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CRSO: Operating dams to support fish passage

Columbia River System Operations EIS
Published Nov. 22, 2019
Spill through two raised spillway weirs (right) and conventional spillways at McNary Dam on the lower Columbia River improves downstream passage survival for juvenile salmon and steelhead.

Spill through two raised spillway weirs (right) and conventional spillways at McNary Dam on the lower Columbia River improves downstream passage survival for juvenile salmon and steelhead.

In addition to several fish passage systems in place at the lower Columbia River and lower Snake River dams, spillway deflectors produce a more horizontal spill flow and limit the plunge depth of water over the dam spillway. This reduces the amount of entrained nitrogen that creates total dissolved gas, or TDG.

In addition to several fish passage systems in place at the lower Columbia River and lower Snake River dams, spillway deflectors produce a more horizontal spill flow and limit the plunge depth of water over the dam spillway. This reduces the amount of entrained nitrogen that creates total dissolved gas, or TDG.

Flow augmentation
The Corps and Reclamation operate Hungry Horse, Libby, Albeni Falls, Grand Coulee and Dworshak dams to store water to reduce flood damages downstream and deliver water for irrigation, among other purposes. However, storing water can interrupt the seasonal river flow patterns. To aid migrating juvenile salmon, in accordance with the National Marine Fisheries Service’s Biological Opinion, the Corps and Reclamation operate these dams to provide seasonal releases to improve flows. This is called flow augmentation.

Flow augmentation can impact operations for the dams’ other congressionally that support fish conservation. To learn more about this team, visit pweb.crohms.org/tmt.

Total dissolved gas

Increased pressure from high levels of spill can result in elevated levels of total dissolved gas in the pool below the dam. This causes air bubbles to dissolve in the water, which supersaturates the water with gases, primarily nitrogen.

TDG is a measure of this air dissolved into water. A buildup of elevated TDG concentrations in the bloodstream and tissue of fish and other aquatic species can cause a condition called gas bubble trauma. Symptoms range from minor injuries to death, depending on concentrations and the duration of exposure.

The Corps and BPA implement a water quality program to monitor TDG during annual fish passage spill operations.

The team at the Corps’ Reservoir Control Center, in coordination with the technical management team, adjusts spill operations in real-time to maintain TDG within state water quality standards to minimize impacts on fish and comply with the NOAA Fisheries’ and U.S. Fish and Wildlife Service’s biological opinions on the operation and maintenance of the Columbia River System.

Spill
Water is spilled over a dam’s spillway when it cannot be stored in a reservoir behind a dam or passed through turbines to generate electricity, such as during maintenance activities, periods of low energy demand or periods of high river flow. Spill for these reasons is generally characterized as involuntary. Involuntary spill is most common in the spring.

In the early 1980s, the Corps began spilling water at several of its projects as one way to divert ESA-listed juvenile salmon and steelhead away from the turbines to aid downstream juvenile fish passage survival. Over many years of testing, the Corps, Reclamation and BPA, with input from the NOAA Fisheries and other federal, tribal and state fish managers, developed a comprehensive spill program tailored to the unique configuration of each of the four lower Snake River dams and four lower Columbia River dams. This program keeps water moving over spillways for fish, generally from April through August each year.

This program has increased downstream migration survival and the overall migration speed of ESA-listed juvenile salmon and steelhead over the years.

Building on of the foundation of the spill program and its benefits to juvenile fish passage, federal, state and tribal partners aligned on a flexible spring spill operation for 2019 and 2020 with the goal of improving juvenile salmon survival while managing costs of hydropower generation (diverting water for spill reduces power generation). For more information on this agreement, a key component of operating federal dams on the Columbia River in 2019 and 2020, visit www.bpa.gov/news/newsroom/Pages/Flexible-spill-agreement-aims-to-benefit-salmon-and-hydropower.aspx.