Increasing heatwave frequency in streams and rivers of the United States

Heatwaves are increasing in frequency, duration, and intensity in ocean, coastal, and lake ecosystems. While positive water temperature trends have been documented in many rivers, heatwaves have not been analyzed. This study examined heatwaves in rivers throughout the United States between 1996 and 2021. Riverine heatwaves increased in frequency over the study period, with the most robust increases occurring in summer and fall, in mid- to high-order streams, and at free-flowing sites and sites above a reservoir. The increase in heatwave frequency was accompanied by an increase in moderate strength heatwaves as well as a doubling of the annual mean total number of heatwave days at a site. Riverine heatwaves were often associated with normal or below-normal discharge conditions and at sites with a mean annual discharge <= 250 m(3) s(-1). These results provide the first assessment of heatwaves in rivers for a large geographic area in the United States.

Automated summary

The frequency of heatwaves in rivers has been increasing in the United States, particularly in summer and fall, in mid- to high-order streams, and at free-flowing sites and sites above a reservoir. This increase in frequency is accompanied by an increase in moderate strength heatwaves and a doubling of the annual mean total number of heatwave days at a site. These heatwaves are often associated with normal or below-normal discharge conditions and occur at sites with lower mean annual discharge.