|Image courtesy of Mike Cochran
1 - What is going on at Green Peter?
At Green Peter, we’re seeing dead Kokanee salmon, something we typically see every year when the Corps draws reservoirs down in the fall. However, we’re seeing greater Kokanee losses this year due to changes in how the Corps operates dams across the Willamette Valley to comply with a court injunction to improve downstream fish passage for juvenile Upper Willamette River spring chinook salmon and steelhead, which are both listed as threatened under the Endangered Species Act (ESA). Kokanee are not a listed species under the ESA. The Kokanee are experiencing barotrauma, which is caused by a rapid pressure reduction as they pass from the reservoir side of the dam to the downstream side. The regulating outlet they are passing through is approximately 100 feet beneath the water on the reservoir side and near the water surface on the downstream side.
2 - Why is the lake level so low?
We are performing “deep drawdowns” at some of our reservoirs, including Green Peter for the first time this year, to comply with a court injunction to improve juvenile spring chinook salmon and winter steelhead passage through the dams. “Salmon are surface-oriented fish, so we’re hoping they'll key into the increased surface outflows from the reservoir [during the drawdown] and swim downstream, where they need to go,” said Kathryn Tackley, a physical scientist at the Portland District who is working to implement injunction measures.
3 - Is this really necessary?
Yes, we've changed how we operate many of our dams as required by a court injunction. The goal is to provide improved downstream fish passage and survival for juvenile spring chinook salmon and winter steelhead, which are both listed as threatened under the ESA.
4 - When will the drawdown stop?
This year, Green Peter Reservoir is being drawdown to El. 780 ft. for one month, from November 15th to December 15th. Depending on the project, we expect deep drawdowns will continue in the near and/or long-term as part of our long-term strategy to improve survival for juvenile spring chinook salmon and steelhead, which are both listed as threatened under the ESA.
5 - Why are all the fish dying?
The cause of death is barotrauma, which is similar to a diver experiencing the bends. Barotrauma is caused by a rapid pressure reduction as the fish pass from the reservoir side of the dam to the downstream side. The regulating outlet they are passing through is approximately 100 feet beneath the water on the reservoir side and near the water surface on the downstream side. We typically see Kokanee mortality every year when the Corps draws Green Peter Reservoir down in the fall. More Kokanee are passing through the dams this year due to the “deep drawdown” the Corps is implementing to comply with a court injunction to benefit ESA-listed fish.
6 - Why doesn’t the Corps stop the drawdown now that you know the fish are dying?
The court injunction requires the Corps to carry out the deep drawdown.
7 - How do you know nothing is wrong with the water?
The Corps and its partners continually monitor the water quality for turbidity, total dissolved gas and temperature. Biologists from the Oregon Department of Fish and Wildlife determined that barotrauma (a rapid change in pressure that fish experience when their depth changes too quickly) is the cause of death for the Kokanee salmon.
8 - Will all the fish in Green Peter die and how will you replace what has been lost?
As of right now, Kokanee are the only fish we’re seeing affected, and they are not an ESA-listed species.
9 - You knew that fish would die as a result of this action, why didn’t you do anything?
The court injunction requires the Corps to carry out the deep drawdown and did not identify the impact to other fish as a constraint on the operation.
10 - What other impacts from the drawdown can we expect?
Elevated turbidity can be expected from the deep drawdown at Green Peter Reservoir. Turbidity is being tracked continuously through real-time monitoring equipment installed upstream and downstream of Green Peter Dam, and near the town of Sweet Home’s water supply intake.
11 - How do we prevent another die-off during next year's drawdown?
There is no provision in the court injunction that allows the Corps to change deep drawdown operations due to kokanee die-off. It's likely there will be a similar die-off next year as the Corps carries out the court-ordered actions for the benefit of the endangered species of spring Chinook and steelhead.
12 - Will this impact our drinking water?
The drawdown will not impact the water quality for purposes such as municipal water supply. The Corps and its partners continually monitor water quality for turbidity, total dissolved gas and temperature.
13 - Will this wipe out Kokanee fishing at Green Peter next year?
We are at the beginning stages of collecting data. It’s too early to tell how the Kokanee fishing season will be impacted.
14 - How detrimental is this to the fish population?
We’re obtaining data on this event to determine the long-term future impact to Kokanee salmon.
15 - Will other fish species also die because of the deep drawdowns?
Barotrauma is caused by caused by a rapid pressure reduction as fish pass from one side of the dam to the other. The reservoir level was approximately 100 ft. above the outlet at the time of the Kokanee mortality event. As the reservoirs are drawn lower, the difference in pressure the fish experience when passing through the dam is reduced.
16 - Will the Kokanee be restocked?
This will be determined by the Oregon Department of Fish and Wildlife. The last year that ODFW stocked Green Peter with Kokanee was 2015.