Large concrete dams, like Grand Coulee Dam, are actually built as a series of immense blocks, not as one solid structure. At Grand Coulee, workers poured concrete five feet at a time into wooden forms at the top of each column. A system of trestles and traveling cranes moved the workers and building materials to where they were needed.

This photo, taken around 1937, shows the huge concrete blocks which make up the foundation of Grand Coulee Dam. Traveling cranes move along the trestle which spans the dam site. The Columbia River had to be diverted around the site while the dam was being built.

Written on original envelope: Grand Coulee Dam - general (no date)