The Bonnet Carré Spillway is a flood control operation in the Lower Mississippi Valley. Located in St. Charles Parish, Louisiana, about west of New Orleans, it allows floodwaters from the Mississippi River to flow into Lake Pontchartrain and thence into the Gulf of Mexico. The spillway was constructed between 1929 and 1931, following the Great Mississippi Flood of 1927, and has been designated as a National Historic Civil Engineering Landmark by the American Society of Civil Engineers. Bonnet Carré Crevasse The Bonnet Carré Crevasse was one of several levee breaches in the Bonnet Carré area in 1871. Local drainage systems were unable to contain the floodwater and strong winds caused the water to enter Lake Pontchartrain and surrounding urban areas. The river levee was not restored until 1883. Overview The Bonnet Carré Spillway consists of two basic components: a control structure along the east bank of the Mississippi River and a floodway that transfers the diverted flood waters to the lake. The control structure, located between the towns of Montz and Norco, is a mechanically-controlled concrete weir that extends for over a mile and a half parallel to the river. 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. Confined by guide levees, the floodway stretches nearly to Lake Pontchartrain, with a design capacity of . The spillway is crossed by U.S. 61 and Interstate 10. 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 twelve times thereafter through February 2019 to lower river stages at New Orleans. The 2019 opening began February 27, 2019, when river levels were predicted to rise to minor flood stage in New Orleans on March 19, 2019. The Army Corps of Engineers initially opened 38 of the 350 bays and increased this number to 206 bays when the flood crest arrived later in March.