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Thompson Falls Dam

First built in 1915, the Thompson Falls dam is a seven-unit hydroelectric plant on the Clark Fork River in Thompson Falls, Montana. NorthWestern Energy owns and operates the dam, along with 10 others across the state. The energy produced and collected at these dams allows us to provide clean, sustainable energy to meet our customers’ needs. NorthWestern Energy values being a good steward of the resources at our projects and works to protect and enhance the fish and wildlife resources around our sites.

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Fish Ladder

In partnership with Montana Fish, Wildlife and Parks, US Fish and Wildlife Service, and Confederated Salish and Kootenai Tribes, NorthWestern Energy operates a fish ladder at Thompson Falls dam which allows native fish, especially threatened bull trout and several other salmonid and non-salmonid species, to migrate above the dam. Once fish ascend the ladder, biologists collect and track important information including population health. Engineers and biologists worked together to design the ladder to be effective at catching and passing fish in a complex environment at multiple flow levels.

Providing fish passage allows access to important upstream habitats including areas used for reproduction. Access to these spawning and rearing areas help to maintain genetic diversity to support the health of fish populations.

Fish Ladder Design


  1. Designing and positioning a fish ladder so fish can find and use it is a challenge - it’s not like you can put up a sign pointing the way. Migrating fish are attracted by a current and can be coaxed into certain areas by manipulating water flow. To lure fish to the ladder, flow up to 80 cubic feet per second (cfs) is discharged in front of its entrance pool.
  1. Once inside the ladder, fish swim from pool to pool against a 6 cfs flow. Pools are about 5 feet wide and 6 to 10 feet long. The total length of the ladder is 356 feet. The ladder was designed to assist fish migrating upriver. Fish traveling downstream pass over the dam or through the powerhouse.
  1. Near the top of the ladder is an area that can be used to trap and hold fish. Every morning, biologists check the holding area and count, sort, tag and measure all fish.
  1. Fish are released into the reservoir upstream of the dam. Most fish using the ladder are traveling to their natal spawing areas. Typically, bull trout, northern pikeminnow, peamouth and suckers pass through the ladder in the spring. Brown trout and mountain whitefish mostly use the ladder during the later summer and the fall.

Fish Numbers

The fish ladder operates annually between March and October. Since operations began in 2011, more than 33,000 fish have been successfully gathered and passed upstream. Here you can review our historical data and observe the ongoing numbers of this year’s fish ladder season.

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Be Dam Safe

At NorthWestern Energy, we’re committed to the stewardship of Montana’s river resources as well as maintaining vibrant and significant recreational opportunities such as boating, waterskiing, swimming and, of course, fishing. However, any water-based recreation near the dams has potential risks. For this reason, we ask that you practice the following safety measures:
Be Dam Safe

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