Environmental

Balancing water spills at hydropower dams for fish passage and energy production

Balancing water spills at hydropower dams for fish passage and energy production
(photo courtesy PNNL)

The amount of water spilling over a dam is not always the biggest factor in successful fish passage; fish activity, swimming ability and time of day play a role, often more so than spill levels alone, according to research from the Pacific Northwest National Laboratory.

Dams that impound water for hydro power provide renewable energy, but they also have a direct impact on the environment, especially fish. Adjusting the amount of water spilling over a dam can help fish navigate passage, but spilling more water means less is available to produce power. Determining when and how much water to spill to help fish pass safely through dams while meeting grid reliability needs are critically important questions.

PNNL’s peer-reviewed research highlights the complex dynamics around spill and fish passage through dams and finds that spill is not always the biggest factor in successful fish passage.

“We found that fish activity, swimming ability, and time of day all play a role in fish passage, often more so than spill levels alone,” explained PNNL earth scientist Ryan Harnish. “Our data show that when young salmon and steelhead are less active in the spring — like at night or in cooler water temperatures — spilling more water can make a big difference in helping more fish avoid passing through the powerhouse. But under other conditions, higher spill levels have little effect.”

This is valuable information for decision-makers who are tasked with balancing water needs for environmental health and energy demand. Knowing when high spill levels best support fish passage, and when they don’t, provides options for balancing the tradeoffs associated with hydropower generation and environmental impacts. To take a closer look at how spill level affects fish passage, researchers needed detailed tracking information showing when and how fish passed a dam. 

The standard method in the Columbia River Basin is to tag individual fish with passive integrated transponder (PIT) tags, which are detected as fish pass through the juvenile bypass system at the dam. While reliable, this approach doesn’t provide information about the various other routes fish can travel through a dam — information needed to determine how spill level affects fish passage. To get this information, studies were conducted between 2008 and 2018 using a different method: acoustic telemetry.

The PNNL team analyzed the 10 years of acoustic telemetry data, including 3D information on fish behavior, from multiple dams along the Snake and Columbia rivers. This allowed them to identify the route fish took and the precise time of their passage. Combined with information about fish behavior and survival along with dam operations, they evaluated what factors were most likely to affect fish passage through different routes and conditions. They looked at spring salmon and steelhead smolts migrating toward the ocean to see how spill levels affected the number of smolts passing through the powerhouse. This number turned out to be related to their activity level and swimming ability, not just spill.

“High spill levels that are intended to reduce the number of fish passing through the powerhouse in the spring are likely to be most effective when fish are less active or have reduced swimming ability, such as at night, during high river flows, or in cooler water temperatures,” explained Harnish. “High spill levels were not the single most important factor in smolt passage across the board.”

These findings point to the need for more studies to provide comprehensive information about exactly when and how fish pass through dams. That information can help decision-makers determine how to best support fish passage and meet energy needs in the future.

The the Bonneville Power Administration (BPA), which markets power from 31 federal dams across the Pacific Northwest, said: “This publication is another piece to the puzzle of understanding fish passage through the federal hydrosystem. It should provide the region’s scientists and managers with additional information to consider when evaluating operations that meet the multiple purposes of the Columbia River System.”

This research was funded by BPA, as part of a cost-share agreement to expand upon analyses originally funded and initiated by the Department of Energy’s Water Power Technologies Office.

Founded in 1965, PNNL is operated by Battelle for DOE’s Office of Science, which is the single largest supporter of basic research in the physical sciences in the U.S.