What the Corps is trying to prevent

For the last 50 years, the Mississippi River has retained its course only because of the Old River Control Structure. Without the ORCS, the Mississippi would basically have been captured by the Atchafalaya River and stopped flowing east past Baton Rouge and New Orleans to take a shorter, more direct route to the Gulf. The ORCS, a series of dams and floodgates designed to control the amount of the Mississippi that diverts down the Atchafalaya, almost failed in 1973 because of a particularly snowy winter across the upper Midwest US and the Canadian plains. (The definitive essay on the ORCS is John McPhee's "Atchalafaya", from the New Yorker in 1987.)

Well, this past year was far, far snowier than 1973. There's massive flooding all along the Mississippi down from Memphis way, and it's just starting to reach Louisiana now. The big news today was that the Army Corps of Engineers has partially opened the Morganza Spillway, a floodgate upriver of the ORCS, to alleviate pressure on the ORCS and help prevent flooding in the more densely populated parts of the state.

It's worth taking a minute to look at the pictures at the Morganza Spillway Wikipedia page:
Picture 1A shows the flooding that the Corps is deliberately inducing. It's directed south and east into the Atchfalaya floodplain. The broad purple swath represents up to 5' of water; the brighter colors are worse.

Picture 2 shows the projected flooding if the ORCS holds but the Morgaza spillway were not opened. The flooding in the Atchfalaya floodplain is less severe, but suddenly there's a vast knife of water cutting through Baton Rouge and dumping up to 40' of flooding into New Orleans.

Sometimes, there is no good option. This is the better one.