US army engineers have opened floodgates in Louisiana that will inundate more than 7000 square kilometres of land in an attempt to protect cities at risk from the rising Mississippi River.
The Morganza Spillway opened at 1500 local time to ease pressure on Baton Rouge and New Orleans.
It's the first time in 40 years that the level of the Mississippi has forced the floodgate to be opened.
About 25,000 people could be adversely affected reports the BBC.
Two other floodgates were opened earlier this month.
The US Army Corps of Engineers warned that if the spillway was not opened, New Orleans could be flooded by about six metres of water.
Colonel Ed Fleming says it's a major operation - "an historic day not only for the entire Mississippi River," he says, "but for the state of Louisiana."
The trigger for the spillway opening was when 42,500 cubic metres of water per second was flowing down the Mississippi River at Red River Landing, just north of the spillway.
The water will be channelled out of the Mississippi into the Atchafalaya river basin, a low-lying area of central Louisiana.
Water will flow south, flooding homes and farms in the state's Cajun country.
Over several days, the water should run south to Morgan City - where workers are rushing to reinforce levees - and then into the Gulf of Mexico.