A complete homogenization of water temperatures duringwidespread flooding

Abstract

Thermal regimes can vary substantially across small geographic distances in rivers,and the factors responsible for creating a diverse thermal landscape can operate onmultiple spatial and temporal extents. This short communication reports on the tem-perature dynamics in seasonal, off-channel habitats in the lower reach of the Gud-brandsdalslågen River in South-Eastern Norway (61 o N). The river is ordinarilycharacterized 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, thedifferences between cooler and warmer sites exceeded 5C (range 3–10C) and con-tinued to diverge until early July (range 6–21C). Differences persisted through theregular rain-driven spates in summer but were entirely diminished during the floodwhich started on August 7. By August 10, the water temperatures at all 14 sites hadconverged at 12.3C. The homogenization was likely the result of the sheer volumeof runoff through Gudbrandsdalslågen, overriding any locally modulating factors suchas groundwater inputs and isolation from the main channel. This suggests a relativelystrong influence of locally modulating factors in the study area under normal hydro-logical conditions in summer, and a complete dominance of runoff temperature dur-ing the flooding event. Disruptive events such as flooding can hence periodicallyrevert the thermal heterogeneity by changing the relative importance of the factorsthat create patchy thermal regimes, with potential ramifications for aquatic organismsthat occupy relatively warmer habitat patches in summer. extreme events, fish community, habitats, precipitation, thermal regime