US Army Engineers May Open Key Louisiana Spillway

* Corps may open Morganza spillway over weekend

* Floodwater diversion threatens crops, rural population

By Kathy Finn

NEW ORLEANS (Reuters) - U.S. authorities may soon open a key spillway to relieve the swollen Mississippi River to avoid flooding Louisiana's two largest cities but potentially swamping thousands of homes and acres of crops.

The U.S. Army Corps of Engineers said Friday it may open the Morganza Spillway, which would divert high water to homes, farms, a wildlife refuge and a small oil refinery in the Atchafalaya River basin, to avoid flooding more densely populated Baton Rouge, the state capital, and New Orleans.

It would be the first time the spillway, located about 45 miles northwest of Baton Rouge, has been opened in nearly 40 years.

Louisiana Governor Bobby Jindal on Friday told residents in the affected areas to start leaving their homes and said the state has plans with the American Red Cross to provide shelters for evacuees.

"It's extremely likely the spillway will be opened tomorrow and no later than Sunday," Jindal said at a news conference after he met with Corps officials.

Flooding could reach 20 feet in the Atchafalaya basin.

About 2,500 people live inside the floodway, and another 22,500 people and 11,000 buildings would be affected by backwater flooding when the Morganza is opened. Backwater flooding is water pushed back into streams and tributaries that cannot flow as normal into what will be an overwhelmed Atchafalaya River.

Some 18,000 acres of cropland could be flooded.

"We always felt like there may come a day that it may happen," said Martin Frey, a farmer in Morganza, the site of the spillway.

Frey farms 1,600 acres inside the spillway, including 450 acres of rice that had just begun to grow. Anticipating the high water, he pulled irrigation wells and motors.

"Now I have a crop sitting out there ready to die because we have no rain and can't pump water. That's about as disheartening as knowing that all of that's going to be under 15 or 20 feet of water soon," Frey said.

SAVING CITIES

The Corps expects the Atchafalaya basin to flood within 10 to 14 days of opening the Morganza. Alon USA Energy on Friday continued building a second levee to shore up existing levees around its 80,000 barrels-per-day refinery in Krotz Springs, Louisiana, which could face supply disruptions or have to shut down amid flooding after the spillway opens.

Thousands of residents in towns along the Mississippi River from Illinois to Mississippi have been evacuated in recent weeks after the river overflowed its banks, fed by heavy spring rains and large snow melt from a stormy winter.

The flow of water at the Red River Landing, about 63 miles northwest of Baton Rouge, was expected to reach 1.5 million cubic feet per second over the weekend, which is the minimum needed to open the spillway, the Corps said in a statement.

As of Friday morning, the flow of water at the Red River Landing was 1.45 million cubic feet per second, according to the Corps.

The Corps said it anticipated opening the spillway to divert up to 150,000 cubic feet per second at peak flow. When fully opened, the spillway can allow 600,000 cubic feet of water per second.

Earlier this week, the Corps issued maps showing that New Orleans, Baton Rouge and other cities along the Mississippi would be inundated if the Morganza was not opened.

Jindal, a Republican, and New Orleans Mayor Mich Landrieu, a Democrat, have both said failure to open the Morganza could lead to flooding in New Orleans.

In addition to threatening densely populated areas, lower Mississippi flooding could force shutdowns of as many as eight refineries and at least one nuclear power plant alongside the river.

The refineries make up about 12 percent of the nation's capacity for making gasoline and other fuels. (Additional reporting by Erwin Seba and Kristen Hays in Houston; Editing by Eric Beech)