Flood of evidence

Furious floods

Too many floods

Fewer trees = more floods?

Wetlands and floods

Flood prevention: the earthmover approach


This reserve, created in the wake of the 1993 flood, returned some of the Missouri River's floodplain to the river.

Courtesy U.S. Fish & Wildlife Service


boat travels down street in St. Genevieve, Missouri. Half-submerged house in background Damage survey, St. Genevieve, Missouri Courtesy FEMA.


Standing a dozen feet above its neighbors, this house in St. Charles County, Mo., may be ready for the next flood.

Copyright David Tenenbaum

Wet wonders
A cattail marsh, located in the Upper Mississippi watershed. Wetlands like this cattail marsh, located in the Upper Mississippi watershed, help store floodwaters during heavy rains.
Wetlands -- swamps, marshes, fens and bogs -- are natural water-storage features on the landscape. Once considered wasted land at best, or lurking-grounds for evil at worst, wetlands are essential wildlife habitat, massive natural water filters, and "natural sponges" that hold water when it rains, then release it slowly.

Wetlands are also under assault. In Iowa, Illinois and Missouri, for example, more than 80 percent have been destroyed, often by draining for farms or development. Currently, in the United States, the pace of destruction is most extreme in the coastal wetlands of Louisiana, which shelter New Orleans from hurricanes.

Levees built by the U.S. Army Corp of Engineers guide the Mississippi River and its silt through a narrow shipping channel to the Gulf of Mexico. Now, when the wetlands are eroded by storms, there's no dirt to rebuild them. "We now have a levee system to nearly the edge of the continental shelf, so the sediment gets dumped off ... into the deep part of the Gulf, and it doesn't build land any more," says Stephen Nelson, a professor of geology at Tulane University, so the barrier wetlands are disappearing. "We're losing our protection from hurricanes, and the biggest worry about flooding around [New Orleans] comes from hurricanes." (The Why Files also covered beach erosion.)

River, floodplain, and hills behind. Wetlands play an equally crucial role in preventing flooding at the other end of rivers. In the headwaters and watershed, they store water during heavy rains, slowing runoff into streams and reducing flood peaks. (Here's info on the upper Mississippi watershed).

Wetlands are natural basins, observes Joseph Larson, director of the environmental institute at the University of Massachusetts. Some of the trapped water returns directly to the atmosphere through evaporation or plant transpiration, and while few wetlands recharge groundwater, they slowly release the water they hold to their outlet stream. It's simple physics: Wetlands typically have a large inlet and a small outlet.

Even better'n concrete!
Wetlands are major flood-control agents in the watershed of the Charles River, which flows through Boston. Larson observes that when the Corps of Engineers arrived to investigate the flooding problem, it was raining heavily, and they saw the marshes filling and "acting as natural flood controls. The rain was after a dry period -- and the water went downstream over three weeks rather than three days."

After calculating the cost of creating that amount of storage with dams, Larson says the Corps concluded it "would be a lot less expensive to buy 8,000 acres of natural wetland, and to use conservation restrictions to buy up development rights" on other floodplain acreage.

The Charles River is considered a classic example of natural flood control, but just because wetlands can store water, they are not a panacea. In a major flood, all bets are off. Just like dams, wetlands can fill up, after which, incoming water simply runs off.

So while wetlands can help reduce the size of minor floods, in giant floods, they may only reduce flood height. During the epic 1993 flood on the Mississippi and Missouri Rivers, wetlands "got swamped," says David Galat, who studies big river ecology at the University of Missouri at Columbia. "You were dealing with a flood that was beyond any ability of wetlands to absorb it."

Wet, but wild? A house stands on tall pilings; in the background, houses are on shorter pilings.
If wetlands reduce flooding, and offer other benefits to wildlife and water quality, why not create or restore wetlands to create a cheaper, more natural flood-control mechanism? The answer depends on the distinction between restoring damaged wetlands from building new wetlands to replace those destroyed by development. Restoration can work if the soils and water flow have not been altered too drastically, says Joy Zedler, a professor of restoration ecology at the University of Wisconsin-Madison.

But the practice of destroying a natural wetland and replacing it with a new one built elsewhere (so-called "mitigation") is much more problematic, says Zedler, who chairs a National Research Council study on wetland mitigation. Unless the new wetland is in the natural stream flow, it may be worthless for reducing floods.

Many constructed wetlands don't work to provide natural habitat either, Zedler adds. In one case, permanent wetlands were built around Portland, Ore., to replace natural marshes lost to development. "Permanent marshes are not a feature of the landscape there," Zedler says, so "they needed a new [geological] classification for them." Among other problems, the ponds attracted exotic bullfrogs, which devour native amphibians.

Overall, Zedler says that while some people say mitigation can succeed, "The scientific, peer-reviewed stuff pretty much points out problems."

If wetlands won't always work, why not control floods with dams and levees?

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