On March 8, the Bonnet Carre Spillway was opened to relieve pressure from the levees encasing the Mississippi River. A flood warning went into effect on March 14, and forecasters predicted it to continue until March 28.

The river was approaching the 17-feet flood stage, but the National Weather Service lifted the warning because its crest projections lowered to half-foot below the flood stage level. Only three days later, the flood warning was put back into effect. The National Weather Service said the Mississippi River reached 17 feet at 10 a.m. on Monday.

For only the 12th time in 90 years, the Bonnet Carre Spillway in New Orleans is being opened to prevent the Mississippi River from flooding #wmc5 >>https://t.co/hkPJ4vR7zZ pic.twitter.com/2zSrXyRLre

— Action News 5 (@WMCActionNews5) March 8, 2018

While the term “flood stage” may seem intimidating, the event has a negligible impact on New Orleanians. Flood stage is 17 feet, but the levees are 20 feet high. The extra height is called freeboard, and gives the city time to prepare for the high water level. It’s only one way that New Orleans is protected from flooding. 

 “[High-river event protection is] mostly in the domain of the U.S. Army Corps of Engineers, with local support coming from parish and regional flood-protection authorities,” Richard Campanella, senior professor of practice in architecture and geography, said

The Mississippi River is high today at 14.5 feet, 2.5 ft below flood stage, in New Orleans at St. Louis Cathedral because of rain upstream. Army Corps of Engineers @TeamNewOrleans will open the Bonnet Carré Spillway on Thursday to relieve flooding risk @theadvocateno @wwltv pic.twitter.com/lY4uoBkt0N

— Matthew Hinton (@MattHintonPhoto) March 6, 2018

The corps began patrolling the river on Feb. 26 because the Carrolton river gauge reached 11 feet. Campanella said flood risk is reduced by opening “safety valves.” Starting with the Bonnet Carre Spillway that diverts “up to one-fifth of the water volume into lateral water bodies.” Since March 13, 130 bays were opened, diverting 132,000 cfs of water away from New Orleans. 

The river is not exceptionally high. It crests every spring, and last year it reached 16.48 feet. Campanella said the Mississippi River reaches this height about every four to seven years. This year, the water height is a result of heavy rainfall in the Ohio River Valley and surrounding areas in late January and early February on top of normal late-winter snowmelt.