Q&A: As Rivers Recover From Last Year’s Drought, Winter May Pose Untold Threats

February 6, 2023 By Eric Williamson, williamson@virginia.edu Eric Williamson, williamson@virginia.edu

The “mighty” Mississippi River has only recently bounced back from last year’s drought, and the Missouri and the Colorado rivers are still struggling to recover.

While most people don’t tend to think about how heat affects water this time of year, a graduate student at the University of Virginia says winter is the time when rivers can undergo their most extreme heat waves.

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Spencer Tassone, a doctoral candidate in environmental sciences who graduates this spring, studies the effects of extreme temperatures on rivers and other less-studied bodies of water. His research has determined that river heat waves are increasing in frequency.

“My study provided evidence that heat waves are associated with times of low water volume, such as during a drought,” Tassone said.

Although heat waves are largely associated with summer and fall, the most extreme river heat waves happen between December and April, he found.

Spencer Tassone in the ocean with scuba suit

Tassone has been interviewed in various media about his research. This screengrab is from a self-made video for the UVA Coastal Research Center.

Related to his findings, Tassone recently won UVA’s Rath Public Service-Focused Research Award. He also is co-author of a technical report for the Chesapeake Bay Program on rising watershed and bay temperatures and was interviewed live on Fox Weather.

He spoke to UVA Today about the threat of a changing climate on our waters, including to fish, and the implications of his research for future waterway management.

Q. Why are these river heat waves occurring?

A. Riverine heat waves across the U.S. are on the rise due, in part, to increasing atmospheric temperatures and low-flow conditions associated with climate change and other manmade causes.

Q. Can you explain why the most extreme heat waves happen during colder weather?

A. People typically associate heat waves with summer, as this is when temperatures are greatest. However, aquatic heat waves are often defined based on the water temperature being above a certain threshold for the season and for a minimum duration of five days. This allows us to identify extreme temperature conditions at any time of the year.

Our results found that the top 92% of the most extreme heat waves occurred between December and April. My study did not examine the mechanisms of those most extreme heat waves and cannot say precisely what caused them. They are likely associated with an increasing trend in anomalous high winter atmospheric temperatures and below-normal flow conditions.

Q. How bad are heat waves for fish and other aquatic life?

A. Riverine heat waves are an ecological problem. While long-term increases in temperature are concerning for species distributions, short-term extreme temperatures, referred to as heat waves, are concerning for the welfare of specific aquatic species.

In other aquatic systems where heat waves have been well-studied (e.g., open and coastal ocean), heat waves have caused mass die-offs of seagrass and kelp, as well as increased coral bleaching. There are likely similar issues occurring in riverine systems, but the study of heat waves in rivers is in its infancy; my paper is the first to document riverine heat waves.

Freshwater fish, including brook trout – the Virginia state fish – and sculpin are likely the most vulnerable to heat waves, as they require small, cold-water streams and rivers to persist.

Furthermore, while warm-water species are more tolerant to temperature increases, they can be sensitive to extreme temperatures, including rapid changes in temperature and declines in dissolved oxygen that co-occur with heat waves.   

Arial image of a river
Times of lower water volume create the conditions necessary for river heat waves. Pictured is the James River in Richmond, also not a part of the study. (Photo by Spencer Tassone)

Q. What are the implications for river management?

A. Riverine heat waves are challenging to identify in real time as the data used to identify them are not typically available. However, there are some management practices that could be implemented to slow the rate of rising water temperature. Those practices include maintaining and establishing forested buffers along the banks of streams and rivers, as they provide shade to surface water. We also need to reduce flow alteration to streams, as below-normal flow is associated with heat wave conditions.

Additionally, policies need to be developed to better protect and maintain healthy watersheds, as well as natural lands that provide cooling benefits, such as forests and wetlands.

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Q. What was the scope of your river research?

A. The riverine heat wave paper used daily average water temperature data collected by the United States Geological Survey from 1996 to 2021 that came from 70 streams or rivers across the U.S. There were no sites in Virginia due to a lack of long-term (greater than 25 years) continuous monitoring data.

Q. Why doesn’t the commonwealth have this information?

A. Water temperature data is being collected by USGS in Virginia. However, these datasets have not been around for as long as other areas of the country.

My study provided evidence that air and water temperature in the Southeast region of the U.S. are warming at a faster rate than other U.S. regions. This underlines the importance of maintaining long-term water temperature records in Virginia and elsewhere.

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Eric Williamson

University News Senior Associate University Communications