June 29, 2020

by Margot Dick

In the 1930s, IIHR’s founding Director Floyd Nagler opened the door for the institute’s fish passage research, which would continue for more than 80 years. Fish migration patterns make fish passage structures vital at many hydropower dams because moving past the dams and their turbines is not only exhausting for the fish, but it can also maim or kill them.

IIHR’s fish passage research began in the Midwest but would later largely focus on the Columbia River system, where major hydropower dams provide clean energy to large portions of the Pacific Northwest. Hydropower is a relatively clean form of energy, but it is not without consequences for the fish that live in the rivers. The Columbia River is home to migrating salmon, many of which return to the upstream areas of the river each year to lay eggs. When the eggs hatch, the juvenile salmon must make the return journey to the ocean. Each year, the adult salmon coming from the ocean face the exhausting task of passing through eight dams going upstream, and then their progeny must do the same on their way back to the ocean. The addition of fish passageways at the dams offer a safer alternative to young fish on the downstream journey than swimming through the dangerous turbines.

Fish passage structures serve multiple purposes, including redirecting fish around dangerous turbines, reducing the total dissolved gas in the water, and easing fish travel past a dam.

Dam turbines pose a threat to fish safety because the fish can get caught, maimed, or killed in the rotating motors. Even the fish that make it safely through the turbines can be so dazed that they are easily picked off by waiting fish-eating birds. When given the choice between going through the turbines and swimming through a fish passage structure, most fish choose the latter—the path of least resistance.

Total dissolved gas (TDG) occurs at dams where the vertical drop is particularly steep. When the falling water hits the still water below, oxygen bubbles are pulled deep into the water, causing it to become supersaturated with the gas. The bubbles then get trapped under the scales of fish or enter their bloodstream, causing gas bubble disease (similar to the bends in humans). Gas bubble disease can be painful or even fatal to the fish. To prevent TDG-related deaths, IIHR models the dam and fish passageway with more shallow exits, allowing dissolved oxygen to disperse rather than getting trapped in the water.

When fish passage work began at IIHR in the 1930s, research staff pioneered the use of physical models. Today, IIHR uses both physical and computational models to determine the most efficient fish passage structures. The computational models, also known as computational fluid dynamics or CFD, are a newer practice, adopted at IIHR in the 1980s, which is particularly useful for TDG and temperature modeling.