"A gripping new history celebrating the remarkable heroes of the Johnstown Flood--the deadliest flood in U.S. history--from NBC host and legendary weather authority Al Roker. Central Pennsylvania, May 31, 1889: After a deluge of rain--nearly a foot in less than twenty-four hours--swelled the Little Conemaugh River, panicked engineers watched helplessly as swiftly rising waters threatened to breach the South Fork dam, built to create a private lake...