When Davis Pond freshwater diversion opened up in 2002, it was a result of decades of work to get additional fresh water from the Mississippi River into Barataria Bay for fisheries enhancement.
As a relatively small diversion taking water from the upper layers of the river without much sand, it wasn’t expected that much land would be built. Small deltas have built, though. Two research projects this spring are going to try to answer how much land has been built and how sediment from the river is moving through this system.
“Despite all the reasons it shouldn’t do this, the river can still build land,” said Alisha Renfro, staff scientist with the Mississippi River Delta Restoration Campaign and the National Wildlife Federation.
“This is a system people thought was too small to build land,” said Alexander Kolker, assistant professor at Louisiana Universities Marine Consortium who teaches through Tulane University.
Lindsay Nakashima, with the National Audubon Society, will use high-altitude photographs to quantify the amount of land gain since the diversion opened. A separate, but companion project by Kolker attempts to determine how sediment moves through the system.
“It’s really an interesting area,” Renfro said.
Early on, there were problems getting enough water out of the collection area fast enough, causing the diversion to operate poorly. The diversion began operating more efficiently after guide levees were raised and more holes were cut in the outflow area to the lake, and now the average flow is about 5,000 cubic feet per second. The diversion has a capacity to flow up to 10,650 cfs.
The diversion was originally authorized by Congress by the Flood Control Act of 1965 to help improve oyster production in the area. The plan was changed in 1974, 1986 and 1996 to eventually become associated with pushing back salt water that was encroaching from the south due to coastal land loss.
Although there have been a number of studies done on the earlier Caernarvon diversion along the Mississippi River, Renfro said Davis Pond seemed to have been largely ignored in comparison.
The research work will examine how the Davis Pond diversion built land in the first place. In other diversions designed to build land, the emphasis is always on how much sand can be pushed into the marsh, Renfro said.
“But I think we fail to talk about the fine materials and that’s 80 percent of what the river carries,” she said.
What these studies will help to shed light on, she said, is how much sediment is coming through the diversion, how and where that fine settlement goes, and how much land can be built when the focus is fine sediments.
“They’re (freshwater diversions) not what we’re going to be building in the future, but they can tell us something,” Renfro said.
Kolker’s project will be an extension of work he’s been doing around the coast in which he uses isotopes to determine if the sediment in an area is recently deposited from the river or sediment that’s been moved around within a system. The isotope Kolker studies sticks to mud and has a half-life of 53 days. Old sediment, he explains, would not have any of the isotope on it because it would have degraded.
“We want to know something more concrete than ‘we have land in that lake that we didn’t have before,’ ” Kolker said.
Past projects have been done in Mardi Gras Pass, Cubit’s Gap and West Bay along the Mississippi River as well as projects in the Atchafalaya River basin.
Time and time again, he said, the research shows that these areas can keep pace with relative sea level rise, a combination of sea level rise and the sinking of coastal land.
“It suggests that the diversions have promise,” Kolker said.
Although there have been questions about how far sediment can actually travel from the river once the water slows down, studies in other areas show that may not be as much of a concern as first thought.
Studies at West Bay showed sediment traveled into the bay for more than nine miles and at Cubit’s Gap more than three miles, Kolker said.
“That’s quite far,” he said. “That’s a lot farther than a lot of people thought.”
Efficiently trapping sediment for land building purposes means not only grabbing as much sediment as possible, but also getting that sediment deep into the marsh where it’s needed.
The study to be done at Davis Pond will provide more information about how well Louisiana coastal marsh can keep pace with relative sea level rise. So far, results look good after seven years of study.
“In Louisiana, we don’t have as much sediment as we used to have, but we still have enough sediment if it’s used wisely,” Kolker said.
Follow Amy Wold on Twitter, @awold10.