Water Flow, Elevation Important to Marsh Design, Resilience

A Louisiana Sea Grant-funded study found that the way we design and build marshes, especially decisions about elevation and water flow, can greatly affect how healthy and resilient they become over time.

“Our findings clearly showed that design matters,” said Tracy Quirk, associate professor in the Department of Oceanography and Coastal Sciences at Louisiana State University. Graduate student Kevin Stoner aided Quirk on her project, Evaluating Ecological Functions of Created Marshes in Louisiana to Inform Decisions about Elevations and Confinement. “Knowing how elevation and water movement affects marsh growth and sustainability can help us build better wetlands in the future,” she noted.

Three marshes along the north shore of Lake Pontchartrain were studied: one that was created and surrounded by dykes (confined), one that was created without dykes (unconfined) and a nearby natural marsh. The two created marshes were built in 2018 using dredged material.

The unconfined marsh, which allowed water and sediment to move more freely, showed higher sediment accumulation, lower salinity during dry periods and a higher variety of plants species. The confined marsh had less water movement. It also had fewer helpful soil processes and was more likely to be taken over by less desirable plants like cattails and woody shrubs. The natural marsh, which was mostly covered with Spartina patens, had fewer plant species overall and relied more on plant roots and organic matter to build elevation, since it didn’t receive as much new sediment.

In the confined marsh, elevation levels also varied with some areas too high and others too low. The unconfined marsh had more stable elevations that better supported wetland plants. While all three marshes had some plant species in common, the mix of plants depended heavily on elevation and how water flowed through each site.

“Including natural water flow in marsh design improved sediment buildup, plant growth and long-term success,” said Quirk. “As we work to restore wetlands across Louisiana, how we build them will make a big difference.”