What can save Gulf's fragile coastal wetlands? Salt water, perhaps.

As the aircraft buzzed the shoreline, the duo spotted two- to three-foot-high walls of what they called "peat balls" – bunches of uprooted marsh grass with large wads of peat still attached to their roots.

The plane flew back and forth over the wetlands as the scientists hunted for the peat balls' source. "What really struck me was that some regions of the marsh were torn up, while other regions seemed very resilient and hadn't changed at all," FitzGerald says.

The yin-and-yang regions were separated by a relatively narrow stretch of high ground that once served as the banks of a Mississippi tributary that emptied into the Gulf.

Freshwater marshes dominate the landscape west of the rise, sustained by the Caernarvon Diversion project. They sustained the most damage. On the other side of the rise, saltwater marshes dominated and sustained far less damage.

The plant-survival pattern the team saw there held for coastal plains and deltas along the shoreline west of the Mississippi as well, the team says. Indeed, in some cases, the salt marshes took the brunt of the hurricane's surge, yet the freshwater marshes sustained far more damage.

The key, the team found, was in the depth of roots from the plants in each type of marsh. There was little difference in the soil types in each – both consist of fine silts.

The denser, deeper roots of the saltwater grasses required significantly higher stresses to shear off the upper layer of sediment than did the shallower, more sparse roots of the plants in the freshwater marshes.

One reason for the difference in root depth: The freshwater plants seemed less able to thrive if their roots penetrated into oxygen-deprived soils, which sit below a relatively thin layer of surface soils, according to the team.

But the nutrient-rich diversion water from the river also played a role, the team suspects. With more nutrients available at or near the soil surface, the freshwater plants have no incentive to send roots deep into the underlying soil.

This mechanism for weakening diversion-fed freshwater wetlands dovetails with research Eugene Turner, a zoologist at Louisiana State University, has conducted in the region. As for allowing a freshwater marsh to toughen up by steering it toward a saltier future, "salinity control is not a documented restoration strategy along this coast," he writes in an email exchange.

At the least, he says, the results reinforce the need for a rigorous effort to monitor the wetlands that diversion projects aim to help.

Even then, he adds, with confirmation, the new results represent "a deal breaker" for diversion projects as wetlands restoration tools in the region.

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