The 377-foot-long Gallatin Street Bridge (formerly known as the Dry Channel Bridge) once linked the town of Thompson Falls to the Thompson Falls hydro facility. It was designed by William Pierce Cowles, a
Minneapolis engineer, and constructed by Missoula bridge builder
O.E. Peppard. With the construction of a smaller bridge nearer the powerhouses, this bridge fell into disuse and is today a footbridge to Power Island Park. It is used on rare occasions by vehicles for maintenance on the Dry Channel and Main Channel Dams. The bridge is one of six contributing structures in the Thompson Falls Hydroelectric Dam Historic District.
Historic Resources of
Thompson Falls, Montana
(Page 2)
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. Of the nine independently eligible residential buildings in the nomination, seven can be categorized as intact examples of the Bungalow style, most having been constructed around 1912.
From the NRHP Nomination Form
In July of 1910, the Commissioners of Sanders County retained William Pierce Cowles, a Minneapolis engineer, to design bridges over the
Clark Fork at Plains, Thompson Falls, and Trout Creek. The Missoula bridge builder, O.E. Peppard, won the contract to build those
bridges for a total of $125,650. Actually, the construction at Thompson Falls involved two bridges: one over the main channel and one over the dry channel. The Dry Channel bridge was evidently built with the Thompson Falls Power Plant in mind, because, although built about one hundred yards upstream of the dry channel dam, the bridge is above the level of the reservoir.
The Thompson Falls bridges were built in 1911. Construction of the power plant began in 1913. The Dry Channel bridge is 377 feet long. The three main spans, supported by concrete piers, are each 90-foot, pin-connected Pratt through trusses. Their superstructures are as follows: lower chords are eyebars; hip verticals are eyebars and the other verticals are two laced channel sections; diagonals are eyebars with turnbuckles; upper chord is a continuous steel plate riveted atop two channels with lacing bars riveted to their lower flanges. Steel I-beam floor beams are connected to the superstructure by means of U-bolts at the hip verticals and are riveted to the superstructure at the other verticals. A deck of bridge plank is supported by wood stringers which sit on the top flange of the floor beams. The main spans are approached at either end by wood stringer spans.
Although the Main Channel Bridge is closed to traffic, the Dry Channel Bridge is still open, providing access to the island which separates the two bridges and which serves as a park. The Thompson Falls Power Plant, which merged with the Montana Power Company in 1929, still generates electricity with the six Allis Chalmers generator units that came on line between 1915 and 1917.
[Update to the above - the High Bridge was rehabilitated and narrowed in 2010 and is now open as a pedestrian bridge. A new powerhouse has been added, more than doubling the power output of the project to 94MW.]
From the Historic American Engineering Record