On November 15th, 2021, the Skagit River in Mount Vernon crested at a height of 36.98 feet, just five inches short of the previous 1990 record. Across the county, rivers replaced roads, riparian zones became stream beds, culverts filled to the brim, and homeowners’ backyards and living rooms were inundated with water. The culprit? A succession of atmospheric rivers that brought several days of heavy rain to the region. As we drove past fully submerged Christmas tree farms, kayakers paddling downtown roads, and cars flipped sideways, I grieved for the widespread damage and loss, but I also couldn’t help but think of the salmon. What does this massive flood event mean for them? Where do they go? Will they survive?

A major consequence of climate change in Western Washington, and the Skagit Watershed in particular, is an increase in flood frequency and intensity in the winter months. As temperatures warm, our atmosphere can hold more moisture. As we urbanize our landscape through paving and development, we increase run-off volumes into streams and rivers, and decrease our resilience in the face of significant precipitation events. As more precipitation falls as rain in the upper reaches of our watershed, streamflow increases more rapidly. While the November 15th event was rare and anomalous, climate change models project that these intense winter flood events will become less rare in the decades to come. Floods that have historically reoccurred every 50 years will likely start to happen at 20, 15, and even 10 year intervals. How will the salmon fare?

While salmon adapt to their local flood regimes and minor flooding can actually be beneficial for salmon (i.e. new habitat, clearing of sand and silt, wood recruitment, and new food sources), they are not yet adapted to the anomalously large flood events that we’re seeing with increased frequency in the Skagit Watershed. In their hydrologic model, researchers Mantua and Tohver (2010) found that Western Washington’s historic 20-year flood return frequency will experience the most substantial flood increases in December and January. Unfortunately, these early winter months coincide with the spawning season for several salmon and trout species, when salmon are in their earliest life stages and thus most vulnerable to flood disturbances.

During spawning, salmon deposit and bury their eggs in gravel nests in the stream substrate. These nests, called redds, protect the eggs from high flows and other environmental threats, like predators or sediment build-up. However, there is a water velocity threshold at which the eggs are washed downstream and unable to survive. Additionally, climate change-induced high flow events can mobilize and transport large volumes of sediment atop the eggs and suffocate them. Because the depth of redds varies across species, certain species will be more greatly impacted by these peak flows. Smaller fish, like pink salmon, are unable to bury their eggs as deep and prefer to build their redds in smaller gravel than other salmon species, and are thus more vulnerable to being washed away through streambed scouring.

 

Flooding can also threaten fish by displacing them in floodplains and isolating them from their river channel habitats. High flows have the potential to flush salmonids downstream and into estuaries or marine water bodies. The draw backs of this are two-fold: if the juvenile salmon rearing in freshwater aren’t killed by the great forces of the flooding Skagit on their way downriver, they can be washed into salt water before they are physiologically prepared for saltwater life stages and perish as a result. Furthermore, increased stormwater runoff can incorporate toxic contaminants into freshwater systems, which is especially deadly for coho salmon. Finally, high precipitation events, like the mid-November storm, trigger landslides and mudslides, which add detrimentally large sediment loads to streams and rivers. These sediment influxes can clog gills, impact feeding, and bury salmon eggs. In extreme cases, landslides can block entire streams or rivers. For example, the 2020 Big Bar landslide blocked a critical migration route in the Fraser River in B.C., and threatened to decimate local sockeye and chinook populations.

The negative impacts of climate change are great, but there are greater reasons to continue the pursuit of salmon population enhancement and environmental restoration. In their 2007 study, researchers Battin and others used models of the nearby Snohomish River to study salmon responses to climate change. While their model’s projections for summer streamflow and winter peak flow don’t bode particularly well for Chinook populations, they acknowledge that their models are unable to take into account the remarkable adaptability of salmon. They speak to potential for salmon to adjust to higher winter peak flows by changing the timing of their egg laying. If salmon deposited their eggs just a month or two later in the winter, would flood mortalities be significantly reduced? In their paper, they encourage research that examines the likelihood of these evolutionary or behavioral responses to climate change, as they’re not yet well-understood.

On an optimistic note, they also speak to the promise of restoration efforts – like floodplain connectivity and riparian planting – as an effective means to further buffer the detrimental impacts of winter flood disturbances for our salmon populations. SFEG and other organizations implement these types of restoration projects every year. Pacific Northwest salmon have co-existed with active volcanoes, earthquakes, and dynamic, ever-changing river systems. With our help and continued efforts, perhaps they can withstand climate change as well.