Heatwaves have increased in frequency and intensity as climate change begins to take effect across our planet. The impacts of this change are still unclear in a variety of ecological contexts.
One area not investigated in any significant manner is the role of heatwaves on rivers. Riverine environments play an important role for terrestrial ecology and the effects of heatwaves on rivers can shape wildlife and communities that depend on them in the coming decades.
Monitoring River Temperature in the United States
In a recent study, the United States Geological Society (USGS) recorded surface water temperature in 70 rivers across the United States.
The period of monitoring, covering a 26-year period between 1996–2021, looked at mean water temperature that exceeded a local, seasonal varying 90th percentile threshold for a period greater than or equal to 5 days. In other words, it monitored water temperature changes well above normal water temperatures.
This is comparable to a method developed for monitoring marine-related heatwaves by Hobday et al. What this research showed was that rivers generally were at average or below average discharge during heatwaves with water temperatures increasing.
Overall, 39% of the rivers monitored exceeded air temperatures, which indicates that other changes and drivers are likely affecting rising water temperatures.
It is possible that shallow groundwater contributions are affecting temperature in some streams, particularly ones that more greatly depend on such sources as shallower water may heat up faster as temperature increases.
Impact of water temperature rise in rivers
Regardless, there are severe consequences anticipated as water temperature increases while discharge decreases.
Increase in concentrated pollutants in rivers
For one, pollution in streams will likely concentrate, worsening ecological conditions in many regions where streams and rivers are critical to communities and wildlife. People who fish river fish may be more directly affected in terms of their health as fish are projected to have more concentration of pollutants.
Contaminants are also more easily mobilized in higher water temperatures, heavier metals or pollutants. Overall, this first study demonstrates a potentially critical problem for terrestrial ecosystems as temperatures increase across the United States.
How different river types respond to heatwaves
Taking the recent study discussed and using it to look at other similar research, we can see that the scale of the problem could be more severe for certain types of rivers. Other works have indicated that various rivers respond differently to heatwaves depending on the type of river.
For instance, low-land rivers are extremely sensitive to heatwaves. On the other hand, high-altitude rivers or rivers depending on snow-fed discharge received cold water that has led to a dampened effect and thermal response as heatwaves have increased.
What this suggests is that mitigation strategies that attempt to dampen severe temperature changes on rivers may need to conduct different strategies based on the types of rivers encountered. 
Heatwaves can lead to river flooding
In addition to temperatures, and particularly in tropical or sub-tropical locations, heatwaves can condition drastic atmospheric changes that lead to increased rainfall. This has been found then to link to or affect severe flooding events, such as that seen in the Yangtze River Valley.
In other words, water temperatures may increase as air temperature increases; however, rivers themselves could become more prone to flooding in certain places given that heatwaves often precede prolonged rains.
Water released from dams affects river temperature
Added to this, many river systems globally are dammed rivers.
Often, selective water release from reservoirs can drastically alter river water temperatures. This has been demonstrated to potentially affected various aquatic species sensitive to water change.
The global river systems are sensitive to climate change
What we see is that river systems globally can be sensitive to many changes, including climate change leading to increasing frequencies of heatwaves and severity of heatwaves.
Unfortunately, the effects of air temperature on rivers has not been extensively studied, in particular during heatwave events.
However, as studies have increased, we can now see that different river types are affected differently. In particular, low-land rivers and those that have shallow groundwater sources are more severely affected.
Pollution appears to become worse as water temperatures increase, as water pollutants concentrate and become more mobile.
Long-term effects on ecology still need to be studied but impacts are evident, particularly on food resources.
In the long-term, rivers will need different mitigation strategies in order to minimize the effects of climate change, including managing ways to reduce increasing water temperatures.
 For more on the study looking at heatwaves and river water temperatures, see: Tassone, Spencer J. et al. 2022. ‘Increasing Heatwave Frequency in Streams and Rivers of the United States’. Limnology and Oceanography Letters: lol2.10284.
 For more on how to capture heatwave variation and effects on marine environments, and by extension the use of this method on river systems, see: Hobday, Alistair J. et al. 2016. ‘A Hierarchical Approach to Defining Marine Heatwaves’. Progress in Oceanography 141: 227–38.
 For more on the effect of heatwaves in a Swiss context and how this has affected different types of rivers, see: Piccolroaz, Sebastiano, Marco Toffolon, Christopher Robinson, and Annunziato Siviglia. 2018. ‘Exploring and Quantifying River Thermal Response to Heatwaves’. Water10(8): 1098.
 For more on the effects of heatwaves and river flooding in China, see: Chen, Yang et al. 2022. ‘Greater Flash Flood Risks From Hourly Precipitation Extremes Preconditioned by Heatwaves in the Yangtze River Valley’. Geophysical Research Letters 49(18). https://onlinelibrary.wiley.com/doi/10.1029/2022GL099485 (October 30, 2022).
 For more on river water temperature change and the relationship to dam reservoirs, see: Ahmad, Shahryar K. et al. 2021. ‘Predicting the Likely Thermal Impact of Current and Future Dams Around the World’. Earth’s Future 9(10). https://onlinelibrary.wiley.com/doi/10.1029/2020EF001916 (October 30, 2022).