UGA paper suggest marshes losing ground to rising seas

Jan. 21—The famed Marshes of Glynn may be underwater in a matter of decades, according to the research of Herb Windom with the University of Georgia Skidaway Institute of Oceanography.

Based on his research and that of others, Windom believes there are a few culprits to blame — upstream improvements and dredging keeping fine-grain sediment from traveling downriver to the coast, dumping dredge spoils in areas where it can't circulate back into waterways and sea level rise.

"The paper is about quantifying or estimating. Look at old data and extrapolate to the present," Windom said. "If you look at sediment delivery now, it's about 2/3 of what is needed for the salt marshes to keep up with sea level rise."

To thrive, a salt marsh needs to be covered by water at high tide and above the water at low tide. This intertidal zone has historically maintained itself, he says, by capturing fine-grain sediment coming downriver to build up the ground in which it grows.

Not all three factors have an equal contribution to the issue, however.

"The main activity that removes sediment from the system is dredging," Windom said.

Whether on the coast or upland, the sediment removed by dredging is typically not used in ways that result in its return to waterways. The U.S. Corps of Engineers has recognized its part in this, Windom said, but has yet to change any policies regarding dredging based on the information.

The smallest factor of the three, although one on which state and local authorities can take action, is likely dams and reservoirs in North Georgia.

As North Georgia continues to grow, the population centers need more and more water for human use and irrigation. Atlanta's population in 2020, for example, has grown by around 10 times what it was in 1960.

"All of a sudden you have big cities like Atlanta where people are needing reservoirs," Windom said.

Coastal improvements to shore up the shoreline against storms and erosion are a fourth factor, he added. Those improvements can keep the marshland from migrating inland, but the benefits gained make it something of a tradeoff.

Sea level rise is an issue public and private entities take seriously, Windom says, but it is not likely something local officials can have much effect on.

The combination of the three factors means the marsh isn't getting enough sediment to keep itself above water.

"The thing is that the salt marshes are required to be within the tidal range. Submerged at high tide and above water at low tide. ... Now, if they're covered constantly, they'll ultimately die off," Windom says.

There is a solution, Windom says — being smarter about dredging and removing unregulated or unlicensed dams would likely be a substantial improvement.

For the Brunswick area, he suggested officials keep close tabs on dredging activity.

"I think what you do is you think about conservation of fine-grain sediment," Windom explained. "You evaluate what dredging is going on. What do you do that's moving sediment? If it's being put in upland, confined areas, I'd start thinking of more creative ways to handle dredge spoil like depositing it in open waters. Not where you're causing another problem, but where it will be dispersed and put back into the ecosystem."

He can't say for sure whether that fixes everything, but it's the best short-term, actionable option.

"There may not be a whole lot we can do about it, but we need to minimize losing suspended solids," Windom said.

Marshes serve a number of purposes, from breeding grounds and nurseries for marine life and functioning as a protective barrier from storms, says the Skidaway Institute. Windom further added that marshes are a bulwark against coastal erosion as well.

As important as they are now, Windom says there's a compelling argument that the marsh wasn't always as expansive as it is now

"Researchers in the northeast argue, and I've often thought this in the past, but my research doesn't get into that, that salt marshes are an artifact of the nearly colonial period of the U.S.," Windom said. "They deforested a large part of the piedmont region, particularly, plowed it up for row crop agriculture with very little attention to soil erosion."

Some estimate upwards of a meter of the soil in piedmont was disturbed, he explained. All of that soil that didn't settle got washed into waterways and carried to the coast over time as sediment, building up the marshland we know today.

"The researchers hypothesize that these salt marshes weren't here when the settlers came, on the East Coast or the Gulf Coast. ... It's difficult to prove that, but knowing about the process it seems like a reasonable understanding of how salt marshes got here in the first place," Windom said.