Rivers in the United States are warming faster than anyone expected. Scientists are warning that all of that extra heat is already hurting some cold-water fish like salmon and trout. A new study in Proceedings of the National Academy of Sciences investigated more records from more than 40 years from around 1,500 monitoring sites run by the United States Geological Survey.
Professor Li Li of Penn State University and the leader of this study shows a transparent increase in the frequency, intensity, and time of “heat waves” in the river.
The biggest part of the problem is coming from climate change, and it’s really not a game; there’s less snowpack and slower streamflows with time. And at the same time, human factors don’t help, making more dams, buildings, and pavement just seems to make local warming worse. The study also includes insights from Jonathan Walter of the University of California, Davis, who was not part of the research team but still offers practical ideas to reduce the damage.
What’s really happening in the rivers
This study is digging into rivers’ “heat waves,” which means that for 5 consecutive days or more, the rivers water temperature is above what’s normal for that season. The team compared data from 1,471 United States Geological Survey stations between 1980 and 2022. With information from the past and the present, they could see how conditions changed across the country. And what did they find?
In 2022, there was an average of 1.8 more river heatwave events per year than in 1980. And those events were even hotter: during these heatwaves, water was about 0.8°F warmer than in 1980. And to keep adding in, they also lasted longer, these episodes stretched three days more (even more on some occasions) compared to before.
“The trend of river heat waves is actually increasing faster than the air heat waves. So, that’s one thing that is very surprising,” said Li Li. The warming connects to climate change (less snow, melt arriving earlier, slower flows) and to local impacts too, with dams slowing water and urban buildings and streets. It’s a cycle; they soak up heat and pass it on to the air and then the water.
How is it affecting fish
Heat reduces dissolved oxygen in water. And when temperatures rise, it makes cold-water species like salmon and trout speed up their metabolism. Basically it means they need more oxygen just when there’s actually less available. This extra heat is adding on about 12 more “thermal stress” days each year for them.
When they were doing the analysis, the authors used 59°F as a stress threshold because, for example, some trout (like the bull trout), the temperature where it begins to struggle.
Jonathan Walter (University of California, Davis) explains, “The way that we use water, the way we manage those resources can have an impact on heat waves… Assuming adequate amounts of water in a reservoir, we might be able to release water in such a way that downstream temperatures would be somewhat cooled and prevent these hot and stressful temperatures.” In other words, if the reservoirs have enough water, there could be some scheduled release, where cold water is added carefully and timed, so the river can cool down a bit and fish get a break during the heat waves.
What we can learn and do about it
The question is not only “How much water is left in the river?” but also “How warm is it?”.
Li puts it like this: “When we think about water, there’s a lot more attention about what quantity, because we can see it… We’re hoping this kind of study will increase the public awareness of the water quality issue related to climate change.”
The next step would be using it to protect the rivers.