Army engineers are preparing to open the gates of an emergency spillway along the Mississippi River in an attempt to divert floodwaters from Baton Rouge and New Orleans. But opening the gates, which haven't been unlocked in 38 years, will swamp the homes of about 25,000 people in Cajun country and inundate Morgan City, an oil-and-seafood hub. Sheriffs and National Guardsmen have begun warning people in a door-to-door sweep, and shelters have begun preparing to accept evacuees. When the gates open, the water they release could submerge about 3,000 square miles under as much as 25 feet of water. But if the spillway isn't opened, officials fear the flood will overcome the levees in New Orleans in a disaster that would be much worse than Hurricane Katrina.