When we in the upper reaches of the Mississippi River system think about of what we send downstream, we typically tend toward notions of bad stuff, or good stuff made bad by excess.
For example, all that extra water from the acreage we’ve covered with roadways, driveways, parking lots and other rain-proof material that speeds rainfall to the Gulf of Mexico.
Or those harmful chemical wastes, whether industrial or municipal or, more recently, in the agricultural runoff that contributes so much to the spreading “dead zone” of oxygen-poor water beyond the Mississippi’s great delta, below New Orleans.
Now comes the startling new scientific finding that one of the Mississippi basin’s positive contributions to the overall resilience of the delta, and adjacent portions of the seafloor just beyond, is being throttled by human activity upstream.
The asset under study is the flow of sand, silt, sediment, clay — fine, waterborne particles whose settling expands deltas and lifts the sea bottom. And it appears that since the middle of the last century, dam-building and other human interventions have held back so much sediment that underwater portions of the delta and adjacent seafloor have essentially begun to erode.
As with the loss of marshes in coastal Louisiana, a much-studied phenomenon, the new findings suggest a rising risk to coastal communities and landscapes from hurricanes and other storm surges. Unlike the surface changes, the underwater shifts also appear capable of threatening the stability of offshore drilling rigs and pipelines.
Erosion may not seem precisely the right word for what’s happening to a sediment-starved delta. Using it to describe underwater shifts is kind of at odds with the everyday sense of long-term weathering. But it’s the one nonscientists are choosing to discuss the new findings, and researchers at Louisiana State University seem to have settled on it to describe the changes they’re reporting this week.
From LSU’s announcement of the findings, which will be published in the June 1 issue of the journal Marine Geology, and appeared online Tuesday:
During the 20th century, thousands of dams were built on Mississippi River tributaries stopping the flow of fine silt, clay and other sediment from reaching the delta and seafloor to offset erosion. Without this sediment, land — in the form of wetlands and the seafloor — is lost, which threatens offshore and inland infrastructure in the face of waves, hurricanes and surge, or flooding, from storms.
Land loss also affects marine plants and animals as well as how pollution is absorbed and broken down. In this new comprehensive study, scientists have mapped the retreat of the seafloor from the Mississippi River Delta into the Gulf of Mexico for the first time.
How deltas grow, or don’t
To appreciate the sheer scale of their findings, you may need to make friends with the word progradation (a marine geologist’s term for a delta’s expansion, or a seafloor’s gain in height, through addition of sediments). Also its opposite, retrogradation (when a delta retreats, or a seafloor drops, and the changes aren’t offset by fresh sediment deposits from inland).
The researchers combed data sets from public and private sources that went back to soundings made in the mid-1700s and extended up to modern satellite readings. From measurements at a standard 10-meter countour line for comparison, they write in Marine Geology, their calculations show these changes throughout the so-called “Bird’s Foot Delta,” where land had been formed by sediment-bearing flows through a set of branching passes for more than a millennium — until a reversal in about 1950:
- At Southwest Pass, which gets 69 percent of the suspended sediment entering the system, progradation dropped from an average 67 meters per year from 1874 to 1940, to about 26 meters per year in the following four decades, “with evidence of further deceleration from 1979 to 2009.”
- At South Pass and Pass a Loutre, “the delta front has entered the destructive phase,” retreating at rates above 20 meters per year since 1979.
- Comparing measurements from the 1874-1940 period to 1979-2005 indicates that sediment accumulation has fallen by 73 percent across the entire delta front.
- These changes reflect a 50 percent decline, since 1950, in the total suspended sediment load of the river as it reaches the delta.
- A key driver of that decline is the construction of more than 55,000 dams in the river’s vast basin.
Underwater landslides
“The outlets of the Mississippi River also known as the Bird’s Foot Delta have been prograding, or spreading, naturally for hundreds of years, but that has now stopped,” co-author Sam Bentley said in the LSU announcement. “The underwater portions of the delta are now retreating like the land loss occurring in our [surface] landscape.”
“This is a big deal,” emphasized lead author Jillian Maloney, “because it can affect so many processes that occur from the coast to the open ocean, including marine organisms’ lifecycles and underwater landslides.”
Those underwater landslides, or mudflows, are a key concern for drilling operations in the gulf because they can push to the breaking point the pipelines that lie on the seafloor. Another issue: Lines that were laid in expectation that sediment deposits would safely bury them can be exhumed by water movement — rapidly, in storm conditions — and drilling rigs whose supports were sunk deep into the bottom could be destabilized as the floor falls away.
On that point, their calculations found that the “vertical seafloor … across the entire delta front shows a decline in accumulation through time,” and while it might seem small, it is steady. Against an expected baseline rate of 13 millimeters a year — from settling and global sea-level rise — this area appears to be dropping 10 times as fast, somewhere between 10.4 and 14.4 centimeters per year.
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The entire paper, “Mississippi River subaqueous delta is entering a stage of retrogradation,” can be found here but access is not free.
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Irresistible aside: One of coastal Louisiana’s other features recently found to be threatened by retreating marshes is Avery Island, an odd, tall salt dome amid the wetlands; you may recognize the name from the label on your bottle of Tabasco sauce, which the McIlhenny family has been making there for only 150 years.
