A new study says river deltas around the world aren’t just disappearing because of rising seas, but also because the land itself is sinking down into the waters, either as fast or faster than the rising oceans.
Researchers found some of the most rapid sinking is happening along the Mississippi River Delta in Louisiana. The study aims to better guide coastal restoration in disappearing river deltas around the globe, helping leaders, scientists and people living in coastal communities with hard decisions on what can — and should — be saved.
“Coastal areas account for less than 1% of the entire land area we have,” said Leonard Ohenhen, a professor at the University of California, Irvine and the lead author of the study. “But a whole significant population, more than 600 million people, live in those areas.”
The study, published this week in the academic journal Nature, found the contributions of subsidence, or slowly submerging land, to disappearing coasts is often overlooked.
The fight to preserve rapidly sinking land has been a decades-long battle in the Mississippi River Delta, as well as a source of contention between scientific and political figures in the state. But deltas across the world are sinking, too, and fast.
“You have a sort of a hodgepodge of different reasons why deltas are sinking,” Ohenhen said.
He said river deltas naturally sink to some degree, with sediment carried downstream by rivers piling up and pushing down on the spongy, soft land already there. Humans can accelerate this natural process by engineering rivers such as the Mississippi and by extracting groundwater or oil.
“Relative sea level rise in the area is also really important. That's the sea level rise plus subsidence,” said Alisha Renfro, a coastal scientist with the National Wildlife Federation. “It really helps us understand where we can make investments in restoration long-term that we might actually be able to hold on to.”
Lack of sediment is the main driver of subsidence in the Mississippi River Delta, Ohenhen said, creating hotspots of rapidly sinking pieces of land amid slightly more stable areas. Most of the deltas studied in the paper, around 70%, have subsidence problems primarily the result of groundwater withdrawal. But some, like the Amazon and Mississippi deltas, had subsidence issues driven by the disappearance of river-carried sediment to replenish the delta’s land.
Putting hard numbers to and pinpointing causes of subsidence — like human activity — is invaluable to restoring coastal land.
“I would say that really validates what, not just my organization, but what a lot of people have recognized for a long time — that this was a significant contributing factor in subsidence,” said James Karst with the nonprofit advocacy group Coalition to Restore Coastal Louisiana, referring specifically to the lack of sediment sent to the Mississippi River Delta.
Decisions about what pieces of the coast can be saved are even more urgent with the cancellation of two large-scale restoration projects in Louisiana.
Known as sediment diversions, the Mid-Barataria and Mid-Breton plans would have diverted freshwater from the Mississippi River into surrounding wetlands. They were scrapped by the state because of the prospective impact on fisheries for oysters, crabs and other marine species. Fish and oyster harvesters celebrated the projects’ demise, while scientists and coastal restoration advocates warned that time is running out to save the coast.
“In light of the cancellation of Mid-Barataria, I think what we, everybody, should be thinking of is, ‘What is the next best thing?’” Karst said. “Clearly it is not going to move forward, but we can't do nothing.”
“People should be aware that we are in a part of the world that is changing and that is changing rapidly,” he added. “If we want to position ourselves as individuals and as communities, we should be anticipating these changes and anticipating how they will affect us.”
While the average rate of subsidence for the Mississippi River Delta is around 3.3 millimeters per year, Ohenhen said, some areas of Louisiana are sinking at a rate of 3 centimeters per year, one of the fastest rates of all the deltas studied. That is paired with sea level rising by at least 7 millimeters per year along the Gulf Coast, he said, also one of the highest rates in the world. This puts some areas of Louisiana’s land at higher risk of loss than anywhere else.
“In the Mississippi River Delta, for example, that is one of the only deltas in the world where you have active relocation of people from the delta due to land loss,” Ohenhen said. “The time that we need to respond to these changes is now before the situation gets significantly worse.”
This story is a product of the Mississippi River Basin Ag & Water Desk, an independent reporting network based at the University of Missouri in partnership with Report for America, with major funding from the Walton Family Foundation.
