According to a new study, the frequency of river and stream heat waves is increasing.
Riverine heat waves, like marine heat waves, occur when water temperatures rise above their normal range for five days or more (SN: 2/1/22).
Researchers compiled daily temperatures for 70 sites in rivers and streams across the United States using 26 years of US Geological Survey data, and then calculated how many days each site experienced a heat wave per year. The annual average number of heat wave days per river increased from 11 to 25 between 1996 and 2021, according to a study published on October 3 in Limnology and Oceanography Letters.
According to Spencer Tassone, an ecosystem ecologist at the University of Virginia in Charlottesville, the study is the first to assess heat waves in rivers across the country. He and his colleagues recorded nearly 4,000 heat wave events, increasing from 82 in 1996 to 198 in 2021, totaling more than 35,000 heat wave days. The frequency of extreme heat increased at sites above reservoirs and in free-flowing conditions, but not at sites below reservoirs, possibly because dams release cooler water downstream.
According to Tassone, the majority of heat waves with temperatures above average occurred outside of summer months between December and April, indicating warmer wintertime conditions.
Human-caused global warming contributes to riverine heat waves, with heat waves partially tracking air temperatures — but other factors are most likely also at work. According to the study, less precipitation and lower water volume in rivers, for example, allow waterways to warm up more quickly.
"These very short, extreme temperature changes in water can quickly push organisms beyond their thermal tolerance," Tassone says. He claims that, when compared to a gradual increase in temperature, sudden heat waves can have a greater impact on river-dwelling plants and animals. Salmon and trout are especially vulnerable to heat waves because they rely on cold water to get enough oxygen, regulate their body temperature, and spawn properly.
The heat has chemical consequences, according to hydrologist Sujay Kaushal of the University of Maryland in College Park, who was not involved in the study. Higher temperatures can hasten chemical reactions that contaminate water, contributing to toxic algal blooms in some cases (SN: 2/7/18).
According to Kaushal, the research can be used to help mitigate future heat waves by increasing shade cover from trees or managing stormwater. Beaver dams in some rivers show promise for lowering water temperatures (SN: 8/9/22). "There is something you can do about it."
S. Tassone et al., Increasing heatwave frequency in US streams and rivers. Letters in Limnology and Oceanography doi: 10.1002/lol2.10284, published online October 3, 2022.