Riverine heat waves on the rise, outpacing air heat waves

Riverine heat waves receive far less attention compared to air heat waves. Our understanding of riverine heat waves has been limited by poorly consolidated water temperature records, despite the substantial advances in river water temperature monitoring. To address this gap, we developed a long short-term memory (LSTM) model for reconstructing historical daily mean water temperature in 1471 sites across the contiguous United States from 1980 to 2022. Using these temporally complete records, we analyzed the characteristics of riverine heat waves and their long-term trends, and evaluated how they compare with air heat waves. Our analysis revealed that riverine heat waves occur less frequently (2.3 versus 4.6 events/year), and with lower intensity (2.6 versus 7.7 °C/event) than air heat waves, but persist for longer durations (7.2 versus 4.0 days/event). More importantly, the frequency, duration and intensity of riverine heat waves have been increasing at rates 2–4 times higher than air heat waves. These increases in riverine heat waves are primarily driven by climate factors including rising air temperatures and declining snowpacks, while anthropogenic activities such as river regulation by dams and agriculture further modulate response of rivers to climate change. The pronounced rise in riverine heat waves highlights the urgent need for global monitoring of river water temperatures and inclusion of riverine thermal extremes in climate risk assessments and mitigation strategies.