As you might have heard from the news, the Army Corps of Engineers has opened the
Morganza Spillway to divert much of the high water on the Mississippi River into the Atchafalya Basin.
Some 15,000 people may be flooded out -- to prevent that happening to some 3 million plus people. Both the Atchafalaya and the lower Mississippi have lots of agricultural land, but the latter also has a series of industries and one of the nation's busiest collections of port facilities.
The Atchafalaya is, hydrologists figured out by the 1940s, where the Mississippi wants to change its course and go (as it kept changing its exact route to the Gulf every so often before humans started messing with it), and it would have shifted there at least by 1973 on its own.
Good article from The New Yorker from back in '87 gives good background:
The Control of Nature - ATCHAFALAYA by John McPheeReally, the roots of the dilemma dates back to the French and Spanish Colonial era. The colonial era engineers really didn't understand the scale of the river-- much larger than anything they'd encountered, as the 3rd largest river system on the planet-- and treated it as the same thing as smaller rivers they were used to, just somewhat bigger. The land grants given up and down the river had the proviso that they had to levee off the river in their area-- slowly attempting to levee the whole system. And as more progress was made towards that goal, the more the Mississippi's high water became physically higher. They should have had minor levees if that for most of the agricultural area, with larger ring levees around towns and cities, just expecting that some of the cropland would be flooded from time to time (with more fertile sediment deposited).
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