The Mississippi watershed is the world's third largest after the Amazon and the Congo. Since record flooding in 1927, the Army Corps of Engineers has spent billions of dollars on an intricate 2000-mile system of levees, spillways and flood zones designed to allow homes, farms and industrial development to thrive along the river and its many tributaries. Now that system is being severely tested, as the Corps is faced with opening spillways to devastate some places in order to save others. At stake either way are homes, businesses, billions in property damage and entire communities. Is the river untamable after all? We hear from the Corps and its critics and from the Mayor of Vicksburg, Mississippi, one city that's bracing for a catastrophe.