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U.S. river heat waves are rising. Why that matters

ScienceU.S. river heat waves are rising. Why that matters

The rivers of the United States are getting into some hot water. According to the findings of a recently conducted study, the number of river and stream heat waves is on the increase.

Riverine heat waves are similar to marine heat waves in that they both occur when water temperatures rise over their normal range for a period of at least five days. Researchers assessed daily temperatures for 70 locations located in rivers and streams throughout the United States, using data collected by the United States Geological Survey over a period of 26 years. They then computed the number of days each location experienced a heat wave on an annual basis. According to the findings of the research, which were published on October 3 in Limnology and Oceanography Letters, the yearly average number of heat wave days per river increased from 11 to 25 between the years 1996 and 2021.

According to Spencer Tassone, an ecosystem ecologist at the University of Virginia in Charlottesville, this research represents the first evaluation of heat waves in rivers throughout the nation. Nearly 4,000 heat wave episodes were recorded by him and his colleagues, which is an increase from 82 in 1996 to 198 in 2021. This brings the total number of heat wave days to over 35,000. The researchers discovered that the frequency of high heat rose at locations above reservoirs and in free-flowing situations, but not at places below reservoirs. They hypothesise that this is because dams discharge cooler water downstream, which may explain their findings.

According to Tassone, the majority of heat waves with temperatures that were the greatest above average ranges occurred outside of the summer months between the months of December and April, which points to warmer winters circumstances.

The rise in world temperature due to human activity plays a part in riverine heat waves, with heat waves largely following air temperatures; nevertheless, it is likely that other causes are also contributing to the trend. According to the findings of the research, the rate at which waterways warm up is accelerated when there is less precipitation and a lesser amount of water in rivers.

According to Tassone, “these very rapid and dramatic variations in water temperature may swiftly drive organisms above the limits of their thermal tolerance.” According to him, unexpected heat waves may have a higher effect on river-dwelling plants and animals than a steady rise in temperature would. This is something that he thinks is true. Fish that spawn in cold water, such as salmon and trout, are better able to maintain a healthy body temperature, get the appropriate amount of oxygen, and spawn successfully as compared to fish that spawn in warmer waters.

According to hydrologist Sujay Kaushal of the University of Maryland in College Park, who was not involved in the research but did comment on its findings, heat has both physical and chemical effects. Chemical processes that pollute the water may be sped up by higher temperatures, which can, in certain instances, contribute to the growth of harmful algal blooms.

According to Kaushal, the findings of the study may serve as a foundation for future efforts to reduce the severity of heat waves via methods such as increasing the amount of shade cover provided by trees or better managing stormwater. Beaver dams have been shown to have the potential to lower water temperatures in some rivers. “You truly do have some control over this situation.”

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