MISSISSIPPI — The Bonnet Carré Spillway is a critical component of the Mississippi River’s flood control system that is designed to protect communities, particularly New Orleans, from devastating floods.  

Constructed five years after the 1927 flood that ravaged Louisiana, the spillway is managed by the U.S. Army Corps of Engineers as part of a broader effort to prevent similar disasters. 

“The Bonnet Carré Spillway is a flood risk reduction measure,” said Deborah Centola, chief of water management for the USACE’s New Orleans District. “It’s part of the Mississippi River and Tributaries project, which its main purpose is for flood control.”  

This process is essential for protecting lives and infrastructure. 

Without the spillway, the levees could be overwhelmed, endangering not just New Orleans but also other communities along the Mississippi River. 

“Life safety is just … you can’t put a price on that,” Centola said. 

Who decides to open it? 

The decision to open the Bonnet Carré Spillway is ultimately made by the president of the USACE Mississippi River Commission — based on recommendations from the New Orleans district engineer. 

The process involves close monitoring of the river’s flow and height, particularly in New Orleans.  

According to the USACE, the levees behind the Bonnet Carré Spillway can handle 1.25 million cubic feet of water per second. 

Once that threshold is met, the spillway opens and is capable of diverting 250,000 cubic feet of water per second. This is equivalent to about 20% of the river’s flow — or about 1.87 million gallons of water every second. 

To visualize this, the amount of water diverted through the spillway could fill the entire Caesars Superdome in New Orleans in just eight minutes and 20 seconds. 

What happens when it’s opened? 

Opening the spillway has significant effects downstream, particularly on the Mississippi Sound.  

The diverted freshwater creates a sudden and drastic reduction in salinity levels, which can devastate marine ecosystems. The rapid influx of freshwater, filled with pollutants from 31 states and two Canadian provinces, causes widespread harm to marine life. 

“When you bring in polluted water from 31 states and two provinces in Canada and push it in large quantities, it disrupts the ecosystem,” said Dr. Moby Solangi, director of the Institute for Marine Mammal Studies in Gulfport. “Every time they’ve opened (the spillway), we’ve had major kills in fisheries: oysters dying, shrimp dying… and dolphins.” 

He likened the ecological impact to a natural disaster: “A hurricane comes in and goes away in a few hours, but it takes years to rehabilitate land structures. The same thing happens when you have a disaster like this, it takes many, many years for the ecosystem to recover.” 

Solangi said the Mississippi Sound is a critical nursery for species like dolphins, oysters and shrimp, as well as the endangered Kemp’s Ridley sea turtle. 

“You’re killing the nursery and then expecting things to be out there for you,” Solangi said.

After the 2011 opening, Mississippi’s oyster harvests dropped by 80-100%, resulting in an estimated $58 million in economic losses between 2011 and 2014. 

Jobs were also affected; the industry lost between 145-324 jobs annually during that period, according to a Mississippi State University study. 

More recently, following the spillway’s 2019 openings, a MSU research team documented extensive losses across Mississippi’s seafood and tourism industries. 

Their research found a complete collapse of the eastern oyster population, with brown shrimp harvests declining by 74%. These losses resulted in $12.2 million in direct economic impacts to the seafood industry alongside substantial job and wage losses. 

Additionally, the tourism sector suffered an estimated $4 million in lost revenue, with ripple effects on local tax revenues and broader economic stability.  

Beach closures due to harmful algal blooms and poor water quality further compounded the losses. Algal blooms, driven by nutrient-rich freshwater from the spillway, led to a reduction in tourism, causing losses of over $3 million in wages across coastal communities. 

Solangi captured the gravity of the situation: “It’s an existential threat. If all our ecosystems collapse, the economy and the marine fisheries will suffer.”   

Seeking solutions through the Comprehensive Lower Mississippi River Study

Recognizing the challenges, the USACE is conducting the Comprehensive Lower Mississippi River Study to reassess its flood control strategies. 

“The river is not the same river in the same climate, in the same atmosphere, that it was 100 years ago. So we really are challenged with how best to operate it,” Centola said. 

The study — which spans seven states — explores ways to improve the operation of flood control structures, including the Bonnet Carré Spillway, the Morganza Spillway in Louisiana and the Old River Control Complex, which is also in Louisiana. 

So far, the USACE has gathered public input, developed models to assess water and sediment dynamics, and started evaluating potential changes to flood control operations. 

The next major milestone is expected in 2026 when the study will identify a “Tentatively Selected Plan” to outline preferred strategies for managing the river system. 

Looking ahead 

The Bonnet Carré Spillway highlights a bigger challenge, according to experts: How do we manage natural systems to protect people without causing long-term harm to the environment?  

With more frequent and intense flooding, the spillway is being opened more often than in the past.  

“It didn’t used to be common. It used to be every five or six years. But now, because of climate change, you’re having it more frequently,” Solangi said. 

Experts agree that the spillway’s importance in protecting lives and infrastructure can’t be overstated, but the costs to Mississippi’s environment and economy are just as real.  

As the USACE’s Centola noted, “We hear everybody’s concerns. We’re looking to see what we can change.”  

The Comprehensive Lower Mississippi River Study, along with ongoing restoration efforts and new ideas, offers hope for finding a better balance between flood control, economic needs and environmental health. 

For now, the Bonnet Carré Spillway remains a critical tool for flood protection, one that saves lives — but also creates significant challenges for the Mississippi Coast, according to critics.