America by Air: Bonnet Carré Spillway

Editor’s Note: This article previously appeared in a different format as part of The Atlantic’s Notes section, retired in 2021.
Eric Bruning

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I took this photo shortly after departure from New Orleans on January 11. The lower part of the photo shows the Bonnet Carré Spillway, which had just been opened to divert excess water from the Mississippi River to Lake Pontchartrain. I was returning from the American Meteorological Society’s annual meeting, and it seemed fitting to have a chance to observe this rare event. I was struck by how many public agencies’ quiet, routine efforts resulted in an accurate forecast of water levels so that action could be taken to protect the citizens of New Orleans.

The spillway was in the news this weekend when a massive fire destroyed the train tracks that run across it. Here’s some background on the spillway itself:

When opened, the control structure allows overflow volume to flow into Lake Pontchartrain. The lake’s opening to the gulf is sufficient to absorb and dissipate any conceivable volume of flood flow. Thus, the flood surcharge portion of the water from the Mississippi is divided between the main river and the diversion channel; with the surcharge bypassing the New Orleans metropolitan area, resulting in the Mississippi being lower (through that area) than it could have been; and reducing the stress on the area’s levees that line the river.

The spillway was built in response to the Great Mississippi Flood of 1927 that inundated much of the Mississippi River basin. It was first opened during the flood of 1937, and ten times thereafter through 2016 to lower river stages at New Orleans. The most recent opening began January 10, 2016, when river levels in New Orleans were predicted to approach the flood stage of 17 feet (5.2 m).

But Amy Wold, writing in The Advocate, Louisiana’s largest daily newspaper, calls the effects of the spillway a “mixed bag”:

In addition to sediment, the colder and fresher river water also carries nutrients from upriver agricultural practices, carries the possibility of invasive species and has at least the temporary effect of moving out certain species of fish that want more-brackish water. At the same time, the additional nutrients can provide a base for better growth of other species, the cold water may mean the impact on oysters will be lessened and nutrient blooms apparent in previous spillway operations may be delayed, if they happen at all. Only time will tell, because a January opening of the spillway hasn’t happened since 1937. Although not unprecedented, the early opening could mean different effects will be seen in the coming months.