Where Do Salmon Go In A Flood?

Blog Number 14

February, 2023

January 2nd, day after New Years, 2023. I caught a flash of red below the deck overlooking Rocky Brook, which here can mean only one thing: coho salmon. And, this time of year meant that they would be wild, not hatchery coho as is so often the case in October.

At the very same time, a Washington Fish & Wildlife biologist was heading upstream. I knew he had just finished a walk from the bridge to the confluence with the Dosewallips River, counting fish, so I shouted out, “How many?”

He yelled back, “Nine coho”.

Wild coho spawning in early January is not unexpected. In the twenty some years I have lived along Rocky Brook, I have noticed a late December, early January small run of wild fish most years.

Ten days later, one of those “atmospheric rivers” that meteorologists talk about nowadays arrived, dumping rain for a week.

Rocky Brook at flood stage is a rolling, roaring, roiling torrent. It completely takes over the landscape, water from bank to bank, new channels forming to help deal with the huge volume of water, and higher areas with standing water as the porous soil is too saturated to allow for movement downward.

What happens to salmon when a river reaches flood stage?

This is a three part answer, since Rocky Brook now contained salmon at three life-cycle stages; eggs, embryos & alevins, fry & fingerlings, and spawning adults.

Eggs, Developing Embryos & Alevins
Recently deposited eggs, developing embryos and alevins are trapped in stream bed gravels. They cannot swim to another, less turbulent part of the river. They have to ride it out, hoping that the forces of all that water will not draw them up into the moving water where they most certainly perish.

Salmon at this stage are impacted by two things, scouring, and sedimentation.

Scouring refers to the situation where flowing water rearranges stream bed gravels. Adult female salmon usually choose a place to excavate a redd and deposit eggs which has good water flow, since the developing embryos need a regular supply of oxygen. Such sites are often below a riffle, where the water is stirred up, resulting in increased dissolved oxygen. Rocky Brook is particularly well oxygenated because of the falls and, below it, steep gradient and narrow, rocky canyon, creating the classic pool and riffle channel.

The problem with these sites is that during flood stage, this location may be subject to severe scouring. Of course, the female cannot know if this is going to be a severe flooding year; most of the time such locations are ideal for embryo and alevin development.

The historically large salmon runs resulted in females spawning in almost any available space, often later arriving females digging up and replacing previous egg deposits of earlier fish with her own. Thus, there would inevitably be places where at least some eggs would survive during a flood.

Today, with the smaller runs, this may not be the case, egg mortality due to flooding is undoubtedly higher.

Sedimentation is the second way that developing embryos and alevins may be killed as a result of river flooding.

At flood stage, a river is able to carry a much greater amount of sand and silt. Rocky Brook and the Dosewallips both go from crystal clarity to thick soupy gray and brown during a flooding event. As the rains slow and the water returns to normal, such sands and silts sink into the stream bed. Such deposits can be quite thick, and bury or smother the embryos and alevins.

Eventually water movement will sluice out these materials, cleaning gravels, but it may take months, or even years. The embryos and alevins cannot wait. By then, they have already died from insufficient oxygen or the inability to move through the gravel when the time comes to become a free-swimming fry.

Fry & Fingerlings
At the time of this year’s flooding, Rocky Brook’s salmon were not all trapped in the stream-bed gravel. There were some fry and fingerlings from the previous year; coho, steelhead, and rainbow trout.

Fry (1 to 2 inches long) and the larger fingerlings (3 to 4 inches) can move during a flood. Undoubtedly they find places along the edge or behind logs and larger rocks where the force of the water is reduced. There they wait. Rocky Brook is a relatively small watershed. Once the rains stop, after two to three days, the water level drops; within a week it is back down to what is “normal” for this time of year.

Spawning Adults
What about those nine spawning adults mentioned at the beginning.

I cannot be sure of their fate. I do know that adult salmon are accustomed to waiting for river conditions to improve. I have watched schools of adults mark time in a deep pool in the Dosewallips at its confluence with Rocky Brook for over a month, waiting for enough rain to clean out summer gunk in Rocky Brook and water levels to rise enough so they can swim up it to spawn. I would guess that the same thing happens during a flood. They find a place less buffeted by the moving water; here, most likely in the larger Dosewallips where there are more side channels.

When the water levels dropped, I did not see these pairs in Rocky Brook, but that does not mean they were not there. Wild coho are very good at hiding and finding places which I may not see. I hope they spawned successfully. If they did, their fry would grow into fingerlings in Rocky Brook 2023, head out to sea in the spring of 2024, live in the ocean someplace and grow for two years, and finally return as adults, 2026.

Of course, the most spectacular thing about a flood here is to see Rocky Brook Falls at this time. At the head of the canyon, a second waterfall, much higher appears on the slope next to it. At flood stage, Rocky Brook Falls has so much water coming down that it creates a misty wind which doesn’t allow one get very close. It is one of those “experience the power of nature” moments, when one feels small and unable to do anything but just marvel at it all and observe from a safe, relatively dryer distance.

Straying
One final point worth mentioning. When a river experiences a significant flooding event or some other major disturbance (landslide, earthquake, volcanic eruption) salmon have a kind of back-up plan. A certain percentage of a population (some estimates put this number at 10%) that returns to a particular river will “stray” meaning that they move up nearby rivers and streams rather than their “home” stream where they started out. This ensures that the population (gene pool) will survive the disturbance and usually return to the river at some future date when things have returned to normal.

The most dramatic example is the Mt. St. Helen’s eruption, in May, 1980. The heat and ash and mud flows essentially killed all the young coho, chinook and steelhead in the North Fork of the Toutle river resulting from the 1979 spawning season. This population’s genetic pool was saved by those spawning adults that strayed and spawned in adjacent rivers and streams in 1979. In succeeding years, when things improved in the Toutle, undoubtedly some of these fish naturally strayed back up the north fork Toutle, in this way, preserving the population.

Hopefully, in late December or early January 2026 another nine (or even more) wild coho will come up Rocky Brook, indicating that in spite of the floods, they successfully spawned. I’ll be watching, and hopefully still writing down what I see and learn.

Dennis Lloyd Kuklok
Blog Number 14
Rocky Brook, February, 2023