U. rivers enter the warm water. The frequency of river and river rise tides, the new analysis shows.
Like ocean tides, river tides occur when water temperatures rise above normal for five or more days (SN: 2/1/22). Using 26 years of United States Geological Survey data, researchers recorded daily temperatures for 70 locations in rivers and streams across the United States, and then calculated how many days each location experienced a heat wave each year. From 1996 to 2021, the annual average number of high tide days along the river rose from 11 to 25, the team reports Oct. 3. Letters on Limnology and Oceanography.
The study is the first assessment of tidal waves in rivers across the country, says Spencer Tassone, an environmental scientist at the University of Virginia in Charlottesville. He and his colleagues matched about 4,000 tidal wave events — jumping from 82 in 1996 to 198 in 2021 — and to more than 35,000 tidal wave days. The researchers found that the frequency of excessive heat is increased in areas above the reservoirs and in free-flowing conditions but not below the reservoirs – possibly because the mothers release cooler water downstream.
Most heat waves with high temperatures above typical ranges occurred outside of the summer months between December and April, indicating warmer winter conditions, Tassone says.
Man-made global warming plays a role in tidal heat waves, as heat waves partly follow air temperatures — but other trends probably drive them as well. For example, the lower precipitation and lower volume of water in medium-sized rivers warms up and becomes easier, the study says.
“These very brief, extreme changes in water temperature quickly push organisms past their thermal tolerance,” Tassone says. Compared to a gradual increase in temperature, a sudden heat wave can have a greater impact on plants and animals living in rivers, he says. Fish such as salmon and trout are particularly sensitive to heat waves because the animals rely on cold water to satiate themselves, regulate their body temperature and spawn properly.
There are also chemical consequences of heat, said 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 contaminate water, in some cases contributing to toxic algal blooms (SN: 2/7/18).
Source research can be used to help mitigate heat waves in the future, Kaushal says, such as by increasing shade trees or managing storms. In some rivers, beaver dams promise to lower water temperatures (SN: 8/9/22). “You can do something about this.”
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