Riverine Heatwaves

News: Several studies have highlighted the increase in intensity and frequency of riverine heatwaves across rivers globally.

About Riverine Heatwaves

• Definition: 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. 
  • An increase in temperatures for a shorter duration is considered a heat spike.
• Causes: Riverine heatwaves are mainly caused by rising air temperatures due to climate change, along with reduced snowmelt, low streamflow, and shrinking river water levels.
  • Human activities such as dams, hydropower projects, wastewater discharge by other water bodies, industrial effluents, and changes in riverbank vegetation also contribute to increasing river water temperatures.
• Status of Riverine heatwaves: It has been observed that between 1976 and 2005, rivers globally experienced, on average, 2.19 heatwave events per year.
  • The intensity has increased over time at a rate of 0.02°C per decade.
  • The duration of riverine heatwaves has increased to 0.09 weeks per decade.
  • 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.
  • Status in the context of India: 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.
    • Impact: It will cause widespread damage to the ecosystem, and the population’s exposure to heatwaves in the Ganges (impacts on drinking water, agriculture, and fisheries) will also be the highest in the world.
• Overall impact: Riverine heatwaves can negatively affect river ecosystems and the organisms dependent on them:
  • The heat stress to the riverine floral and faunal species can affect reproduction rates, reduce migration, and alter food webs.
    • Example: Studies 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), even short heat spikes cause changes in the production of proteins involved in energy metabolism and immune system regulation.
  • Threat to vulnerable species: 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
  • Indirect Impact: 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.
    • Changes in river temperature can affect processes such as decomposition of leaf litter, carbon processing, and carbon storage, which may further impact aquatic insects, crustaceans, fish, and aquatic mammals.
• Prevention:
  • Restoration approach: There is a need to move beyond local-scale interventions and adopt restoration approaches that consider the entire river catchment.
    • Extreme events occurring in headwater regions can affect the entire river network, with ecological and biological impacts spreading downstream, making catchment-level restoration essential.
  • Rehabilitating and maintaining riparian forest cover: Riparian vegetation provides shade that reduces the heating of water and also stabilises riverbanks, improving aquatic habitat.
    • However, care must be taken in selecting appropriate species and planting locations. he adds.
  • Proper operation of reservoirs: There is a need to maintain sufficient flow in rivers by operating reservoirs appropriately for hydropower, drinking, and irrigation water demands.
  • Locate and protect thermal refuges: There is also a need to locate and protect thermal refuges in riverine systems. It would require some basic on-the-ground monitoring, where one would explore the riverine network for natural cold-water refuges.