Eco-engineering on a giant scale

The disagreement underscores a debate that has been raging over marsh restoration for decades. Even before the spill, the marshes were disappearing at an alarming rate - the consequence of the dams, levees and canals built to provide shipping channels and protect New Orleans from flooding. Without intervention Louisiana's bayous could become open water within 50 years.

As a result, support has been growing for a suite of projects to resurrect the marshes (see map). At one end of the spectrum are those who advocate minimal intervention and letting nature take its course. At the other, the call is for yet more engineering: new, hardier breeds of grasses, seeding from the air and artificial reefs to shore up the sinking sediment.

Ultimately, the marshes are vanishing because sediment is in short supply. Wind and waves constantly erode the shoreline, and dams and levees hold back sediment flowing down the Mississippi. What's more, a network of canals dredged by the oil and gas industry carry saltwater inland, killing freshwater marshes. Add to all this rising sea levels and the largest oil spill in US history and the situation is desperate. Without its marshes, Louisiana's thriving seafood industry would crumble and the state's coast would lose its natural defences against the powerful storms that blow in from the Gulf of Mexico.

Turner advocates small-scale intervention: filling in thousands of kilometres of abandoned canals with the dredged sediment that is still piled up alongside them. He also favours helping sediment flow to the marshes. Historically, when the Mississippi's waters ran high, "crevasses" appeared in the river banks and carried sediment into the deltas. Dams and levees now prevent this, so Turner suggests punching holes in the river's embankment to spur the process. "So many marsh restoration ideas assume we can do better than nature," says Turner. "I think that's pretty arrogant."

Others say we need to think big. "Over the years, we've done all kinds of patchwork projects," says Harry Roberts, a retired sedimentary geologist at LSU. "They're not long-term solutions." Roberts has calculated that 18 to 24 billion tonnes of sediment will be needed to maintain the delta as the sea level rises in the next century (Nature Geoscience, DOI: 10.1038/NGEO553). Small crevasses can never meet that demand, he says.

For researchers like Roberts the question has become not how to restore Louisiana's lost Eden but how to create a new, improved one. As part of that, in April, a $23 million project kicked off to pipe mud more than 6 kilometres from Cote Blanche Bay to Vermilion Bay's Marsh Island Wildlife Refuge. The aim is to recreate 160 hectares of marsh.