Louisiana Governor Bobby Jindal said the trigger for the opening of the spillway would be when 42,000 cubic metres of water per second was flowing down the Mississippi at Red River Landing a point that had nearly been reached on Friday.
The opening of the floodgate will release water over thousand of acres of farmland and rural towns prompting warning of flash floods from forecasters and urgent evacuation orders in Louisiana and Mississippi.
According to flood predictions, a flood as high as 4.57 metres is set to hit the small Louisiana community of Butte La Rose when the spillway is opened.
The worst floods to hit the central United States in more than 70 years have already swallowed up thousands of homes, farms and roads in Illinois, Missouri, Kentucky, Tennessee and Mississippi.
Heavy rains last month filled rivers and creeks already swollen from the melting of a thick winter snow pack and which are now backing up because the Mississippi is so full.
The Mississippi is the third-longest river in North America and its watershed is the fourth-largest in the world, according to the US National Park Service.