Dams & Salmon Survival

For decades what to do about the survival of the salmon in the Columbia and Snake River basin in relation to the dams has been a contentious topic. A district judge in Oregon recently made a ruling that would the raise spill levels over the dams, so we talked to Fish & Wildlife Biologist John McKern and Aquaculture & Fisheries Management Professor Melissa Wagner to get their take.

Verbatim:

(John McKern) My name is John McKern. I was hired by the Corps of Engineers in 1971. I have a fisheries science degree, master's degree that I got in 1971, and a wildlife degree I got in 1968 from Oregon State University.

John McKern worked as a fishery biologist with the Walla Walla District Corps of Engineers providing oversight and technical support to the operation of fish facilities at the lower Snake River dams, McNary Dam, and other dams in the district.

McKern retired in 2000 and has been a technical advisor to Citizens for the Preservation of Fish and Dams, a local environmental organization in Lewiston.

(Melissa Wager) My name is Melissa Wagner. I am the Aquaculture and Fisheries Management Assistant Professor at the College of Southern Idaho, and I also run our fish hatchery as well. I have a bachelor's degree in environmental science and biology, actually from the University of Dubuque out in Iowa.

Melissa Wagner says even though the eight large dams on the Lower Snake and Columbia rivers were built to provide fish passage and have been modified with multiple improvements over the decades, they are still very different from a free flowing river for the salmon when they are navigating to the ocean and then back to their spawning grounds.   

(Melissa Wagner) You know, they're navigating waters with all of these barriers that were never there because they run back based on, you know, their home waters.

What to do about the survival of the salmon has been a contentious topic for literally decades.

In February of 2026  District Judge Michael Simon issued a ruling requiring changes to the operation of federal dams on the Columbia and Snake Rivers, which would, among other things, increase the levels of water going over the spillways of several dams. The intent was to allow more juvenile salmon, or smolts, to bypass the dam turbines safely.

But McKern and Wagner both say those higher levels of water plunging down over the steep spillways can cause a serious threat to the smolts that most people have never heard of: Supersaturation.

(John McKern) Supersaturation occurs when Water entraining air plunges down to depth and the air is forced into solution in the water.

That plunging water forces atmospheric gases like oxygen and nitrogen to dissolve in the water. The measure of how much is known as the Total Dissolved Gas levels, or TDG. When those levels are very high or supersaturated, it can be deadly for fish, especially when exposed for longer periods of time like the ruling requires.

(Melissa Wager) When you have that spillway over, think about like a waterfall, this happened, it does happen naturally. What that does is it creates a plunge pool down below. And so in our, like the atmosphere, it's 100% saturation. But as you're like forcing that water down, it's taking extra atmospheric pressure in, and it creates higher levels than normal. And that can actually cause, I think one of the biggest things that I see in adult salmon is like nitrogen burns. So their faces will turn black. And a lot of them can recover if they're adults, but smolts can't really recover from that because it affects their gills. And if they're currently in transition from freshwater to saltwater, any impact on their gills is going to significantly reduce their chances of survival.

(John McKern) The deeper you go, the more water pressure there is on those bubbles and more goes into solution. So at the bottom of a stilling basin, like at one of these lower Snake River dams, it can be 150%.

It's bad for fish because the fish living in it take that supersaturated air in through their gills into their bloodstream. And once it's there, if it's at the supersaturated level, if they come up into a shallower area, it forms bubbles. That can be bubbles in their skin, bubbles behind their eyes, bubbles in the filaments of their fins. And if it's severe enough, causes an air embolism and kills them.

(John McKern) What happens with gas supersaturation in the string of reservoirs is that once it goes in at the dam upstream like Lower Granite, it stays high all the way down to the next stream dam. At the next dam, it's raised up again, stays high, and all the way downstream through 7 reservoirs and eight dams.

And so it doesn’t dissipate after a short distance?

No, it doesn't. It does in nature where you have water flowing over a riffle and the water is turbulent and the air in the bubbles can leave through the membrane of the bubble and return to normal saturation. But in this situation in the reservoirs, it stays in the water until it comes up to the surface. At the surface, if you're out there when there's supersaturation, it sounds like Alka-Seltzer.

 The supersaturation is caused by the normal spillway bays. Water going through the turbines does not increase gas supersaturation. In 1999, we came up with a raised spillway weir to divert water from the surface over the dam, and it does not plunge down like the water that spilled under the normal spillway bays, and it doesn't aerate like the water spilled under the normal spillway bays. So it doesn't increase supersaturation. So if you have a combination of powerhouse and spillway, They blend together and it gives you a lower level. And the fish can tolerate, juvenile fish and adult fish can tolerate up to about 110, 12, 13%. If you go over 115%, you're getting them in trouble.

So, is there disagreement over what’s a safe level for the total dissolved gas in the water, especially over longer periods?

(Melissa Wager) There's definitely disagreement. I know that there's been studies done where 110% can show negative impact if that's sustained for too long. For example, at our facility, if we have even like 102 to 105% for even just a little bit too long, we'll end up seeing that like when we raise sturgeon, we'll have popeye and exophthalmia and some of the negative side effects at 105%. But it's more about like the environment. So like you said, like how long it is, how warm the water temperature is, are they able to recover? And timing, of course, because our smolts are significantly at a higher risk than our adults are. especially because that can delay their migration time, and they might not be able to avoid predation out in the estuaries and things like that.

McKern says historically up to 16 million salmon would return annually up the Snake and Columbia, but their decline began well before any of the large dams were built.  

(John McKern) What caused it was commercial fishing, the gill netters, the trap men, the seiners, all caught Columbia River salmon, and at one time there were 53 canneries between the Dalles and Astoria canning Columbia River salmon. It peaked in 1880 with 41 million pounds of salmon caught, which was primarily Spring Chinook.  

McKern says it wasn’t just commercial fishing that caused the decline, but also habitat issues. The discovery of gold in Weippe, Idaho led to a gold rush that turned miles of salmon habitat upside down as they looked for gold in rivers and creeks. They sometimes used huge gold dredges, which did enough damage that it required tens of millions of dollars to restore the habitat.

The river fishery has petered out to almost nothing. but ocean catch is still happening from Alaska to British Columbia, Washington, Oregon, and  it's being controlled better, but it's still a major impact to the fish.

And then of course the dams. Not just the large one’s, but hundreds of smaller dams built on tributaries around the state that blocked salmon habitat.

The smolts also faced losses from natural predators like the northern pike minnow and many others.

Over the years the Corps of Engineers worked to improve fish passage and juvenile survival by decreasing the mortality rates due to turbines, improving fish ladders and more.

McKern also says transporting fish around the dams on barges to avoid gas supersaturation has been very successful with high survival rates. But the fish have to go through the powerhouse and the collection system to be captured and  transported into those barges. But high flows over the normal spillways make that difficult.

Wagner says there may be some benefits with the higher, faster spills, but also negative aspects, too.

(Melissa Wager) One of the things we had talked about was they tried to barge the smolts over. And part of that is to, protect them and make sure that they can make it through. But if you're forcing higher flows through, that kind of reduces their ability to even capture those fish in order to put them on the barge. And so, and that's a huge drop regardless of which dam they're going over. That's a big drop for a small fish.

(John McKern) What the fisheries people have concluded and convinced fisheries, I should say, and environmental people have concluded that spill is the safest passage route. It is not. It's not for two reasons. One is because of the gas supersaturation, and it also, the more you spill, the fewer fish go through the powerhouse and are bypassed into collection systems where the juvenile fish can be transported downstream. around seven other dams and release down below Bonneville Dam.

(John McKern) In-river survival is measured every year by the National Marine Fisheries Service, and it runs between 40 and 60 percent for the Chinook salmon. For 50 years the transport's been going on, the survival rate has been about 98% for those transported fish. It's gotten better as the years have gone by. But the more you spill, the fewer you collect. So you've got a trade-off here of 40 to 50% mortality in the river or 98% survival in the barges going downstream.

McKern says one the main issues he sees is when the allowable Total Dissolved Gas levels for Salmon passage were raised to 125%.

125%, according to all that research that was conducted back in the 70s and 60s is not a good idea. So you say, maybe things have changed. No, they haven't changed. There's new research that comes out of Scandinavia with Atlantic salmon, which are most like steelhead. And it says that if you go above 110, 115%, you are in trouble. And 125%, the way this river is being operated just astounds those people.

Is the fix that they're recommending that this judge ordered actually doing more harm than good?

(John McKern) My opinion, yes, it does more harm than good. If you really want to do what's best for the fish, let the fish facilities at the dams operate, let the dams operate so that they don't create gas supersaturation, collect and transport fish, If you want fish to pass in river, use the overflow spill.

Do you think this ruling was based on bad science?

(John McKern) I'm not going to say bad science. I'm going to say ignoring or not even considering the science. Because like I said, the gas supersaturation problem is pretty well known on this side of the world and the other side of the world, but we have a group down here that thinks it's okay. They think it's okay if these fish can come up in the water column or go down to equilibrate that gas in their bloodstream. Doesn't happen that way.

(John McKern) And I think that the decision to spill that water as they're doing now is arbitrary and capricious. What's it taking away from the navigation industry? What's it taking away from the power pool? That's arbitrary and capricious in my mind.

(Melissa Wager) It's probably not a secret, especially for what I do. I am a huge supporter of the salmon. I love seeing the salmon come back. I want to see the salmon recovered. But at the same time, I understand like those generate power, the dams generate power. So there's two factors leading into this is 1, hydropower is one of the most green types of renewable energy that we can have. And the other is we already have them established. If we were to remove those dams right now, all of that sediment that's built up, you're going to remove all of that spawning ground for other species. I think ones that constantly get overlooked are sturgeon.

And so if we were to wipe out even remotely any of their spawning grounds, then that's a major decline.

Wagner says she’d like to see continued improvements to the turbines, the fish ladders and building side channels, as well as adjusting both the spill levels and when the spills happen.

(Melissa Wager) Big picture, I would want to say  we really work to try to find a balance. You know, that's how we have to do everything at this point. But, we want to have that safe passage for our salmon, our sturgeon, our lamprey, all of our important species that require that navigation. But we don't want to have like a reliance on just one renewable energy. So if hydropower is a possibility and it is a really good one, we don't want to be able to lose that.