River water temperatures swing from hot to cold: major discovery shows value of observational records

Their research was based on national scale, open data from the US Geological Survey. Such data are an essential scientific resource for understanding environmental variability between different places and over time, especially in a warming and increasingly human-dominated world.

In turn, these river water temperature patterns are important to manage habitats for wildlife and biodiversity conservation, as well as understanding the impacts of hot and cold water pulses on socioeconomic activity such as energy production, recreation, fisheries, and drinking water.

Most scientific research on river water temperature to date have focussed on extreme or average values, but other thermal factors are also important for maintaining healthy and productive waterways. One of these is how quickly river water temperature changes, which dictates whether animals and people can adjust or find safe places to survive during thermal shifts.

In the first study of its kind, experts at the University of Birmingham and Indiana University analysed 15 years' worth of high-resolution temperature data nationally and observed 6,507 surges (temperature rises) and 4,787 plummets (temperature falls) at 88 of the 102 stations assessed.

This research -- which has been recently published in Environmental Research Letters -- not only revealed the prevalence of rapid temperature changes, but also how variable these events can be.

One striking finding was that both surges and plummets ranged between nearly freezing temperatures to conditions exceeding 40°C over some geothermal hotspots. The authors went on to describe water temperature changes of over 18°C within a single event, transcending extreme thermal shifts previously described worldwide.

Lead author Dr. James C. White, from the University of Birmingham, commented: "Our findings provide critical insights into the dynamics of rapid water temperature changes rivers, which will help future studies unravel how different water sources and weather patterns affect the likelihood and characteristics of these events."

"Evidence presented in this research is critical for informing more effective environmental management solutions, such as controlling dam flow releases or planting along riverbanks. especially as the climate changes.

The scientists reported several regional differences across the US, with rivers in western regions like California and Oregon experiencing fewer surges and plummets overall, which were often confined to droughts and heatwaves.

In stark contrast, these rapid temperature shifts were more commonplace in the Upper Colorado river basin across central-west US, where combinations of high aridity and mountainous snowmelt likely played a part. Frequent surges and plummets also manifested across southeastern regions like Florida and Georgia, with these probably a result of persistent storm events.

Co-author David Hannah, Professor of Hydrology and UNESCO Chair in Water Sciences at the University of Birmingham, commented: "By demonstrating how widespread these river water temperature surges and plummets are using long-term national-scale data, we have highlighted where and when rivers are most prone to rapid temperature fluctuations. This provides a critical step towards being able to pinpoint where and when interventions that can best mitigate river temperature volatility to safeguard and climate proof vulnerable water courses."