As climate change continues to worsen, heatwaves like one that killed hundreds of people as it rolled across the Pacific Northwest last summer have become more common and more extreme. These heatwaves can lead to mass die-offs of fish in streams that become too warm for them. However, some salmon have found a surprising savior in this battle to survive: glaciers.
Rising atmospheric temperatures and extreme heat events have spelled disaster for Pacific Northwest communities. Heat-related illness and deaths for people are becoming more common, and environmental resource managers are dealing with droughts during summer months that make water allocation more difficult. Warming stream temperatures and low flows put cold-water species like salmon at higher risk of heat stress, disease, and even population die-offs, and heatwaves have sometimes been the tipping point between tolerable and deadly stream temperatures for salmon.
Glaciers can alleviate some of these effects by producing cold meltwater for the streams and rivers that receive their meltwater. In this way, they create buffers against high temperatures and low streamflows. However, the precise contributions of glaciers to streams and the effects of climate change on those contributions remain questions that can only be answered through long-term studies.
Mauri Pelto, the director of the North Cascade Glacier Climate Project (NCGCP), has been working to answer questions like those throughout his research career. An environmental science professor at Nichols College, he founded the NCGCP in 1984 to monitor the long-term response of glaciers to climate change. Along with a changing team of researchers including undergraduates, graduate students, and his own children, he has been studying these glaciers and tracking changes in their mass, terminus location, and water runoff for almost four decades. Though the main focus of Pelto’s project is tracking and understanding glacier retreat, his work also includes collaborating with community partners to study the potential impacts of glacier retreat on wildlife, including salmon, ice worms, and mountain goats.
The latest study from the NCGCP, published in the journal Water this April, analyzes how the contribution of glacier runoff to streams during heatwaves can change stream conditions and potentially protect salmon populations at risk of heat stress-related harm in rivers. The research team compared two sections of the Nooksack River in northwest Washington—the heavily glaciated North Fork and the unglaciated South Fork—and tracked stream temperature, water discharge, and glacier runoff during 24 heat events over the past decade.
The study quantified the effects of glacier runoff on stream temperature and discharge during a heatwave, confirming trends that water resource managers are familiar with. Heat events typically lead to warmer stream temperatures and lower flows. However, in glaciated streams, glaciers melt quicker and contribute more cold runoff to streams, buffering the temperature and water volume rates. While this general trend is understood, these differences have never been quantified, making it difficult to accurately predict the impacts heatwaves might have on rivers and species living in them.
The new study found that during heatwaves, water volume in unglaciated streams like the South Fork Nooksack declines by about 20% while water volume in glaciated streams like the North Fork Nooksack increases by 20%. The water volume in a river or stream affects the amount of dissolved oxygen and the temperature of the water, as well as the discharge levels, all of which impact salmon populations.
Pelto told GlacierHub that the impacts of glacier runoff for stream temperature during heatwaves were less dramatic than differences in water volume, but still significant. In the unglaciated fork, water temperatures rose by about 2 degrees Celsius while only rising 1 degree Celsius in the glaciated fork. However, Pelto points out that “the stream that doesn’t have glaciers is already starting at a higher temperature, so it does end up being more of a problem for salmon.”
The differences between these two forks are already clear for salmon. The South Fork has been exceeding tolerable temperatures for salmon (which is about 19 degrees Celsius), and last summer, there was a mass die-off of 2,500 Chinook salmon in the South Fork Nooksack River following a heatwave. Pelto explains it’s not that the salmon die of the heat itself but that “their systems are stressed out, which makes it easier for an algae or bacteria explosion to harm them.” In the North Fork, stream temperatures did not rise nearly as much and the salmon emerged from the heatwave unscathed.
As glaciers retreat and disappear, more rivers will begin to resemble the South Fork Nooksack. This means salmon populations will be more susceptible to the negative impacts of heatwaves and heat events, and large die-offs like the one seen last summer may become more common.
Salmon are a vital cultural, economic, and ecological resource in the Pacific Northwest, and their loss would be devastating to the region. Understanding the quantifiable ways in which glaciers buffer stream temperatures and discharge rates will allow fisheries managers to protect salmon and prioritize at-risk salmon populations in the future.
