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Today the Army Corp of Engineers, led by Major General Michael J. Walsh (president of the Mississippi River Commission) gave the go-ahead on the crucial decision to open the floodgates at the Morganza Spillway.

The Corps was waiting for an important threshold. The trigger for opening the Morganza is when the flow of water by the Spillway exceeds a threshold of 1.5 million cubic feet per second.  That occurred today.

It is only the second time the Morganza Spillway has been opened and the first time since 1973.

Read: What caused the river flooding?

The decision was a long-time coming as the millions of residents that make up Baton Rouge and New Orleans waited patiently the majority of the past week  They were well aware of the significance of this spillway and its consequences when opened (or not).

Still, the decision is not easy one to make.

When the Morganza is opened, you are purposely flooding some to save many. Yes, you may save Baton Rouge and New Orleans from waters spilling into those cities however you are creating widespread inundation of the Atchafalaya Basin. 

The basin is the heart of Cajun country in south-central Louisiana.  The AP reports it is home to some 25,000 people in an area known for small farms, fish camps, crawfish and a drawling French dialect.

Now opened, the waters from the bloated Mississippi River will surge south to the Gulf of Mexico through the Atchafalaya River and divert floodwater from the river into the basin's swamplands, backwater lakes and bayous.  Several thousand homes will likely be flooded.

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On the other hand, if the spill was not opened the bulging waters of the Mississippi would have likely caused levees to fail along the river from Morganza to Plaquemines Parish, including the majority of Baton Rouge and New Orleans.

The Washington Post quotes Oliver Houck earlier this week, a law professor at Tulane University who has long been involved with the spillway, "They’re going to have to open up Morganza, not for the height, but for the pressure on the levees, it's going to remain high for weeks and weeks. And that’s the scare. All it takes is one weak spot"...in a levee to flood New Orleans.

The imminent flooding of the Atchafalaya Basin was known for days before today's decision.

On Thursday, during a news conference in Baton Rouge, Gov. Bobby Jindal urged residents in affected areas (especially the Atchafalaya Basin) to prepare for evacuations if called for by their local elected leaders.

"Based on various inundation maps, you're looking at roughly 3 million acres that will be impacted, be underwater," when the floodway opens, Jindal said. "That includes about 18,000 acres of cropland just within the Atchafalaya basin." Morgan City and other population centers in Acadiana are protected by floodwalls.

Now we wait and track the progress of both the floodwaters filling up the basin and the downstream Mississippi River waters that will now be fortunately less bloated as they pass through Baton Rouge, New Orleans and Plaquemines Parish.

Senior meteorologist Jon Erdman has more on the consequences of Morganza Spillway opening and analysis on other spillways in the Lower Mississippi River basin including the Bonnet Carre.

The Weather Channel will have continuing coverage on this developing situation and weather.com will post updates on our Flood Tracker page.