- Several studies have highlighted the increase in intensity and frequency of riverine heatwaves across rivers globally.
- Modeling studies reveal that by the end of the century, more than 50% of the Ganges will experience year-round heatwaves, causing one of the biggest population exposures worldwide.
- Lack of sufficient data and the complexities of studying river water temperatures hamper our understanding of the phenomenon.
River water temperatures have come under the scanner in several global studies as researchers raise alarm bells about the increase in heatwaves across riverine ecosystems.
Riverine heatwaves are defined as periods where daily mean river water temperatures exceed the 90th percentile threshold of the locally defined and seasonally varying river temperatures, for at least five consecutive days. Put simply, when river temperatures are higher than 90% of its past recorded observations for that location and time of the year, it constitutes a heatwave. An increase in temperatures for shorter duration is considered a heat spike.
In a modelling study published earlier this year, researchers revealed that between 1976 and 2005, rivers globally experienced, on average, 2.19 heatwave events per year. The intensity and duration of riverine heatwaves have increased in this time at a rate of 0.02°C per decade and 0.09 weeks per decade, respectively. Under the high climate emissions scenario, projections reveal a 95-fold increase in the duration of riverine heatwaves by the end of the 21st century.
The study also predicts that in India, more than 50% of the Ganges will experience year-round heatwaves under the high emissions scenario by the 2090s. Not only will this cause widespread damage to the ecosystem itself, the population exposure to heatwaves in the Ganges (impacts on drinking water, agriculture, and fisheries) will also be the highest in the world.
A multitude of impacts
Riverine heatwaves can have multiple implications for riverine ecosystems and the organisms that are dependent on them. The heat stress to the riverine floral and faunal species can affect reproduction rates, reduce migration, and alter food webs. As different organisms respond to thermal stress differently, existing research shows that these implications vary vastly.
For example, a study on the Indian freshwater fish, Magur (Clarias magur) showed that the species displays compromised immune responses along with severe tissue and physiological damage when exposed to extreme heat stress. In rohu (Labeo rohita), studies show that even short heat spikes cause changes in the production of proteins involved in energy metabolism and immune system regulation among other physiological responses.
Additionally, global reports also reveal that riverine heatwaves can have indirect effects, such as promoting the spread of pathogens and reducing the overall quality of river water.
“Water temperature is a key driver of biological productivity, food-web integrity, and overall ecosystem functioning in rivers. Temperature variation influences processes such as the breakdown of leaf litter and the processing and storage of carbon. Changes in these processes can cascade through the food web, affecting aquatic insects, crustaceans, fish, and aquatic mammals,” explains Shishir Rao, Doctoral Student at Odum School of Ecology, University of Georgia, USA. Rao’s research focuses on the tropical rivers of India’s Western Ghats.
Research on the impacts of small hydropower projects (SHPs) on some of the river systems in Karnataka’s Western Ghats indicates that such projects could contribute to riverine heatwaves. “Our studies found that the stretch of the river with reduced water flow due to SHPs was the most ecologically altered part of SHP-regulated rivers. The reduced depth and wetted width in this reach increased water temperature and lowered dissolved oxygen, especially during the dry season. Also, fish assemblages shifted away from specialist, native, endemic, fast-flowing water-adapted species toward generalist, slow water-adapted, pool-dwelling species, indicating a community shift,” shares Rao.
Like marine heatwaves, riverine heatwaves also add undue pressure on endemic and endangered species that have narrow thermal tolerances, and benefit invasive species that tolerate broader temperature ranges, Rao explains.
Changing river temperatures and rainfall patterns have shown an increase in invasive species such as common carp, tilapia and African catfish in the Ganges. The research showed that between 2009 and 2019, the annual mean temperature of the river’s mid-stretch saw an increase from 0.9 to 1.88 ºC. Quarterly and annual river water temperature recordings revealed that the temperature increase was not linear but observed as a sudden rise over a short period of time. These temperature and rainfall changes had a compound effect on the river’s invasive species populations.
Data deficiency and the complexities of studying riverine heatwaves
“Riverine heatwaves are a threat that is not as visible as other types of water-related extreme events, such as floods and droughts. They are becoming more frequent with climate change because of their strong link to air temperature, which is why they deserve more attention,” shares Manuela Brunner, Assistant Professor, Institute for Atmospheric and Climate Science, ETH Zürich and the WSL Institute for Snow and Avalanche Research SLF, Davos, Switzerland. Brunner is one of the co-authors of a 2025 perspective article on riverine heatwaves in Nature Water.
In the paper, the authors explain that apart from air temperatures, river water temperatures are also affected by interactions with other water bodies, such as lakes and streams, and the mixing of ground and surface water. The presence of vegetation along riverbanks and anthropogenic effects due to the addition of wastewater and industrial effluents are also factors to account for when studying riverine heatwaves. “Heatwaves are also favoured by a lack of stream flow and snowmelt contributions, both of which can alleviate the development of riverine heatwaves if abundant,” she adds.
An added complication is that riverine heatwaves often do not occur in isolation, explains Niko Wanders, Associate Professor, Department of Physical Geography, Faculty of Geosciences, Utrecht University, the Netherlands. Wanders is a co-author of the January 2026 research article on the increasing intensity of riverine heatwaves worldwide.
“During riverine heatwaves, we often also see normal heatwaves and possibly droughts occurring. These have a potentially bigger impact on society than the riverine heatwaves, but the latter have a devastating impact on biodiversity and industrial cooling water outputs,” says Wanders.
“The biggest challenge is monitoring, obtaining sufficient records to understand the complex interactions, as well as understanding the impact of a variety of sources of high-water temperatures,” he adds.
For Indian rivers, data insufficiency is a critical drawback that stymies discussions on changing river water temperatures and resulting heatwaves. However, the Hydroclimatic Research Group at the International Institute of Information Technology (IIIT), Hyderabad, is working on filling some of the missing pieces of the puzzle.
The team has developed hybrid models to study river water temperatures and predict future trends. A study of seven river catchments in India revealed that the highest annual increase in river water temperature for the winter season is observed in the Musi River at 0.16ºC/ year. For the summer season, the Tunga-Bhadra catchment showed the highest annual temperature increase at 0.35ºC/ year.
The study revealed that not all river water temperature increases are directly proportional to increases in air temperatures. For the Ganges, Cauvery, and Godavari catchments, increases in air temperatures negatively impacted water temperatures (Ganges: -0.07 ºC/year, Cauvery: -0.06ºC/year and Godavari: -0.03ºC/ year). This observation underscores the importance of accounting for the myriad factors that influence river water temperatures and heatwave observations.
The study also found that a 1ºC increase in river water temperature reduces the dissolved oxygen saturation levels by 2.3%. Dissolved oxygen concentrations are critical for biodiversity support and to maintain water quality.
While modeling studies can aid in addressing some of the data deficiencies, an interdisciplinary approach is essential to understand the overarching effects of rising river temperatures and heatwave occurrences, explains Rehana Shaik, Associate Professor, IIIT and head of the Hydroclimatic Research Group.
“River water temperature is not purely a hydrological variable. It sits at the intersection of hydrology, climatology, environmental engineering, data science, and public policy. For example, in northern India, glacier melt from the Himalayan mountains influences downstream thermal regimes in the Ganges. In semi-arid basins like the Musi River, evaporation and low flow amplify warming. Pollution increases thermal stress in basins like the Narmada and Godavari, and climate change further complicates this system,” she adds.
Finding proactive solutions
Riverine heatwaves are emerging topics of concern amid the current climate change crisis. While data generation is crucial to understand these phenomena, proactive measures are also needed to protect rivers from extreme weather events, explains Jonathan Tonkin, Professor and Rutherford Discovery Fellow, University of Canterbury, New Zealand. In a recent review article, Tonkin and other global researchers highlight that extreme weather events are drastically altering the biodiversity of rivers, making it imperative to consider solutions that promote resilience.
“The most important thing right now would be to move away from local-scale interventions and focus on restoration approaches that incorporate the entire catchment. An extreme event that occurs in the headwaters can impact the entire riverine network. Ecological and biological responses can also spread downstream. Thinking about the entire catchment, or the network, is crucial in terms of prioritising restoration at scale,” shares Tonkin.
One way to do this would be by rehabilitating and maintaining riparian forest cover, shares Rao. “Riparian vegetation provides shade that reduces the heating of water while also stabilising riverbanks and improving aquatic habitat. However, care must be taken in selecting appropriate species and planting locations, since some non-native trees can consume large amounts of water and reduce overall water availability in the river,” he adds.
Rao also emphasises the need to maintain sufficient flow in rivers by operating reservoirs appropriately for hydropower, drinking, and irrigation water demands. “By ensuring that sufficient water remains in the river channel, environmental flows can buffer rivers against climatic extremes,” says Rao.
Tonkin further adds that locating and protecting thermal refuges in riverine systems is another need of the hour. “This would require some basic on-the-ground monitoring, where one would explore the riverine network for natural cold-water refuges. Pinpointing these ‘safe havens’ is going to be fundamentally important for the ecosystem’s future,” he shares.
Read more: 1 in 4 freshwater species around the globe at risk of extinction
Banner image: The drying Tunga River in Karnataka during summer. Representative image by Lenin Moorthy via Wikimedia Commons (CC BY-SA 4.0).
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