Temperature trends in Norwegian rivers from 1990 to 2025 are strongly amplified by latitude

Study region Mainland Norway Study focus Based on measured water temperatures from 73 Norwegian rivers in near natural catchments from 58°N to 70°N, we analyzed warming trends from 1990 to 2025 and assessed seasonal and latitudinal variation in warming rates using linear mixed effect modelling. Furthermore, we assessed the coupling between river and air temperature trends, and changes in the likelihood of high-temperature events potentially stressful to cold-water adapted salmonids. New hydrological insight for the region River temperatures increased significantly in all months except December–February, with the strongest warming in June and July (0.60 and 0.52°C decade⁻¹). Warming was strongly amplified at higher latitudes in July, August, and September, consistent with Arctic amplification. For example, estimated July warming rates at 70°N (0.72°C decade⁻¹) were more than twice those at 58°N (0.32°C decade⁻¹), equivalent to 2.5°C and 1.1°C of warming over the 35-year period, respectively. Mean summer (June–September) Sen's slopes for water temperature (0.43°C decade⁻¹) were comparable to those for catchment air temperature (0.40°C decade⁻¹), indicating broad thermal coupling, though water and air temperature trends at individual sites were not correlated, suggesting local modulating factors. The probability of high-temperature events more than doubled across the dataset, increasing from ~20% to ~50% at 58°N between 1990 and 2025. The accelerating thermal change in northern rivers has significant implications for cold-water species and freshwater ecosystems.