In an attempt to control the Mississippi River, the Army Corps of Engineers created a relief valve of sorts above New Orleans — the Bonnet Carré spillway. The idea for the spillway came about after the Great Mississippi Flood of 1927. At a site in St. Charles Parish where the Mississippi River had breached the levee several times before, the Corps built a floodgate and a floodway that would take excess floodwater from the river to Lake Pontchartrain, reducing stress on the levees through the city. In 1928, the Corps started building a 1 ½-mile-long stretch of floodgates running parallel to the river, that could be opened in the event of a major river flood. The project was finished in 1931, and the gates were first opened in 1937.
The concrete gates have 350 bays with a total of 7,000 wooden needles. When needed, cranes lift the needles up to let the river water travel through. It takes about 36 hours to move all of the needles. Guided by levees in the spillway, the water travels six miles to the lake. The gates been opened a total of 11 times, the last time in 2016.