The Mississippi -- Atchafalaya System

Wednesday, March 11, 2009
New Orleans, LA

The Mississippi River basin, the land the river drains, is more than 12 million square miles, more than 40% of the land area of the USA, all or parts of 31 states. It’s the third largest river basin in the world, behind only the Amazon and the Congo. It’s average discharge volume is almost a half million cubic feet of water per second — every minute, hour, day, week, month… all year long. It’s an amazing and awesome thing.

All that water races toward sea-level here in Lower Louisiana. Throughout geologic history the river has built a massive delta area where the silts and sediments carried south are deposited at the point the river meets the sea. All the land area of Southern Louisiana was built up in this manner. But rivers naturally change. In delta areas like this when the main flow of the river has deposit sediments in one place for a period of time, building up and raising the river bottom, other nearby potential channels are now relatively lower and the river will naturally shift it’s flow to a new channel in a shorter and easier path to the sea.

In Lower Louisiana the Mississippi has a major distributory, another outlet to the sea — the Atchafalaya River. On Tuesday, as we drove from Abbeville to New Orleans, we drove through the various swamps and stream channels of the Atchafalaya basin, an area that makes up the largest swamp in the USA. Few roads traverse these lowlands. Both I-10 on the northern end of this area and US90 to the south (the road we took yesterday) are mostly elevated roadways, bridges really, for dozens of miles at a stretch.

Over the eons of time the main flow of water has changed, alternated, between the current Mississippi channel and the Atchafalaya, numerous times. As one becomes silted up the main channel shifts to the other.

After hundreds of years of flowing down past Baton Rouge and New Orleans, and silting up the delta in that area, the river has begun to shift more and more water to the Atchafalaya. During the 1950’s the US Army Corps of Engineers built a series of floodgates and control structures where the Atchafalaya River flows from the Mississippi. Called the Old River Control Structure, it’s goal is to maintain a 70%/30% split in the amount of water that flows down the Mississippi Channel and the Atchafalaya Channel respectively, and prevent the Atchafalaya from becoming the main channel. There’s a tremendous amount of development and economic activity that would be left high and dry if that happened.

But you can’t fight Mother Nature and win, even if you’re the US Government. As the current old river delta continues to silt in, it becomes harder each year to prevent the Atchafalaya from capturing the main flow of the Mississippi. Eventually, the Mississippi will decide these man-made structures aren’t up to the task of keeping the river controlled. When that happens the river will make it’s move to favor the Atchafalaya.

It’s only a matter of time.

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