This is a very small transformer house, more commonly referred to today as a substation. Like the powerhouse built at the dam at the same time, this building/structure was made of pieces of local bedrock blasted during construction of the two dams on the site.
This transformer house was built as part of the Thompson Falls Hydroelectric Project, constructed on the Clark Fork River, the largest river by volume in Montana. The project was constructed between 1910 and 1916 at a spot opposite the town of Thompson Falls at a large natural waterfall. The project required the construction of two dams, one above and another below what came to be known as Power Island. Originally producing about 45 megawatts, a second powerhouse was added later, upping the project's output to 94 megawatts.
Though no longer an integral part of the hydro project, it still stands at its original location. Though many are chipped or broken as a result of vandalism, all of the original insulators which passed wires through the walls are still in place.
It's not known how long the transformer house was in use and in what year it was retired from service.
Historic Resources of Thompson Falls, Montana
The Thompson Falls Multiple Resource Area nomination consists of one historic district and a variety of individual residential and commercial buildings constructed between 1900-1916. The Hydroelectric Dam Historic District is made up of five buildings and six structures, all of which date from 1910-1915 and are in good condition.
The six structures in the district consist of the Dry Channel
and Main Channel Bridges (1911), the concrete Dry Channel and Main Channel Dams (1915), the masonry Powerhouse and
the small, masonry Transformer House.
From the NRHP Nomination Form
Planning for the design and construction of the Thompson Falls Hydroelectric Project began in earnest after die Thompson Falls Power Company entered into an electric power contract with the Chicago, Milwaukee, and St. Paul Railroad (Milwaukee Road) in February 1913. The Milwaukee Road intended to use hydroelectricity generated at Thompson Falls as one source of power for a proposed project to electrify the Rocky Mountain Division of its new "Pacific Coast"
transcontinental mainline. The Rocky Mountain Division was a difficult, 440-mile-long section of rail line that passed through the mountainous country between Harlowton, Montana and Avery, Idaho. Officials of the railroad believed that the steep grades and mountain passes on the division could be more efficiently handled by electric power than steam. It was hoped that the improved service would generate increased traffic for the line.
The engineering team of Henry Herrick and Max Hebgen held charge of the design of the Thompson Falls Hydroelectric Project.
After much consideration and debate, by late-summer 1913 Herrick and Hebgen had devised a development scheme for Thompson Falls. The scheme took advantage of a former channel or bend in the river which basically looped to the north and west of the falls. A dam built just upstream of the falls would divert and impound the river into the former and long-since-dry channel, creating a large supply reservoir. A second but smaller dam necessarily was required to impound the reservoir at the lower or downstream end of the dry channel as well. Although requiring construction of two dams, the use of the dry channel as the reservoir site afforded a rather simplified intake system. An earthen canal only would be needed to convey water from the reservoir down to the powerhouse.
From the Historic American Engineering Record