With working-class towns like Leeville and Golden Meadow partly overrun by an encroaching Gulf of Mexico, Cajun Country is in full retreat from its historic home in the deep swamps of southern Louisiana.

Now, a bold plan put forward by the US Army Corps of Engineers – and currently being discussed in the new Congress – would build a semipermeable "Great Wall of Louisiana" from the Mississippi River to Texas to block the advancing Gulf and, at the same time, do the opposite of what a levee is supposed to do: Allow water through to keep marshlands from drowning in the kind of brackish backwaters that are killing off Louisiana's signature swamps at the rate of more than 30 acres a year.

For some 120,000 people along Louisiana's blue-collar coast, the "Morganza-to-the-Gulf" levee – a sort of intertidal Maginot Line – is seen as salvation, especially since the 2005 storms. But critics say that such a "leaky levee" is a false hope, a taxpayer-funded Louisiana hay wagon that is scientifically unproven and even detrimental to both the region's ecology and economy.

What is certain, however, is that this storm-blocking proposal promises to test the political fortitude of lawmakers and scientific wisdom of the nation's levee-builders, with deep ramifications for this ancient delta.

"The idea is to plan for protection and restoration together, and [the Morganza plan is an example of] where the two ideas can benefit each other," says Windell Curole, of the South LaFourche Levee District, who manages the levees there. "But there are also places of conflict."

With a price tag of $886 million, 65 percent of which would be federal money, the Morganza section of the great wall would zigzag from St. Mary's Parish to LaFourche Parish, enclosing some 550,990 acres of the Terrebonne Basin and walling off the oil depots of the port of Houma. In the mayhem of a storm, nine 56-foot-wide gates and three even larger floodgates would close to keep out tidal surges, but on calm days the locks would open and allow the region's natural intertidal flows to nourish the marshes.

Planned since the late 1990s, the new levee won't directly protect New Orleans, but its proponents say it will stop the long-term degradation of the marshes that could one day turn the Crescent City into a beach resort, unprotected by surge-slowing swamps.

Currently at stake, experts say, is one of America's most critical ecosystems.

About 50 percent of the Lower 48's seafood comes from the area that would be protected by the Morganza levee. Eighteen percent of the nation's daily fuel also is pumped through these marshes as the chief port to the offshore platforms.

Still, more than 10,000 people have had to migrate northward to escape the Gulf, a disturbing shift for an Acadian culture of pungent fishing villages and gritty petro towns fanning out across low alluvial ridges. "Our communities have been retreating and continue to retreat," says Mr. Curole. The new levee is their final stand, he says.

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