Freshwater Diversions Help Wetlands, Fight Encroaching Oil [w/video]

NWF investigates how river diversions, when done properly, can help in oil spill situations.

10-26-2010 // Craig Guillot

 

Long used as a way to partially restore the natural connection between the Mississippi River and the wetlands, Louisiana's freshwater diversions found a new use in fighting oil during the Gulf oil disaster.

Before levees were built around the Mississippi River in 1920s, Louisiana's wetlands were nurtured by the natural flow of the river through its tributaries and annual flooding. John Lopez, PhD, director of the Coastal Sustainability Program of the Lake Pontchartrain Basin Foundation, said there are approximately 25 diversions on the state's coast. They take on a variety of shapes and sizes and range from small channels that were originally cut for navigation or fisheries to complex structures solely built for water diversion. While none were originally designed to carry sediment, minor sediment flow combined with the freshwater impact of stimulating plant growth has helped slow the rate of land loss.

Monitoring data from the early '90s showed that that due to the Caernarvon diversion in St. Bernard parish, the number of freshwater marsh plants dramatically grew over a period of three years. That, in turn, reduced marsh loss with a net increase of over 406 acres in a sampled area of 2,289 acres of marsh. Its benefits have accrued over time and other diversions such as David Pond in St. Charles Parish have shown similar results. Lopez said the diversions can be utilized in even more ways than they are now.

"There is a lot of engineering that can be done to improve the efficiency,” said Lopez. “Where the structure is located, how you operate it and the maximum propulsion are all very important."

Diversions also help bring marsh salinity levels to a more natural state, said Karla Raettig, director of National Wildlife Federation's Coastal Louisiana Restoration Project . As saltwater moves further into the wetlands, it can have a lethal effect on some of the reeds, shrubs and trees that help hold the land together. And when that goes, erosion accelerates.

"There are some differences of opinion in how successful they have been but there are many indications that [lowering] the salinity levels help save some of the marshes from saltwater intrusion," said Raettig.

Diversions are operated by the Louisiana Coastal Protection and Restoration Authority and were also used during the Gulf oil disaster to stop oil from entering the marshes. Some diversions such as the Caernarvon were opened to full capacity and allowed to flow at almost 8,000 cubic feet per minute. CPRA chairman Garret Graves said that the flow of freshwater helped keep some of the oil at bay and create a buffer zone around parts of the river delta.

"There is no question that the diversions can be beneficial,” said Graves. “The Corps models and observations indicated that they were, so I think there's another potential use for diversions in oil spills scenarios."

However, the diversions are not without complications. Some fishermen pointed out during the disaster that the surge of freshwater caused damage to their oyster beds and inshore fisheries. If the salinity of the naturally brackish areas declines, it can cause damage to the area’s saltwater species.

"It's a careful balance,” Lopez said. “You don't want to completely freshen the entire system. They need to be controlled to allow salinity to rebound."

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