Hydroelectric power reservoirs have imposed a new challenge on rivers; one that they would have faced alone if left untouched.
Evolution has allowed young salmon migrating downstream to reach the ocean under the influence of flow.
Large reservoirs will alter this natural progression.
Salmon will travel slower in the water.
As a result, salmon will have greater exposure to predators, diseases, and other environmental stresses than in free-flowing water.
The process proved to be much more complex.
Why the reservoir was gradually drained during 2024
To address these concerns at Oregon’s Lookout Point Project, managers attempted an unorthodox strategy.
They lowered the reservoir over a six-month period to allow for the least amount possible by late fall.
Their goal was to expedite the downstream migration of juvenile chinook salmon so that they may survive their downstream migration better.
The Lookout Point Dam forms part of a high-head hydropower dam in Oregon.
Beginning in July 2024, operators began to lower the reservoir gradually.
The water level decreased continuously until the reservoir was at its low water mark in mid-November to mid-December.
The purpose behind the lowering of the reservoir was to decrease the time required for juvenile chinook salmon to pass through the facility.
Dams with large reservoirs can impede fish migration and cause increased predation, disease, and mortality.
It was hoped that a significant drawdown of the reservoir would shorten the time fish spent passing through the system.
When survival reached its highest levels during drawdown
This experiment provided researchers with a unique opportunity to assess how fish respond to large scale changes in the way that a reservoir is operated.
Researchers assessed salmon migrations using acoustic telemetry techniques.
Data regarding water temperature and hydrologic conditions were collected.
The results varied significantly among the different periods of the drawdown.
The U.S. Geological Survey reported that regardless of which stage of the drawdown occurred, larger fish consistently had a higher apparent survival rate compared to smaller fish.
Survival of apparent fish through each dam occurred at a maximum during late November when the elevation of the reservoir was at its lowest point.
During this time frame, fish spent less time “wandering” or lingering in the fore bay area of the dam.
How reservoir conditions changed outcomes for juvenile salmon
Between September and October, as the reservoir levels fell, the reservoir changed from a thermally stratified system (layered vertically) to a warm isothermal reservoir (homogeneous).
The transition negatively impacted reservoir survival.
The results demonstrated that draw-down operations can produce very different outcomes depending on reservoir conditions and water levels.
Later in the experiment, fish passage improved as the reservoir levels declined closer to minimum levels.
The primary objectives were achieved once the reservoir levels were at their lowest points.
This success came after a period in which fish survival was negatively impacted prior to the completion of the full draw down.
What the experiment means for future drawdowns ahead
Several current efforts exist within the broader Willamette Valley System that utilize operational methods that aid fish passage through dams.
These future efforts include late-fall deep drawdowns and spill operations at multiple facilities, including Lookout Point.
The experiment also raised questions that need further research.
More studies are needed to understand how the various ways juveniles use their environment for survival are impacted by drawdown processes.
It would help researchers determine whether the long-term benefits of drawdown activities outweigh the negative impacts experienced by juveniles during transitional periods.
To date, it remains unclear.
This type of understanding could then be used to optimize future management decisions concerning the recovery of Pacific salmon in the area affected by the Lookout Point Project.
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