The study published in Nature Reviews Earth & Environment shows that river water quality tends to deteriorate during extreme weather events. As these events become more frequent and severe due to climate change, ecosystem health and human access to safe water may be increasingly under threat.

The research led by Dr. Michelle van Vliet of Utrecht University analysed 965 cases of river water quality changes during extreme weather such as drought, heatwaves, rainstorms, and flooding, as well as under long-term (multidecadal) changes in climate. “We looked at various water quality constituents such as water temperature, dissolved oxygen, salinity and concentration of nutrients, metals, microorganisms, pharmaceuticals, and plastics,” says Van Vliet.

The analysis shows that in most cases water quality tends to deteriorate during droughts and heatwaves (68%), rainstorms and floods (51%) and under long-term changes in climate (56%). During droughts, less water is available to dilute contaminants, while rainstorms and floods generally result in more contaminants that run off from land to rivers and streams. Improvements or mixed responses in water quality are also reported for some cases owing to counteracting mechanisms, for example when increased transport of pollutants is offset by more dilution during flood events.

Impact on nutrients and micro-organisms in river water

Dr. Maryna Strokal and Dr. Nynke Hofstra from Wageningen University & Research contributed to the extensive study, looking into nutrients and micro-organisms in particular. “We saw that changes in land use and other human factors (such as population and wastewater treatment) have a larger impact on nutrients and micro-organisms in river water than long-term climate change,” says Hofstra. “But still, we did find high nutrient and micro-organism concentrations in rivers caused by extreme weather events such as rainstorms and flooding.” Plastics were one of the other water quality constituents studied, but for those insufficient data are still available to draw a generalised conclusion. The study also calls for more data collection and studies of water quality in non-Western countries. “We need a better monitoring of water quality in Africa and Asia. Most water quality studies now focus on rivers and streams in North America and Europe,” says Van Vliet.

Towards effective management strategies for clean water

The results of the study underline the urgent need for a better understanding of water quality changes during extreme weather events, and the mechanisms underlying this. “Only then will we be able to develop effective water management strategies that can safeguard our access to clean water and ensure ecosystem health under climate change and increasing weather extremes.”