Dec. 22, 2008, was filled with anticipation of Christmas in Roane County, Tenn., 85 miles northeast of Chattanooga. What no one anticipated was the failure of a retaining dam, and 5.4 million cubic yards of coal ash spilled into the Emory River. Also called “fly ash,” this byproduct of power generation covered the entire floodplain with black sludge, knocking houses off their foundations and uprooting trees.