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Historic spillway opening sparks debate over potential environmental impacts

By Chaffin Mitchell, AccuWeather staff writer
May 15, 2019, 3:32:50 PM EDT

The historic second opening in a year of the Bonnet Carré Spillway last week has sparked concern about the effects it will have on Lake Pontchartrain, after marine wildlife washed ashore following the first opening.

The first opening of the spillway this year occurred on Feb. 27 and led to controversy regarding the impact on wildlife, because the spillway is known to have effects on the water quality in Lake Pontchartrain. Officials in Mississippi and Louisiana have been debating whether that contributed to the deaths of 45 dolphins and 25 turtles that washed ashore earlier this year, The Advocate reports.

Spring Flooding Lousiana

Workers open bays of the Bonnet Carre Spillway, to divert rising water from the Mississippi River to Lake Pontchartrain, upriver from New Orleans, in Norco, La., Friday, May 10, 2019. Torrential rains in Louisiana brought such a rapid rise on the river that the Army Corps of Engineers is opening the major spillway four days earlier than planned. Spokesman Ricky Boyett says the river rose six inches in 24 hours, with more rain expected through the weekend. (AP Photo/Gerald Herbert)

Now the debate is getting more attention because the Army Corps of Engineers reopened the Bonnet Carré Spillway again on May 10. This is the 14th operation of the structure since 1937.

When the spillway is opened, materials are deposited in Lake Pontchartrain and Lake Borgne. Since the lake is an estuary with a mix of fresh and salt water, releasing fresh water into these brackish and saline lakes has an immediate, short-term, adverse environmental effect.

The Army Corps of Engineers also try to limit spillway openings to minimize the impact of invasive freshwater species entering the Lake Pontchartrain basin.

However, according to the Army Corps of Engineers, the long-range effect is favorable because it simulates the natural flooding cycle of the river and provides a replenishment of valuable nutrients to the ecosystem. Spillway openings are strongly associated with increased oyster, crab and other fisheries production in both lakes for several years after the flood events.

“Trillions of gallons have come down from the Bonne Carré Spillway. That has changed the habitat, the food source and all sorts of changes for these animals,” Moby Solangi with the Institute of Marine Mammal Studies in Mississippi told WAFB.

Too much fresh water can stunt shrimp growth, which could lead to a later or less impressive season, a Louisiana Wildlife and Fisheries assistant secretary, Patrick Banks, told The Advocate. The rapid changes in temperature and salinity can push already stressed animals over the edge, such as dolphins and their calves. Some people are also concerned about the impact of fresh river water on oyster fisheries.


With each opening, the river deposits an average of 9 million cubic yards of sediment, mostly silts and sand, within the floodway. According to the Corps, these deposits are removed by private contractors and local government agencies for use as fill material in residential and industrial developments. This sediment is a valuable local resource since most of the surrounding region is near or below sea level.

While water levels on the Mississippi remain high, the flood control system in place is capable of handling more water.

"We have a system in place and that system is able to pass high water events far greater than what we have today," Ricky Boyett, an Army Corps of Engineers spokesman, told the AccuWeather Network. "We've never actually maximized what the system can do. So when we talk about the flood, that is a term for the high water; however, fortunately we're able to keep the water between the levees and prevent it from getting into homes and property."

The Lake Pontchartrain Basin Foundation (LPBF) and environmental groups monitor the lake during and after an opening to determine the effects the opening has on salinity and nutrient levels.

"The Lake's water quality is directly related to rainfall runoff. If it has rained in your area in the past three days, the runoff could pollute the lake," the LPBF website states.

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Any negative effects and imbalances could lead to toxic algae blooms that are harmful to humans and animal health.

"It's really important to get metrics and try to understand with each opening [of the spillway] what the level of environmental impact is," Dr. Brady Skaggs, the foundation's water quality program director, told the AccuWeather Network.

In the past, the lake has experienced blooms including blue-green algae. Large algae blooms also can cause low-oxygen dead zones at some locations on the lake bottom. When the algae dies, it sinks to the bottom and decomposes, which uses up oxygen.

The LPBF has been planning on gathering information about the effects of opening the spillway two years in a row.

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