The ecological impact of Hurricane Katrina, five years later.

Five years later.
Aug. 17 2010 7:36 AM

After the Flood

The ecological impact of Hurricane Katrina, five years later.

Also in Slate, read an article about some Louisiana National Guardsmen who were stationed in Baghdad during Katrina and again during the recent oil spill. Read our complete coverage of Katrina from 2005.

It's been five years since Hurricane Katrina. How much environmental damage was done to New Orleans and the Gulf Coast, and what sort of recovery has there been?

New Orleans in 2005 after Katrina hit.
Flooding in New Orleans after Katrina

In the early days after Katrina hit and the levees broke, there were fears that floodwater would poison the New Orleans area, carrying as it did a so-called "toxic gumbo" of nasty substances—everything from petroleum to lead to household pesticides. Five years on, there may still be pockets of elevated toxicity (for example, near the sites of oil spills caused by the storm), but as noted in a 2007 paper from the journal Cityscape, "the environmental problems in the city are not significantly different now from environmental conditions before Hurricane Katrina."

In fact, the floods seem to have done a bit of good: Researchers from Tulane recently discovered that soil lead levels in New Orleans fell off dramatically after Hurricane Katrina and Hurricane Rita, which followed less than a month later. That's because relatively clean sediments washed into the city in the wake of those storms, blanketing older, more toxic soils. Despite those changes,however, lead researcher Howard Mielke notes that the city is "still in terrible shape" when it comes to lead contamination. And indoor pollution from mold growth in flooded houses remains a potential health hazard.

A more pressing concern for environmentalists is the degradation of the coastal wetlands—a long-standing problem exacerbated by Katrina and Rita. Wetlands are important for a number of reasons: Like rainforests, they support a wide range of plant and animal species. During storms, they buffer inland communities by slowing and absorbing floodwaters; they also help purify groundwater by filtering out sediment and pollution.


Wetlands all along the Gulf Coast have been shredded by oil and gas development, rising sea levels, and a decades-long invasion by ravenous, beaverlike rodents known as nutrias. The levees along the Mississippi River also contribute to erosion, since they prevent the river from overflowing its banks—a natural process that would normally replenish the wetlands with fresh sediment. In the two decades before Katrina, Louisiana's coastal wetlands were already losing roughly 10 to 14 square miles a year.

Katrina and Rita took a big bite out of these weakened ecosystems. Storm surges physically expanded the areas of erosion, and the salinity of that Gulf water killed off vegetation in the freshwater wetlands. Rita probably did even more damage in that regard: Whereas Katrina drenched the coastal zones with a quick blast of Gulf water—the surge lasted just a few hours—Rita's waves lingered for many, many hours, marinating the wetlands in brine. All told, U.S. Geological Survey researcher John Barras estimates that Hurricanes Katrina and Rita caused about 17 or 18 years' worth of wetland loss in Louisiana.

The hurricanes may also have hastened some changes in what are known as cypress-tupelo swamps. These forested areas—named for the huge trees that dominate the landscape—used to stretch from New Orleans all the way down to the coastline, but now only a few thousand acres in that region remain in good health. These pockets were already being threatened by invasive tree species from Asia, which are much more prolific seed-producers than the slower-growing native trees. Katrina's wind blast aided their incursion by spreading those seeds around. It also sheared the tops off of big native trees, which allowed more sunlight to reach the invaders growing from the swamp floor.

Sean Anderson—a California-based ecology professor who's been working in Louisiana for the past five years—explains that if hulking cypress and tupelo trees get replaced by smaller, thinner plants, there won't be as much space for birds to nest. The dense growing patterns of the Asian species would mean that habitat for land-dwellers, like deer, would shrink as well. Finally, the loss of these forests would also be a loss for climate change: Scientists suspect that a swamp filled with these smaller interlopers won't be able to sequester nearly as much carbon.

You might think that the recent BP oil spill disaster would only have made things worse for the wetlands. But according to Anderson—who's chairing a working group attempting to characterize likely impacts from the spill—the oil doesn't seem to be causing much additional damage, since most of what's washed ashore is getting caught up on the outer fringes of the coastline. Plus, by the time it hits land, natural processes have weathered the oil, making it less toxic than when it first came gushing out of the wellhead.(Contamination has made its way farther inland in some places, however, and data are still coming in.)

Anderson suspects that fisheries in the Gulf are likely to be the long-term victims of the oil. That's ironic, he notes, because fish populations had been doing rather well, post-Katrina. Certain seafood species, like oysters and some kinds of shrimp, were harmed by the hurricanes, but these populations bounced back relatively quickly. Fish actually got a bit of a break after Katrina: Since the storm wrecked the fishing industry's human infrastructure, marine animals enjoyed five years of reduced fishing pressures. This was supposed to be the year that the industry ramped back up to its pre-2005 catch capacity—but the BP spill has probably put the kibosh on that.

Is there an environmental quandary that's been keeping you up at night? Send it to, and check this space every Tuesday.

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