U.S. rivers are running hot.
A new analysis of nearly 1,500 river locations over more than 40 years found that the frequency, intensity and duration of heat waves is increasing in streams across the country, posing a threat to many species that are adapted to cooler temperatures.
“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, a professor of environmental engineering at Penn State University and an author of the study.
The new analysis, which was published in the peer-reviewed journal Proceedings of the National Academy of Sciences, is the first in-depth study of riverine heat waves, which are defined as five straight days of high temperatures in comparison to seasonal averages.
The authors found that human-caused climate change is the primary driver of the trend, as snowpack dwindles and streams flow more slowly.
Other human factors also affect the trend. Dams slow the flow of water downstream. Buildings and pavement absorb heat that warms the air and then the water.
Scientists expect that species that rely on cold water, including salmon and trout, will increasingly struggle as streams heat up. Li said it will be important to document fish kills to see if and what trends are emerging.
Heat decreases the amount of dissolved oxygen in water. Meanwhile, cold-water species often see their metabolism rise in warmer waters, meaning there’s less oxygen available.
The findings could inform how to manage watersheds and whether changes to irrigation or dam releases could help species hang on during hot weather.
“The way that we use water, the way we manage those resources can have an impact on heat waves,” said Jonathan Walter, a research scientist at UC Davis’ Center for Watershed Sciences, who was not involved in this research. “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.”
Heat waves have been studied extensively in oceans and lakes, but until now researchers only had limited information on how higher temperatures were affecting streams.
Satellite data drives research of heat in oceans and lakes. Studying river temperatures is more difficult because it’s reliant on stream-gauge sensors that are often in and out of service, leaving data gaps that can be hard to parse.
For the new study, the researchers collected data from 1,471 United States Geological Survey stream monitoring sites from 1980 to 2022.
The data allowed the researchers to assess changes in streams nationwide. They found that riverine heat waves in 2022 were happening more often than in 1980 — with an average of 1.8 additional heat wave events taking place per year. The heat waves were also more intense, with temperatures during heat events about .8 degrees Fahrenheit higher, on average, in 2022 in comparison to 1980. Heat wave events lasted more than three days longer than in the past.
Li said the additional heat is causing about 12 more days of heat stress, on average, for species that need cold water. The study’s authors used 59 degrees F as its threshold for heat stress because some species, like bull trout, struggle to survive at about that marker.
“When we think about water, there’s a lot more attention about what quantity, because we can see it,” Li said. “We’re hoping this kind of study will increase the public awareness of the water quality issue related to climate change.”
