A complete homogenization of water temperatures during widespread flooding

Thermal regimes can vary substantially across small geographic distances in rivers, and the factors responsible for creating a diverse thermal landscape can operate on multiple spatial and temporal extents. This short communication reports on the temperature dynamics in seasonal, off‐channel habitats in the lower reach of the Gudbrandsdalslågen River in South‐Eastern Norway (61 oN). The river is ordinarily characterized by a snowmelt‐driven hydrograph but was subjected to widespread, rain‐driven flooding in August 2023. At the onset of spring runoff in mid‐May, the differences between cooler and warmer sites exceeded 5°C (range 3–10°C) and continued to diverge until early July (range 6–21°C). Differences persisted through the regular rain‐driven spates in summer but were entirely diminished during the flood which started on August 7. By August 10, the water temperatures at all 14 sites had converged at 12.3°C. The homogenization was likely the result of the sheer volume of runoff through Gudbrandsdalslågen, overriding any locally modulating factors such as groundwater inputs and isolation from the main channel. This suggests a relatively strong influence of locally modulating factors in the study area under normal hydrological conditions in summer, and a complete dominance of runoff temperature during the flooding event. Disruptive events such as flooding can hence periodically revert the thermal heterogeneity by changing the relative importance of the factors that create patchy thermal regimes, with potential ramifications for aquatic organisms that occupy relatively warmer habitat patches in summer.