Over the next few days, water will crawl through the swamps of Louisiana's Cajun country, chasing people and animals to higher ground.
The floodgate was opened this weekend for the first time in nearly four decades, shooting out like a waterfall, spraying 6ft into the air. Fish jumped or were hurled through the white froth and what was dry land soon turned into a raging channel.
The opening of the spillway diverted water from Baton Rouge and New Orleans, and the numerous oil refineries and chemical plants along the lower reaches of the Mississippi. Shifting the water away from the cities eased the strain on levees and blunts the potential for catastrophic flooding.
The water will flow 20 miles south into the Atchafalaya Basin, and from there it will roll on to Morgan City, an oil and seafood hub and a community of 12,000.
Most people had cleared out of the town of Butte LaRose last night, as the waters closed in. No evacuation orders had been issued there, but last week residents were packing their bags.
"It's almost like a ghost town," said Chalmers Wheat, 54, who, along with his twin brother, Chandler, was doing what he could to protect his home before going to stay with his father in Baton Rouge 40 miles away.
The Morganza spillway is part of a system of locks and levees built after the great flood of 1927, which killed hundreds and left many more without homes. When the Morganza opened, it was the first time three flood-control systems have been unlocked at the same time along the Mississippi River, a sign of just how historic the current flooding has been.
Earlier this month, holes were blown into a levee in Missouri to employ a similar cities-first strategy, and a spillway north-west of New Orleans was also opened.
Snowmelt and heavy rain swelled the Mississippi, and the river has peaked at levels not seen in 70 years. About 25,000 people and 11,000 structures could be affected by the oncoming water, and some people living in the threatened stretch of countryside - an area known for fish camps and a drawling French dialect - have already fled. Cajuns are descendants of French-speakers who fled Acadia, an area covering what is now Canada's Maritime provinces and northern Maine in the US, as the British took control in the 18th century.
In the nearby community of Stephensville, rows of sandbags were piled up outside nearly every home.Merleen Acosta, 58, waited in line for three hours to get her sandbags filled by prisoners, then returned later in the day for more bags.
This is the second spillway to be opened in Louisiana. The army used cranes to remove some of the Bonnet Carre's wooden barriers, sending water into the massive Lake Ponchatrain and eventually the Gulf of Mexico.
By yesterday , all 350 bays at the 7,000ft Bonnet Carre structure were to be open. The Morganza, a 4,000ft long structure built in 1954, was expecting to only open up about a quarter of its 125 gates.