Opinions clash on the effectiveness of diversion projects

An article in Nature Geosciences magazine touts diversions of freshwater and sediment from the Mississippi River as the savior of Louisiana’s coastline.

An article from Nature magazine touts these same diversions as a cause of destruction of coastal marsh.

Both magazine covers graced one slide in Chris Swarzenski’s presentation during a discussion of diversions Friday in Baton Rouge to make a point.

“There’s no contradiction,” said Swarzenski, research hydrologist with the U.S. Geological Survey’s Louisiana Water Science Center.

Each view can be defended because the effectiveness of river diversion differ largely because of where it is located.

“Diversions have to play a role in coastal restoration. This is not a yes or no.” Swarzenski said. The question is how that should happen.

In a discussion about the merits of river diversions, there has been a tendency to say the diversions are all good or all bad, but both river and sediment diversions have benefits and downfalls, Swarzenski said.

His presentation was one of six given on diversions during the annual meeting of the Louisiana Association of Professional Biologists and the Louisiana Chapter of the Wildlife Society in Baton Rouge on Friday.

The discussion about river diversions, especially as the state moves forward with plans for a large diversion near Myrtle Grove in Plaquemines Parish, needs to focus on how the work can be done for the best benefit.

He said diversions fall into two categories: One is for water quality purposes to push back intruding salt water to help rejuvenate marshes, and others that focus on bringing sediment into coastal areas.

“We have this concept that if we just throw river water on it, it will rejuvenate a marsh,” he said. “The reality has turned out to be quite opposite.”

Eugene Turner, professor in the Department of Oceanography and Coastal Research at LSU, said his research shows that large amounts of nutrients in the water coming down the Mississippi River can mean less root growth in the marsh because roots do not have to reach deep into the ground, leading to soil instability and more damage during tropical storms.

“We should be testing our assumptions,” Turner said. “And we need to go back and look at why these haven’t worked, because they haven’t.”

Still others pointed out that coastal marsh areas that seemed to fare poorly were flooded too much, even if they did receive more sediment than other areas.

“I would say this is more important than the nutrient issue,” said John Day, professor emeritus in the Department of Oceanography and Coastal Research at LSU.

The key, Day said, was to make sure marshes had time to consolidate sediment.

Others at the conference disagreed with that assessment, claiming fresh water helps coastal plants grow because the lower salinities help the plants use more nitrogen in the water.

Andy Nyman, associate professor with the LSU Agricultural Center and the School of Renewable Resources, said that for areas of the coast where sediment diversions have occurred, such as sections at the mouth of the Mississippi River and the Wax Lake Outlet by the Atchafalaya River, “we know sediment diversions work. That’s not the issue.”

In addition, Nyman took issue with the idea that nutrients cause plants to produce fewer deep roots in favor of growing foliage above ground. He said research going back to the 1970s shows that’s not the case.

“You add nutrients and plants make more roots, not less,” he said.