Farmers Along Mississippi See Crops Washed Away

"Protecting lives is the No. 1 priority," Army Corps of Engineers Maj. Gen. Michael Walsh said aboard a boat from the river at Vicksburg, Miss., hours before the decision was made to open the spillway.

The opening will release a torrent that could submerge about 3,000 square miles under as much as 25 feet of water in some areas but take the pressure off the downstream levees protecting New Orleans, Baton Rouge and the numerous oil refineries and chemical plants along the lower reaches of the Mississippi.

Engineers feared that weeks of pressure on the levees could cause them to fail, swamping New Orleans under as much as 20 feet of water in a disaster that would have been much worse than Hurricane Katrina in 2005.

Instead, the water will flow 20 miles south into the Atchafalaya Basin. From there it will roll on to Morgan City, an oil-and-seafood hub and a community of 12,000, and then eventually into the Gulf of Mexico, flooding swamps and croplands.

The corps said it will open the gates when the river's flow rate reaches 1.5 million cubic feet per second and is predicted to keep rising, which is expected sometime Saturday. Just north of the spillway at Red River Landing, the river had reached that flow rate, according to the National Weather Service.