Damming the River
The 913-foot-long Main Dam was built in two stages. This end of the dam was constructed first, after a temporary cofferdam was built upstream to confine flow to the south half of the river channel. After this side of the river was dewatered, rock was excavated to provide a firm foundation for the dam, and massive lumber forms were built and filled with concrete. When this end of the Main Dam was complete, the cofferdam was removed, and the process was repeated from the opposite shore to construct the southern portion of the dam.
Building the 450-foot-long Dry Channel Dam was less complicated. A cofferdam built just upstream of the Gallatin Street Bridge kept water from the dam site and other areas under construction, including the powerhouse canal, forebay and foundation. Narrow-gauge railways connected construction sites to a concrete mixing plant, sawmill, rock quarry, and spur lines of the Northern Pacific Railroad.
In late June 1915, both dams were operational. Gates were closed and water behind the dams rose and flowed over the crest of the Main Dam. Although a system of flashboards was added to the tops of the dams over the next few years, sufficient head was now available to begin generating power at the Thompson Falls Project.
From the Plaque