estimated a comprehensive wetland plan alone, without rebuilding the levees, would cost $14 billion. The Bush administration rejected that plan three years ago and told the state to come up with a smaller, more affordable plan instead, $1.8 billion worth.
Yet after Katrina, scientists like Twilley say the larger-scale program is more important than ever.
ROBERT TWILLEY: I don't think it's fair to put $1.8 billion into a program and sell this expectation that we're restoring the coast; it's not going to do it.
And, in fact, I would even argue that, you know, until you reach some threshold of funding and commitment, then you really are not going to have an impact. And what we feel here in Louisiana is the challenge and the commitment needed to sustain this coast.
BETTY ANN BOWSER: Recently, the Army Corps of Engineers began asking the people of New Orleans to comment on a new, single plan to combine flood protection and wetland restoration into one package.
COL. RICHARD WAGENAAR, District Manager for Army Corps of Engineers: We've got input from a lot of the professionals. We are in a partnership here with the state of Louisiana. And we continue to get input, solicit input from them and many of the other people that are on the team, but we need the public's input.
BETTY ANN BOWSER: Twilley agrees; levees alone won't protect New Orleans from future flooding.
ROBERT TWILLEY: The guarantee of protection, related to that barrier system, when water is lapping at your heels, I feel is a real, almost futile effort that, unless you actually replace those water areas with land that is much more stable, then I think there's going to be a real false sense of security, relative to what we're going to tell the public in relation to the degree of protection that we can guarantee in the future.
BETTY ANN BOWSER: So far, neither the federal government nor the state of Louisiana has indicated it will undertake a massive wetlands restoration.