When Chinook salmon begin returning to the Snake River each fall, thousands of fish gather below Hells Canyon Dam to complete the final stage of their life cycle. These fall Chinook salmon will dig out the redds where they deposit their eggs.
That is where the eggs will develop, but they are not left entirely to chance.
Water flow greatly impacts the success or failure of any given spawning season.
Therefore, Idaho Power makes adjustments to the operation of its Hells Canyon Complex each fall. The goal is to help maintain suitable conditions during spawning and incubation.
Keeping river conditions steady is not always simple
Once eggs are laid below Hells Canyon Dam, they must remain submerged to develop into juvenile fish.
The first step to achieve that goal is for Idaho Power to manage the release of Brownlee Reservoir between Oct. & Dec. During the spawning season, Idaho Power maintains the outflow of Brownlee Reservoir between 8,500 cfs and 13,000 cfs.
Idaho Power may draw down Brownlee Reservoir during the spawning season. This allows the reservoir to absorb increased inflow. It also helps maintain more consistent flows downstream.
Idaho Power operates this program to decrease the risk of fish nesting areas being exposed due to changes in river flow.
Helping salmon can come with tradeoffs
There are some negative consequences associated with managing rivers for fish.
Reduced reservoir levels can impact Idaho Power’s hydroelectric generation capacity for the dams within the Hells Canyon Complex. This means Idaho Power may purchase additional electricity elsewhere.
In addition to generating hydroelectric power, Idaho Power must maintain suitable river conditions during the spawning season. Decisions made during the fall can affect both hydropower generation and fish management for months afterward.
These decisions require Idaho Power to consider both river operations and fish management.
Electricity generation remains a primary purpose of the Hells Canyon Complex. At the same time, river conditions become an important consideration once spawning begins.
Finding the shallowest nests shapes every decision
Protecting fish habitat begins with knowing where the fish are born.
The U.S. Geological Survey, Idaho Power, U.S. Fish and Wildlife Service, and the Nez Perce Tribe conduct surveys throughout the Snake River system each year. These surveys count the number of redds and determine shallow spawning areas in the river.
The information collected during these surveys helps determine future minimum flow requirements for spawning fish.
One of the most important measurements is the depth of the shallowest nests found each year. After establishing that level, Idaho Power maintains that minimum flow throughout incubation and emergence periods.
Fall Chinook seasons are managed by the Idaho Fish and Game Commission.
Methods for conducting surveys have changed over the years. When Idaho Power initiated this program, initial aerial surveys were done via helicopters.
Most current aerial surveys are done using drones that provide images for further review. Biologists also utilize underwater video equipment for surveys, as many spawning sites are located at depths too great to view from the air.
Some sites have been documented at depths as high as 22 feet.
Spawning season is only the beginning
Idaho Power’s Fall Chinook Salmon Program was developed as a voluntary operation plan for the Hells Canyon Complex in 1991. Since then, an annual survey of spawning sites has taken place throughout the Snake River and its connected tributary systems.
Recent surveys indicate there are thousands of redds along the Snake River and its tributaries. Flows established during autumn can help protect eggs until juvenile fish emerge from the riverbed.
Additionally, anglers pursue fall Chinook salmon in portions of the Snake River where angling seasons remain open.
Dam operators, fishery managers, and biologists all share the same objective each year.
Provide stable river conditions that allow juvenile fish to survive to adulthood. Work continues long after the spawning season ends.
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