The Morganza Spillway opened at 1500 local time (2000 GMT) on May 14 to ease pressure on Baton Rouge and New Orleans.

This is the first time in four decades the level of the Mississippi has forced the floodgate to be opened. About 25,000 people and 11,000 buildings could be adversely affected.

Fed by rainwater and the spring thaw, the Mississippi and its tributaries have caused massive flooding upstream, and officials have said the flooding in Louisiana is the worst since 1927.

The US Army Corps of Engineers warned that if the spillway was not opened, New Orleans could be flooded by about 20ft (6m) of water.

Two other gates - the Birds Point-New Madrid Floodway and the Bonnet Carre Spillway - were opened earlier this month.

The Morganza Spillway, 45 miles (72km) north-west of Baton Rouge, was last opened in 1973.

The flooding is approaching records set 84 years ago when hundreds of people in the region died.

The American Red Cross has prepared shelters for thousands of people who are fleeing the region. Farmers in the region are expecting to lose their entire crops in a year of high prices for farm produce.