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Washington's Lower Monumental Dam Has a "Bubbler" like an "inverted hot tub" by Emile Perreira The Pulse, August 5, 2026

You may have passed Washington's Lower Monumental Dam without realizing it works
like an "inverted hot tub," cooling the water so salmon and steelhead can survive below the surface

Fish have had to navigate the waters of the lower Snake River for millions of years. Salmon and steelhead travel long distances upstream to reach their spawning grounds. They rely on powerful instincts to navigate through the river system.

However, man-made barriers such as Lower Monumental Dam pose an enormous threat to this long-standing tradition.

Behind the dam's massive concrete walls is a complex system that generates electricity while allowing boats and fish to pass through.

Conditions within the water were changing.

A method was needed to cool the water without refrigeration or pumps.

A hidden problem was forming inside the fish ladders

Fish migrate through the fish ladders built into Lower Monumental Dam to get past the dam.

These passageways enable fish to go over the dam so that they do not get trapped against it.

When warmer water forms a barrier to fish, it disrupts fish migration.

During summer, the Snake River separates into two layers; warmer water stays near the surface, and colder water remains deeper. This creates a problem because the upper parts of the fish ladders pull water from the warmer layer, which raises the temperature of the water. Officials report that migrating salmon and steelhead often stop inside the ladders when the water becomes too warm.

Those delays can reduce migration success during the warmest parts of summer. Helping fish move through the ladders has become an important part of improving survival during migration.

Existing fixes came with a major drawback

For many years, engineers have studied different methods to maintain a cooler temperature in fish ladder water during the summer.

One solution was a series of very large cooling chimneys that drew colder water from deeper in the river. Although successful, the structures were expensive to build and install. Engineers were searching for a cheaper solution that would produce similar results.

Engineers ultimately focused on a solution that moved water rather than adding more infrastructure. It relied on creating water currents instead of building another large structure.

That concept has now been installed at Lower Monumental Dam by the Army Corps of Engineers, Walla Walla.

Engineers believe similar systems could also be used at other dams on the Snake and Columbia rivers if they prove successful.

Thousands of bubbles began moving cold water

The hidden system beneath the river at Lower Monumental Dam is called a fish bubbler.

Instead of pushing cold water into the ladder, it injects compressed air into the bottom of the river. As the bubbles rise to the top of the river, they cause an upward-flowing current that moves colder, heavier water toward the fish ladder. At the same time, the flow pushes warmer surface water away from where the fish enter the ladder.

In essence, the end result is cooler water at the point where salmon and steelhead start traveling upstream.

Therefore, the system operates similarly to an "upside-down" hot tub. Hot tubs circulate heated water upwards toward people sitting in them.

The system produces downward-moving cold water that helps migratory fish during warm weather.

Could the same approach work at other dams? According to officials from the United States Army Corps of Engineers, one of their main objectives is to lessen heat-related stress on fish.

Another is to promote continued movement through the ladder instead of entering it and resting.

Thousands of tiny air bubbles now help reshape conditions beneath the dam.

Elsewhere in Oregon, a hydroelectric dam released 950 steelhead 600 miles away, and more than 600 somehow found their way back after two years in the ocean.