Thompson Falls Dam
Generating capacity: 94 megawatts
1610 Maiden Lane
P.O. Box 667
Thompson Falls, MT 59873
Thompson Falls Dam is a seven-unit hydroelectric plant on the Clark Fork River in Thompson Falls. The units have a total generating capacity of 94 megawatts. (One megawatt can satisfy the average energy needs of 750 households.)
The Thompson Falls plant, which began operation in 1915, consists of a main dam and a dry-channel dam with an island in between. Both dams are used to regulate the reservoir, which has a storage capacity of 8,300 acre-feet, and control flow during high spring runoff. The main dam is 913 feet long and 32 feet high.
The dry-channel dam is a concrete gravity structure with an overflow spillway and an overall length of 289 feet. The structure has an average height of 17 feet above the riverbed. It is raised by flashboards and 8-foot drop panels. Thompson Falls is classified as a “run-of-river” project because it can generate electricity using the water that flows down the river, without the need to store additional water supplies.
The Clark Fork River at Thompson Falls was originally a natural waterfall. After the dam was built, westslope cutthroat trout (a species of special concern), the threatened bull trout and other fish species instinctively jumped on the rocks to get upstream. PPL Montana has installed a mechanism to help fish reach the river on the upstream side of the dam. This temporary 43-foot-long, 2-foot-wide ladder is attached to a trap. Fish captured in the trap can be trucked above the dam and released to assist in their passage to upstream locations.
In 2005, PPL Montana began using surgically implanted radio transmitters to track the fish during their migration and their approach to the dam. The process is helping PPL Montana and state fish biologists to develop a recommendation for a permanent passage for the trout.
The island at Thompson Falls contains a public park, and there are several campgrounds nearby.
Thompson Falls is one of two PPL Montana dams west of the Continental Divide. The Clark Fork River empties into the Columbia River, which in turn empties into the Pacific Ocean.