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Built between 1908 and 1910, Rainbow’s original hydroelectric facilities made a major contribution to the development of Montana’s mining industry and electrification of its railroads. By the late 1800s, copper production was the leading industrial force in Montana, and its expansion relied on ample and inexpensive electricity. To meet this need, companies built hydroelectric plants on the Missouri and other rivers to power the mines and copper processing facilities. Rainbow was a good site to make electricity because the falls provided a natural drop in water elevation.

The original Rainbow Development used an intricate system of facilities to make electricity from flowing water. The original dam, which was built on the crest of Rainbow Falls, was constructed of heavy timber cribbing that was filled with stone. Two large steel flowlines carried water from the intake at the top of the dam to a surge chamber on a hilltop above the original powerhouse. Water delivered through the penstocks from the surge chamber or tank to the powerhouse spun turbines that powered the electrical generators.

The original brick and steel-frame powerhouse housed six turbine and generator sets, electrical transmission equipment, maintenance shops, control rooms and electric transmission equipment. To boost power production in 1917, another water intake, flowline and a steel surge tank were added to supply water for two more turbine-generator sets. The 1917 powerhouse expansion, and a one-story equipment storage and carpentry shop added in the 1930s, were done with similar brick construction and windows and are not readily identifiable as additions. In 1990, a concrete dam replaced the original timber crib dam and in 1989, an inflatable rubber section was installed atop that on the right side.

A new powerhouse was completed in 2013 just downstream of the historic powerhouse. Work on that project included the construction of an elevated intake canal for conveying water from the dam to the new powerhouse. The historic powerhouse was decommissioned in 2014 with all of the equipment left in place. NorthWestern Energy is working with the Rainbow Powerhouse Repurposing Committee in Great Falls and the Montana Historic Preservation Office on the future of the historic powerhouse.

Original Rainbow Dam during construction, 1910

Concrete surge chamber and outlets to 8'-diameter penstocks under construction

Construction of the original powerhouse interior, 1910

The Historic Rainbow Hydroelectric Development: Electricity from Water
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