The main channel dam contributes to the historic district at the Thompson Falls Hydroelectric Project. It is a fairly well-preserved example of a concrete arch dam, one the most common types of dam utilized for new hydroelectric development in the early-twentieth-century American West. Two of the dam's more notable engineering features are a concrete gravity section, in addition to the arch, and log sluice.
The Thompson Falls Hydroelectric Project has two dams. The larger of those two structures, the main channel dam, spans the river just upstream of the Thompson Falls, a series of small falls and rapids which drops the river about 30 feet in elevation over a distance of about ¼ mile. The reservoir impounded by the main channel dam occupies a former channel or bend in the river which was dry at the time of project construction. That former dry channel had its inlet on the north side of today's river channel just upstream of the dam, while the outlet was about ½ mile downriver near the lower end of the falls. The facility's second and much smaller dam impounds the
reservoir in the dry channel. It is known as the dry channel dam.
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...
...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.
Construction work at Thompson Falls began in earnest on October 1, 1913, with the main channel dam the first project at hand. It was to proceed in two distinct phases. The first phase entailed work on the west half of the dam. A coffer dam was erected mid-river to divert the flow away from and past that area of the river that fall, and early in the following year labors completed the concrete work on the dam's west half. By then, a new coffer dam was in place and the east half
the dam site dewatered. Unlike the west half, a significant amount of gravel, estimated at 50,000 tons had to be removed from the east before construction could proceed. This proved to be much more arduous and time consuming task than expected. Excess flood waters set work back further that spring. Thereafter, crews were run round the clock all seven days of a week with the goal of completing the project before the 1915 spring flood. That schedule was met, but only with a few days to spare.
The main channel dam has sustained few alterations over the years. Most significantly, a large section of the flashboard system at the west end of the spillway was removed and replaced by two large taintor gates. The log sluice also has been shortened considerably.
From the Historic American Engineering Record