U.S. rivers are in hot water. A new analysis has shown that stream and river heat waves are on the rise.
Like Marine heat wavesRiverine heat waves are when water temperatures rise above their normal range for five days or more (SN: 2/1/22). Researchers used 26 years’ worth of United States Geological Survey data to compile daily temperatures for 70 streams and rivers throughout the United States. They then calculated how many days each site had experienced a heatwave per year. The annual average number of heat waves experienced by each site in the United States was 71, which is from 1996 to 2021. The heat wave days in rivers rose from 11-25The team reports October 3, 2012. Limnology and Oceanography Letters
Spencer Tassone (an ecosystem ecologist at University of Virginia in Charlottesville) said that this is the first time heat waves have been assessed in rivers throughout the country. He and his colleagues tallied nearly 4,000 heat wave events — jumping from 82 in 1996 to 198 in 2021 — and amounting to over 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.
Tassone points out that most heat waves that had temperatures above average occurred between December-April, pointing to warmer wintertime weather.
Human-caused global warming plays a role in riverine heat waves, with heat waves partially tracking air temperatures — but other factors are probably also driving the trend. According to the study, waterways that receive less precipitation or have lower water volumes will warm up faster.
“These very short, extreme changes in water temperature can quickly push organisms past their thermal tolerance,” Tassone says. He says that sudden heat waves are more damaging than a gradual rise in temperature. Because fish like trout and salmon are dependent on cold water for oxygen and regulation of their bodies temperature, they are more sensitive to heat waves.
The heat can also have chemical consequences, according to Sujay Kaushal, a hydrologist at the University of Maryland in College Park. He was not involved in the study. Higher temperatures can accelerate chemical reactions that can contaminate water. In some cases, this can lead to a reduction in the effectiveness of treatment.Toxic algae blooms (SN: 2/7/18).
Kaushal states that this research can be used to help prevent heat waves in the future. This could include increasing shade and managing stormwater. In some rivers, Beaver dams are promisingFor lowering water temperaturesSN: 8/9/22). “You can actually do something about this.”
