Water quality extremes in mountain rivers

WAQUEX – a project on water quality extremes in mountain rivers – investigates the occurrence and drivers of water temperature and suspended sediment extremes, especially under conditions with increasing droughts, heat waves and floods.

Mountains are an essential source of fresh water to rivers and lowland areas, with vital importance for human societies and for maintaining aquatic biodiversity and ecosystem services. Since mountain rivers are particularly sensitive to climate change, projected future changes in precipitation patterns, air temperature, snow and glacier cover, and flow regime are expected to increase stress for mountain river ecosystems and water quality.

The aim of this project is to:

• Gain insight in the temporal and spatial variability of water temperature and suspended sediment extremes for different mountain regions within Europe.
• Identify the main drivers for water temperature and suspended sediment extremes.
• Address climate change impacts on these drivers and predict the impact on future extremes.

Results related to water temperature extremes and its drivers

To assess the temporal changes and spatial variability in water temperature extremes, we analysed water temperature observations of 177 catchments in four mountain regions in Europe. Within this study we tried to understand why and how much rivers have warmed over the past 30 years. We focussed on both the long-term trends and individual extreme situations. Our results show that:

• Mountain rivers in Europe have warmed with a rate of +0.38 ± 0.14 °C per decade over the past 30 years.
• The number of extreme water temperature events has increased, especially in high-elevation catchments and in spring and autumn.
• Non-extreme water temperature is mainly driven by air temperature, but extremes are also impacted by soil moisture, baseflow and meltwater.

Link to the news article:
https://www.wsl.ch/en/news/hot-rivers-climate-change-warms-mountain-streams/

Link to the research article:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024WR037518

Results related to suspended sediment concentration extremes

Suspended sediment is a natural component of rivers, but extreme concentrations can have significant impacts on hydropower generation, reservoir sedimentation, water use, and aquatic ecosystems. We identified the main factors influencing the spatial and temporal variability of the annual suspended sediment concentration (SSC) regime and the occurrence of extreme SSC events in 38 catchments in the Alpine region. Our results show that:

• The presence of glaciers and the magnitude and timing of snowmelt are important factors influencing the annual SSC regime.
• Rainfall is the most dominant driver of extreme SSC events and responsible for 80% of all events.
• Events with the highest SSC and specific suspended sediment yield are caused by glacial melt or a combination of glacial melt and high intensity rainfall.
• A considerable proportion of the extreme SSC events (24% of the total) cause peak SSC values which are very likely to be harmful or lethal to many aquatic species.

Link to the research article (preprint):
https://egusphere.copernicus.org/preprints/2025/egusphere-2024-3985/

Ongoing work: the impact of future heatwaves and heavy precipitation on water quality

Our next study will focus on the effect of climate extremes on water temperature and the suspended sediment concentration in Alpine rivers. We will particularly focus on heatwaves and heavy precipitation, as their magnitude and frequency will likely increase towards the future.

Do you have ideas or suggestions? Or would you like to connect? Feel free to contact Amber van Hamel.