2023-09-15 11:13:29
A flooded river in the Wanzhou district of China, July 4, 2023. AFP
Extreme weather events do not only cause material destruction and human losses. Droughts, heat waves, excessive precipitation and floods lead, in the majority of cases, to a physical, chemical and biological deterioration of the quality of river water. These are the conclusions of Michelle van Vliet, from Utrecht University, based on a review of scientific literature, i.e. 389 publications published between 2000 and 2022, with an international team of a dozen researchers, the majority Dutch.
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The analysis, which appeared in Nature Reviews Earth & Environment, Thursday September 14, covers 965 rivers around the world. According to her, extreme events produce varied and even contrasting effects. The impact is clearly negative in 68% of the cases studied when it comes to droughts and heat waves, and in 51% during heavy precipitation and floods.
The scientists based their work on eleven criteria, present in the publications studied: temperature, sediment suspension, organic pollution, plastics, dissolved oxygen level, salinity, microorganisms, pharmaceutical substances, algae, nutrients (nitrates and phosphates) and metals.
Reduced dilution capacity
Heat accelerates evapotranspiration and therefore reduces the capacity to dilute the various pollution that many rivers already receive. Result: an increased concentration of chemical and pharmaceutical substances and nutrients, particularly from agriculture. Temperature also strongly influences the biochemical processes of water, the growth of algae, and the level of dissolved oxygen. Thus the development of cyanobacteria which accompany proliferations of harmful algae, some of which are toxic, reaches its maximum between 25°C and 35°C.
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Rising temperatures contribute to a significant increase in salt levels – researchers report an increase of 21% in this level on average in the rivers of the southern United States during an episode of drought. This increase influences the development of microorganisms. The salt can then enter the estuaries and be introduced in fresh water bodies. Researchers cite several examples: Mekong (Asia), Rhine (Europe), Valdivia (South America), Tigris and Euphrates (Middle East).
However, “the answers of the rivers”, as researchers call them, can be complex. Due to a lack of water, the slowdown of the current will slow down the transport of sediments and microplastic particles which themselves serve as a support for organic compounds, metals or antibiotics. These can therefore decrease temporarily, but they can then be retained on the banks during droughts.
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