Open Wide

River water hurts and helps Pontchartrain

Todd Masson

The U.S. Army Corps of Engineers opened the Bonnet Carré Spillway on April 11 and 12, unleashing 67,000 cubic feet per second of river water and raising the ire of some environmentalists.

The move was necessitated by heavy spring rains that flooded much of the Upper Mississippi Valley and pushed river current velocities in New Orleans and the River Parishes to extreme levels.

On the day of the opening, river volume in New Orleans was expected to move beyond the 1.25 million cfs level that is the benchmark for an opening, according to corps officials.
Leaving the spillway closed at such levels makes navigation hazardous from New Orleans to the mouth of the river, and it puts the Crescent City at risk of flooding from a levee breach.

The Bonnet Carré Spillway is located 28 miles upstream of New Orleans.

Corps workers removed 24 percent of the structure’s 7,000 needles, which allowed 67,000 cfs of river water to flood the spillway and, eventually, Lake Pontchartrain.
That’s what concerns Carlton Dufrechou, executive director of the Lake Pontchartrain Basin Foundation.

His organization has worked successfully over the last two decades to clean up Lake Pontchartrain, and Dufrechou is leery of anything that could jeopardize the health of the 630-square-mile lake.

“Just because we’ve done it like this for the last 80 years doesn’t mean it’s the best way to do things,” he said Monday.

Mixed with the turbid waters of the river is fertilizer run-off from the farmlands of the Midwest and Upper Mississippi Valley. This nitrogen and phosphorous super-charges algae in the water, resulting in massive algal blooms.

As this algae dies and decomposes, it robs the water of dissolved oxygen, and places at risk any marine life that is unable to escape to more favorable waters.

The most unfortunate aspect of this, in Dufrechou’s opinion, is that none of it need occur.

“In an abundance of caution, opening the Spillway was the right thing to do,” he said. “If the lake has to take a hit, the lake has to take a hit.

“But if we’d just think outside the box, we could do a lot of good with this water rather than a lot of harm.”

Dufrechou would like to see the waters diverted into the LaBranche and Frenier wetlands to the east and northwest, respectively, of the Spillway.

That way, the waters would slow and drop their sediment, which would build back the eroding wetlands, and the wetlands would, in turn, filter the fertilizer from the waters.

“We could turn this flood fight into a coastal-restoration project,” he said. “The algal blooms would be reduced significantly.”

Such a project could benefit the wetlands annually.

“The Bonnet Carre would still be a flood-protection project, but we’d be doing it in a way that is much more environmentally sound,” he said. “It would be an opportunity for us to capitalize on the high river stage every year.”

Mark Schexnayder, a marine biologist with the LSU AgCenter and Louisiana SeaGrant, agrees with Dufrechou that filtering the water through the LaBranche and Frenier wetlands would be a positive.

However, his research shows that the benefits of Spillway openings — even with the water funneled directly into the lake — greatly outweigh the negatives.

“The lake system is not sustainable with salinity levels high enough to kill cypress trees all the way up in Lake Maurepas,” he said. “(Opening the Spillway) is more of a return back to the native, natural state of the lake.”

Schexnayder would like to see the Spillway opened annually to help transform the lake back into a thriving, brackish-water environment.

“Right now, opening the Spillway is a shock to the system,” he said, “because it’s not open all the time.

“If (the lake) had the annual riverine influence, the whole system would compensate.”

With annual openings, Schexnayder said creatures would thrive that eat the blue-green algae that causes such problems during sporadic, episodic openings.

In the short run, however, Dufrechou is hoping for dry conditions up north, which will allow the corps to close the facility soon, thereby limiting the potential negative impacts to the lake.

“At the current rate of flow, it’ll take about 30 days to completely displace all the water in the lake,” he said.

Shrimp, crabs, speckled trout and other marine species will move out of the lake via Chef Pass, the Rigolets and the Industrial Canal, and they won’t return until conditions are favorable.

How long that takes depends on weather conditions after the closure.

“We want to see lots of wind,” Dufrechou said. “What we don’t want to see is a hot June and July with a flat-calm lake.”

Late-season cool fronts would also help to keep the lake cooler than it would get in normal years, which would limit algal growth, Dufrechou said.

Either way, Schexnayder expects fisheries productivity to rise in the lake as a result of the Spillway opening.

As evidence, he points to a study that found shrimp harvests rose in 1997, the year of the last Spillway opening