What was the role of the physical environment in causing the Johnstown flood of 1889?

Johnstown is located in a river valley. The Little Conemaugh and Stoney Creek rivers come together to form the Conemaugh River. A man-made dam constucted by the South Fork Fishing and Hunting club held back a lake 2 miles long, almost a mile wide and 60 ft deep at the earthen dam structure. A combination of poor dam construction and maintainance and very heavy rains caused the dam to burst, sending water 40 ft high down narrow valley river beds and right into the center of town. see http://www.jaha.org/FloodMuseum/history.html