As extreme heat and prolonged dry weather become the norm, we must confront another challenge: rising river water temperatures.
Drought is no longer just a story of too little water. As extreme heat and prolonged dry weather become the norm, we must also confront another challenge: rising river water temperatures.
That may sound like a more specialist concern than wilting crops, near empty reservoirs, water restrictions or curbing household demand. But river temperature is central to the health of freshwater ecosystems and socio-economic activity, and it should be much more visible in the way we plan for drought and climate resilience. From power generation and manufacturing to fisheries, agriculture, recreation and public water supplies, warmer rivers can disrupt operations, increase costs, and threaten livelihoods.
As the Environment Agency and Defra renew their focus on drought preparedness and water resilience, I believe we need to look beyond water supply alone. Low river flows and high air temperatures can combine to create hidden but serious pressures for wildlife, communities and the services healthy rivers provide.
Why drought is also a river temperature problem
In our research, we have focused on one climate- and water-related risk that is still too often overlooked: when river levels fall, rivers can warm more quickly and remain at damaging temperatures for longer.
This matters because temperature shapes almost every aspect of river health, from oxygen levels and habitat quality to the survival, behaviour and reproduction of freshwater species. During heatwaves, low flows and high air temperatures can place ecosystems under intense stress at precisely the point when they are least resilient. In terms of human impacts, for example, power stations use river water for cooling. During hot, low-flow conditions, elevated river temperatures can reduce cooling efficiency and restrict power production, creating threats to energy security.
We are particularly concerned about the emergence of so-called "riverine heatwaves" – sustained periods of unusually high river temperatures that are increasingly recognised as a significant climate-related risk. These events are driven by a combination of atmospheric conditions, hydrological drought and processes occurring within river networks themselves. Existing monitoring and modelling approaches do not always capture these riverine heatwaves effectively.
Mitigating and adapting to riverine heatwaves
We believe better prediction and management of extreme river temperatures should sit alongside wider efforts to strengthen water resilience. This means understanding not only how much water is moving through a river system (and from where it is sourced), but how heat spreads through river networks and how local conditions can intensify or reduce that risk.
In practice, factors such as riparian shade, river habitat, groundwater inputs, reservoir releases and the duration of dry weather can all influence whether a river experiences a short-lived temperature spike or a sustained period of unusually high temperatures.
For policymakers and practitioners, the challenge is to bring these insights into drought planning, nature recovery, catchment management and climate adaptation. If we want action to protect water resources to be effective, it also needs to protect the ecosystems and communities that depend on healthy rivers.
A moment for public understanding
The issue is timely. I recently discussed this research at the Institution of Civil Engineers and British Hydrological Society conference, Drought hydrology: 50 years since 1976, where researchers, water managers and policy specialists considered how the lessons of the 1976 drought apply in a changing climate.
Yet, the bigger opportunity is to use moments like this to widen public understanding of drought. As extreme heat and dry weather become more frequent, the question is not only whether we have enough water, but whether our rivers can remain cool, connected and healthy enough to support wildlife, people and the services society relies on.
Deputy Pro-Vice-Chancellor (Sustainability)
Professor David Hannah studies hydroclimatology in cold regions, focusing on river dynamics, climate interactions, and ecohydrology.
