Spillways and Floodways

Why do we open the spillway and what does it affect? Update


Picture of a previous Bonnet Carre Spillway opening courtesy of the Army Corps of Engineers.


President Trump has approved a federal disaster declaration for the state of Louisiana flood fight.

Disastrous flooding farther north along the Mississippi River, caused the Bonnet Carre Spillway to open for a second time this year. After closing the spillway on April 11th, the US Army Corps of Engineers began reopening it on May 10th.

Soon after, the river rose again, due to continued rainfall in the Mississippi River Valley. In reaction, the Army Corps announced that it will open the Morganza Floodway on Thursday, June 6th.


Spillways and Floodways

The Bonnet Carre and the Morganza floodway are two very different structures.

Built in just two years, the Bonnet Carre Spillway was engineered in response to the devastating 1927 flood of the Mississippi River Valley. It's made up of 350 concrete bays which are pinned with 7,000 wooden needles or slats. When the decision is made to open the Bonnet Carre Spillway structure, two cranes move along the top of the concrete bays removing one needle at a time. Once open, sediment-laden floodwaters from the Mississippi first flow through the control structure and six miles of levee-lined floodway before reaching Lake Pontchartrain. Located in St. Charles Parish, the spillway structure was designed to help prevent the Mississippi River from overtopping levees adjacent to populated communities like New Orleans. The river levees near New Orleans were only constructed to withstand a flow of 1.25 million cubic feet per second- anything faster and the levees might fail.

Similarly, the Morganza Floodway was designed in reaction to the 1927 river flood with the intention of protecting the communities of Southern Louisana. Built in 1954, the control structure is about 4,000 feet long and is outfitted with 125 gate bays. The Morganza structure directs river water through the Morganza floodway, the Atchafalaya river basin floodway and ends at the Gulf of Mexico. At its maximum flow, it can divert almost 2.5 times more water than the Bonnet Carre.

Bonnet Carre Side Effects

When the river reaches its flood stage due to rain and snowmelt throughout the Mississippi River Valley, the opening of the Bonnet Carre Spillway is necessary to protect communities downstream.

Although changes in salinity caused by spillway floodwaters will not cause permanent damage to the estuarine environment of Lake Pontchartrain, recreation on the lake may suffer. Recreators should be mindful of risks due to possible algal blooms in the lake.

The amount of river water that flowed through the open spillway was nearly enough to fill the lake two times. Lake Pontchartrain is an estuary that fluctuates between fresh and salt water, and that offers a unique habitat. Consequently, a flush of fresh water from the spillway has temporarily altered that habitat and changed the fish populations in the lake.

Although river water has displaced a great deal of Lake Pontchartrain, the lake will soon start to return to its usual habitat once the spillway is closed and the elevation of the river falls below the height of the structure. In the past, this process required four to six months to complete.

A potential algal bloom is one consequence of fresh nutrient-packed waters filling the lake, in warmer weather. Such as in 2011, when an algal bloom followed the opening of the spillway. Since then, we've improved our understanding of bloom dynamics, thanks to new methods for testing and regulatory guidance. Additionally, citizen science helps many across the world to crowdsource information on potential blooms. Help us stay vigilant, download the BloomWatch App! With the BloomWatch App, you can report algal blooms, upload pictures to a database, and compare the images to confirmed blooms. In effect, you can help your community respond to blooms by photographing and sharing the location of any algae you might see growing in the lake.

Additionally, because the lake drains into the Gulf of Mexico, the effects of fresh water may be felt in the bayous and bays further south. Through our biweekly Hyrdocoast maps, we have been reporting on the movement of the Spillway discharge since the first opening. The maps illustrate contours of salinity and other water assessments that can aid fishermen and regulators alike in the movement and mixing of freshwater across the basin. Learn more about how dolphin may be affected.

Sidebar on Sediment


Sediment is a crucial resource for our coast and the landloss crisis we are facing. Similar to the coast, the marshes that line the western shore of Lake Pontchartrain suffer from landloss and continue to weaken. Levees built along the Mississippi, as well as the guide levees along the Bonnet Carre Spillway, starve these marshes from the replenishing flow of sediment-laden river water.

Fortunately, in the current Coastal Master Plan, there are two projects that plan to address this. One will dredge a conveyance channel in the western guide levee of the Bonnet Carre Spillway, to allow floodwater and sediment to flow into the adjacent wetlands. The other will erect a new levee to help to protect neighboring LaPlace communities from the flow of the planned channel.

A collection of satellite imagery from the January 2016 Bonnet Carre Spillway opening, showing the movement of sediment from the river, through the spillway and Lake Pontchartrain, and out to the gulf. See more here.

Documenting Change Citizen Science Resources

What if you spot a potential algal bloom?

Help us stay vigilant, download the BloomWatch App! With the BloomWatch App, you can report algal blooms, upload pictures to a database, and compare the images to confirmed blooms. In effect, you can help your community respond to blooms by photographing and sharing the location of any algae you might see growing in the lake.

What to do if you spot a marine mammal or sea turtle: If you see an injured or sick marine mammal or sea turtle immediately call the CWN stranding hotline at (504) 235-3005.


  • Contact information
  • Location including coordinates
  • Nature of the report
  • Pictures (if possible)
  • Estimated the length (if possible)
  • Report if the dolphin is floating “free” or stranded on a beach.
  • Report directly through email or phone (do not use social media).


Audubon Nature Institute: Gabriella Vazquez (Marine Mammal and Sea Turtle rescue Coordinator)

(504) 235 3005gvazquez@auduboninstitute.org

LDWF Marine Mammal Stranding Coordinator: Mandy Tumlin mtumlin@wlf.la.gov