American rivers are getting hot. The frequency of heat waves in rivers and streams is increasing, a new analysis shows.
I like it marine heat waves, River heat waves occur when water temperatures slide above their typical range for five days or more (Serial number: 2/1/22). Using 26 years of data from the US Geological Survey, the researchers compiled daily temperatures from 70 sites in rivers and streams across the United States, then calculated how many days each site experienced a heat wave per year. From 1996 to 2021, the annual average of river heat wave days rose from 11 to 25the team reports on October 3 at Limnology and oceanography charts.
The study is the first assessment of heat waves in rivers across the country, says Spencer Tassone, an ecosystem ecologist at the University of Virginia in Charlottesville. He and his colleagues counted nearly 4,000 heat wave events, jumping from 82 in 1996 to 198 in 2021, and adding up to more than 35,000 heat wave days. The researchers found that the frequency of extreme heat increased at sites above reservoirs and in free-flowing conditions, but not below reservoirs, possibly because dams release cooler water downstream.
Most of the heat waves with higher temperatures above typical ranges occurred outside of the summer months between December and April, pointing to warmer winter conditions, Tassone says.
Human-caused global warming plays a role in riverine heat waves, with heat waves partially tracking air temperatures, but other factors are likely driving the trend as well. For example, less rainfall and less volume of water in rivers means that waterways heat up more easily, the study says.
“These very brief and extreme changes in water temperature can quickly push organisms beyond their thermal tolerance,” says Tassone. Compared to a gradual increase in temperature, sudden heat waves can have a greater impact on plants and animals that inhabit rivers, she says. Fish like salmon and trout are particularly sensitive to heat waves because the animals rely on cold water to get enough oxygen, regulate their body temperature and spawn properly.
Heat also has chemical consequences, says hydrologist Sujay Kaushal of the University of Maryland in College Park, who was not involved in the study. Higher temperatures can accelerate chemical reactions that pollute water, in some cases contributing to toxic algal blooms (Serial number: 2/7/18).
The research can be used as a springboard to help mitigate future heat waves, says Kaushal, for example by increasing tree shade cover or managing stormwater runoff. in some rivers, beaver dams show promise to reduce the temperature of the water (Serial number: 8/9/22). “You can actually do something about it.”