Members of the US Army Corps of Engineers open the Morganza Spillway in Morganza, Louisiana, May 14, 2011. [Photo/Agencies]

MORGANZA, Louisiana – Army engineers on Saturday opened a key spillway to allow the swollen Mississippi River to flood thousands of homes and crops but spare New Orleans and Louisiana's capital Baton Rouge.

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The Army Corps of Engineers opened one of the 125 floodgates at the Morganza Spillway 45 miles northwest of Baton Rouge shortly after 3 pm, sending a flume of water onto nearby fields.

The move, last taken in 1973, will channel floodwaters toward homes, farms, a wildlife refuge and a small oil refinery in the Atchafalaya River basin to avoid inundating Louisiana's two largest cities.

Weeks of heavy rains and runoff from an unusually snowy winter caused the Mississippi River to rise, flooding 3 million acres of farmland in Mississippi, Tennessee and Arkansas and evoking comparisons to historic floods in 1927 and 1937.

It could take three weeks for the enormous flow of water to pass through a system of levees and spillways to the Gulf of Mexico about 100 miles to the south, said Major General Michael Walsh, president of the Mississippi River Commission.

"It's putting tremendous pressure on the entire system as we try to work this amount of water through the Mississippi River tributaries," Walsh said before the floodgates opened.

Some 3,000 square miles of land could be inundated in up to 20 feet of water for several weeks. When flows peak around May 22, the spillway will carry about 125,000 cubic feet per second, about one quarter of its capacity.

About 2,500 people live in the spillway's flood path, and 22,500 others, along with 11,000 buildings could be affected by backwater flooding - the water pushed back into streams and tributaries that cannot flow normally into what will be an overwhelmed Atchafalaya River.

Some 18,000 acres of cropland could be flooded as waters rise, hitting their crest in about a week and remaining high for several weeks before subsiding.

"The land's going to wash away, but that's life," said Hurlin Dupre, who represents Krotz Springs on the St. Landry Parish Council. "The worst of it is we are in a drought and we can't use none of that water."