LAKE PROVIDENCE, La. — Army engineers prepared Saturday to slowly open the gates of an emergency spillway along the rising Mississippi River, diverting floodwaters from Baton Rouge and New Orleans, yet inundating homes and farms in parts of Louisiana’s populated Cajun country. About 25,000 people and 11,000 structures could be in harm’s way when the Morganza spillway is unlocked for the first time in 38 years.