Recent analysis reveals that rivers across the contiguous United States are experiencing heat waves with greater frequency and intensity than air heat waves. Since 1980, these rivers have seen an alarming average increase of 11.6 days per year when water temperatures exceed 15°C (59°F). This temperature limit is extreme stress to a wide variety of aquatic species, especially cold-water fish such as salmon and trout.
These alarming findings add evidence that heat waves in U.S. rivers are occurring up to four times faster than those in the atmosphere. On top of that, these river heat waves persist almost twice as long. Kayalvizhi Sadayappan, the lead author of the study, underscored the urgent necessity to monitor and mitigate this rapidly escalating phenomenon. The research underscores the importance of shifting precipitation patterns, particularly wintertime snow, in driving river heat waves.
The Impact of Human Activity
Our study found that human activities are the dominant drivers of river heat wave frequency and intensity. Urban rivers are subjected to heat waves that are more frequent, longer, and more intense. Rivers in largely undeveloped basins are less likely to suffer such extremes. Rivers in more natural landscapes are rapidly closing the gap, both in frequency and intensity of heat waves.
Sadayappan noted, “Dams have been accelerating trends in riverine heat waves. In particular, large dams have been contributing towards elongating riverine heat waves.” This sentence illustrates the complicated relationship that built environments and infrastructure have with nature’s infrastructure, namely water.
Li Li, the corresponding author of the study, remarked on society’s perception of rivers: “Rivers are often thought of as safe and cool havens protected from extreme temperatures.” This study should certainly be a wake-up call to the deep vulnerabilities rivers across the country are facing.
Regional Variations and Trends
Additionally, the study finds that the Northeast, Rocky Mountains and Appalachian regions have experienced the fastest increase in warm-water days. These areas are heating up faster than the national average. Specifically, 82% of the sites weren’t just above the threshold, but actually increasing their number of days exceeding 15°C by more than 6.8 days/year!
Sadayappan elaborated on the implications of these trends: “This raises the risk of rivers experiencing both extreme heat and low water flows at the same time, which can cause conditions that can lower oxygen levels, stress aquatic life, and even trigger large-scale fish die-offs.”
The impact of the increasing number and severity of heat waves is concerning. They pose a grave threat to aquatic ecosystems and adversely affect the people who rely on these rivers for both recreation and their economic well-being.
The Need for Monitoring and Mitigation
“In light of this substantial increase in river temperatures, scientists highlight the immediate need for regular monitoring and river-wide treatment efforts. ‘Our findings show that riverine heat waves are increasing faster than air heat waves, a trend likely happening worldwide,’ said Sadayappan.”
You can plant new trees, or help existing trees grow along riverbanks to provide shade. Second, better dam management, more use of green infrastructure, and less diversion of water from rivers. These are important steps to fight the growing threat of riverine heat waves.
Sadayappan further explained that “this information can provide warning signals and support adaptive management during riverine heat waves.” Our research points to the power of agriculture in addressing riverine heat waves. It can supply cooler air during the day and water through irrigation during hot weather.