The dam is 420 feet above bedrock and 2,900 feet long at the crest. It is a straight-axis, concrete gravity structure that holds back the water of Lake Koocanusa by its own weight. Besides road construction and railroad relocation a 7-mile-long railroad tunnel was required. The Flathead Tunnel is the world's 7th longest railroad tunnel. Guided by laser beam, workers drilled from both ends of the tunnel with the "holing through" taking place in 1968.
