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What are the impacts of flooding on Salmon?

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In early December, floods devastated many communities here in Washington state. What are the impacts on salmon from such flooding?
from a 3-4 Grade Teacher in Poulsbo, Washington

Professor Salmon’s response:

Flooding creates many dangers for salmon, just as it does for humans.

• Flood waters are dangerous and fast, full of obstacles. Flooding washes salmon up to areas on land with many dangers.  The water is moving very fast and is as opaque as chocolate milk with sediment and debris. The salmon cannot see and swim away from obstacles, so they get smashed into homes, gas stations, churches, playground equipment, trees—everything under the water.  Like humans, they are helpless.  They can get impaled on sharp objects, knocked unconscious and drown from not breathing with their gills.
• Salmon can drown, the same as a human.  They drown from exhaustion. Swimming constantly, they become too tired and hungry to do their osmo-regulation- keeping water out of their tissues. In a sense, they drown.
• Flood waters are full of poisons and toxins that kill fish and humans.  During the flood, all the water helps dilute the effect somewhat. But, as a means to find their way home to their natal streams, salmon are extremely sensitive and can detect chemicals in parts per billion.  These sensitive salmon are exposed to a wide variety of toxins washed from flooded sites such as gas stations, cars, the local dump, septic tanks, sewage treatment plants, pesticide storage areas. All the washed away chemicals and toxins can kill adult and young salmon and can change the juvenile’s DNA.  (See epigenetic impacts.)  Diesel and gasoline can turn a juvenile male salmon into a female salmon. 


Toxins washed from flooded sites can kill salmon and can change their DNA.  Also, salmon can become disoriented as the water recedes.
Photo: The Olympian

• Because their sense of smell is overloaded and the waters are full of chemicals as described above, salmon cannot detect which receding waters to follow. They can get stranded in a field or in a tree when the water recedes. Many become confused and fight against the current of the receding water (a natural salmon reaction). They swim towards the dry land, apparently perceiving that the water is flowing from a lake when,in fact, it flows from a parking lot.

 

• After the flood, salmon may be blocked from their stream or even adjacent streams due to remaining debris and washed out culverts.

 

• Many fish ladders and beneficial works put there by people that care about salmon are washed away.

 

• Millions of juvenile salmon are chopped up by pumps, because flood pump houses (usually beside a dyke) are running at full capacity during the flood and flood restoration workers operate thousands of smaller pumps.

 

• A flood will cause “egg scour”, in which the precious salmon eggs in the reeds are washed away and destroyed by the swift water, debris and tumbling boulders and cobbles.

 

• The salmon’s food supply is washed away in the flooding.  Juvenile salmon eat smaller fish and invertebrates, often aquatic insects in their unwinged water stage.  They are gone with the flooding and scouring.  Imagine having your refrigerator and the groceries in your cupboards washed away.

 

• Many salmon die from abraded gills (worn away from the friction and sucking in of silt and sand) from all the sediment and bruising even weeks after the event is over.

The impacts of flooding are particularly harsh in settled areas.  In a completely natural watershed, the impacts on humans, salmon and their habitat are far less severe.

Insurance and dykes has given us false security and lead us to make unwise land use decisions. The wisest thing humans can do is to NOT build on the flood plain.  Dyking and culverting will not protect us, and climate change is making these works less effective.

Instead, we can farm on flood plains, grow good food and forests, and live on the lower slopes.  This allows the natural watershed function of soil renewal and water cleansing to take place.  It will protect our water supply and keep it clean and cool, and we will not get harmed nor our homes destroyed.  We can take our animals and pets to higher ground during flooding events, as humans have for thousand of years.

We are touched that your class cared to ask about the salmon when there was so much human loss associated with this flood disaster. One way to explore this sad topic with young students is to ask them what impacts they experienced and know about, and explain that the salmon had very similar impacts.  Children who want to help salmon can volunteer for a stream clean up or planting project with a local Streamkeeper group. Maybe the school can start one!

Our warm wishes and kind thoughts for you and your community as you complete your recovery efforts.

Professor Salmon