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Fish Ladders

February 2011: View of the fish ladder in Longview, Wash., where steelheads swim up.

Image courtesy Tess McBride/USFWS (under CC by 2.0 license)

If you're a freshwater fish like a trout, the last thing you want to see is a dam. Dams prevent the upstream movement of juvenile fish to the waters where they'll spawn and live out the rest of their days. Environmentalists have long bemoaned the havoc dams can wreak. Enter fish ladders.

One of the most technologically advanced fish ladders is in Montana at the Thompson Falls hydroelectric plant on the Clark Fork River. It's the first full-length fish ladder in the continental U.S. designed specifically for bull trout, a threatened species. The $8 million, 72-foot (22-meter) high ladder has 48 steps that the fish may climb [sources: Holyoak, PPL Montana].

Fish swimming upstream are attracted to the small opening at the base of the ladder by discharged water. They begin their ascent up the steel and concrete fish ladder, battling rushing water each step of the way. Each of the 48 steps, or pools, is about 5 feet (1.5 meters) wide and 6 to 10 feet (1.8 to 3 meters) long and has openings at the top and bottom. The fish eventually reach a 17-foot (5-meter) gathering pool and then a holding tank. Once inside the tank, scientists examine and tag the fish, which are then released above the dam [sources: Holyoak, PPL Montana].

Fish ladders don't always work as well as they're designed to though. A separate University of Massachusetts Amherst study found that only a small percentage of fish populations are safely passing through the dams researchers observed [source: Adams].

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