Warming Temperatures Are Impacting Salmon Streams
Climate change is threatening the clean, cold, and plentiful water in rivers that salmon need to survive.
Climate change has warmed the air across Washington by 0.15 degree Fahrenheit every decade during the past 100 years,23 a trend that is projected to continue but at an even faster rate, reaching increases of up to 5.3 degrees Fahrenheit every decade by 2090.24
As the air warms, glaciers, which store much of the freshwater in the Pacific Northwest, melt and have less cold water to feed streams in the summer when salmon need it the most.
Glaciers are Melting
In Washington, nearly a quarter of Mount Rainier’s and more than half of the north Cascade and Olympic Mountain ranges’ glaciers have melted,30 and the rate of melting is only expected to increase.31 Scientists estimate 80 percent of western North America’s glacier ice volume will be lost by 2100.32
Scientists already are seeing less water in summer streams,25 and for longer periods of time. Scientists estimate that the amount of water that was released from melted snow declined by 21 percent in the western United States from 1955 to 2016.26 Compounding the problem during low-flow periods is when more water is removed from rivers to irrigate farmland and accommodate demands of an increasing human population.
As the amount of cold water from glacier-fed rivers declines, the water temperature in rivers will continue to increase. For example, the temperature in the Snake River, through which many salmon species migrate, rose 1.4 degrees Fahrenheit from 1960 to 2015.27
Streams with temperatures greater than 64 degrees Fahrenheit can stress salmon, and when rivers reach 70 degrees Fahrenheit, salmon begin to die.33 Without actions to address impacts from climate change there will be fewer salmon and fewer rivers where they can survive.
Climate Affects Water in Streams: The amount of snow in the mountains is decreasing. Scientists project that the average spring snowpack in Washington will decline by 56-70 percent by the 2080s.28 Some Washington streams are getting hotter, stressing salmon. Scientists estimate that 1,016 Puget Sound river miles will exceed 64 degrees Fahrenheit for up to 7.5 weeks.29
The changing climate also is bringing rain instead of snow to some areas. With more rain, scientists predict winter streamflow will increase by 25-34 percent by the 2080s,34 increasing the likelihood of severe flooding in the winter.35 This issue is compounded when floodplains, which are natural storage areas for excess water, are developed for other purposes.
Not only is the amount of water in streams predicted to change but so is the timing. Scientists already are seeing annual peak flows occurring 1 to 4 weeks earlier.36
As peak flows change in volume and season, the delicate life cycle of salmon will be disrupted. Glaciers and snowpack are melting earlier in the season. Faster and stronger running rivers can destroy redds (salmon nests) and flush young salmon out of their calm-water habitat, reducing their chance for survival.37 While some juveniles may survive this premature transition from freshwater to saltwater, many won’t be large enough to catch prey or avoid being eaten.38
Changes in Climate Impacting the Ocean
Human activities have resulted in record-breaking levels of carbon dioxide in the atmosphere and in the oceans.39 Excess carbon dioxide is absorbed by the ocean, forming carbonic acid, which has driven up the average ocean acidity level by 30 percent since the Industrial Revolution.40 Increased acidity damages the phytoplankton, zooplankton, and crustaceans salmon eat. Excess carbon dioxide also can change the way salmon use their sense of smell to find food, avoid being eaten, and find their natal streams.41
The climate also impacts ocean temperatures. During the past 50 years, the near-surface waters off Washington’s coast have warmed by roughly 1.8 degrees Fahrenheit.42 Warmer water has fewer nutrients and less oxygen than colder water and creates conditions less beneficial for salmon. For example, warmer water favors sub-tropical zooplankton, which are poor food for juvenile salmon and the fish they eat.
Salmon have demonstrated during the past 10,000 years that they can adapt to a changing environment. For example, salmon have coped with low streamflows and high temperatures by waiting in cold water pools until they are able to continue their journey. Climate change has exacerbated these problems. When combined with fewer natural buffers, degraded habitats, and lost genetic diversity within the salmon themselves, the salmon are challenged to change quickly enough and their survival is at risk.43
Photograph of Elk Mountain fire by John McColgan, U.S. Forest Service