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Flood the farms to save the cities.

That's the trade-off staring at the Army Corps of Engineers in Louisiana this week as a historically high Mississippi River rolls south, flooding towns in Mississippi on Wednesday, prompting evacuations farther south, and threatening the heavily industrialized petrochemical corridor running from Baton Rouge to New Orleans and beyond.

About a million people live along that corridor, in addition to the New Orleans residents who endured a flooding nightmare six years ago when levees failed after Hurricane Katrina.

So the corps is faced with an unpalatable choice: cause a flood that would drown the livelihoods of central Louisiana farmers and fisherman, or let the high river roll and frantically sandbag 200 miles of levees to prevent flooding in the state's two biggest cities.

If the swollen Mississippi is allowed to run full bore through the state, the water would eat away at levees and could overtop sections, drowning some districts of New Orleans under about 25 feet - an inundation even greater than Katrina, according to a worst-case map prepared by Walter Baumy, chief of engineering for the corps' New Orleans office.

But the corps has an ace up its sleeve. It can throw open a relief valve 35 miles northwest of Baton Rouge, the huge Morganza Spillway. Designed to siphon a fifth of the Mississippi's mightiest flow and sluice it west, the spillway has been opened just once in the 57 years since its completion, during the flood of 1973.

If it's opened again, the Mississippi would fall, as would the threat to Baton Rouge and New Orleans. In exchange, a flood would shoot down the gut of central Louisiana and join the already high Atchafalaya River, which would further swell and flood. For 200 miles, farmers and fishermen would pay a steep price as a torrent greater than Niagara Falls would inundate crops, crawfish hatcheries and, possibly, the small cities of Houma and Morgan City. Sensitive oyster beds in the gulf would be imperiled by the pulse of freshwater.

"These are very high stakes," said Bob Thomas, a professor at Loyola University New Orleans who studies the politics and ecology of the spillway. "The Morganza will cause a lot of damage if it's open, a major flood. But it's designed for that."

The Mississippi has flooded3 million acres in Arkansas, Tennessee and Mississippi and has forced thousands from their homes. On Wednesday, the river set an all-time record at Natchez, Miss. Heavy snows in the upper Midwest this winter and record rains in April across the Ohio River valley triggered the floods.