Watch The Johnstown Flood for Free: How to Stream the Movie Online at 123Movies. . On a stormy day in May of 1889, the South Fork Dam impounding Conemaugh Lake exploded, unleashing a 40-foot wall of water. The bustling industrial city of Johnstown, PA, in the valley below was reduced to a wasteland, killing more than 2,200. This heavily dramatized documentary reviews the factors that led to the dam's collapse, while dramatic reenactments and survivors' personal testimonies detail the horror.