Water Thermal Regime of River‐Floodplain Systems in the Uruguay River Basin and Influence of Hydroclimatic Factors

ABSTRACT Thermal regimes are a key aspect of river ecosystems, yet information on the thermal regimes of rivers in tropical and subtropical systems is relatively limited. In Southeastern South America, we characterized freshwater thermal regimes of 10 waterbodies located in the Queguay River Basin (Uruguay) over a period from 2019 to 2024. We characterized thermal regimes using 26 thermal metrics and applied Principal Component Analyses to evaluate the relative contribution of numeric metrics to variation among waterbody types (lakes, rivers, and streams), seasons, and ENSO phases. Mean daily water temperature, which represented a main component of variance, ranged from 6.4°C to 30.2°C, with summer water temperatures averaging 23.6°C to 26.3°C and winter temperatures averaging 11.1°C to 13.5°C among sites. Daily temperature range and standard deviation represented a secondary dimension of variance, such that streams and rivers displayed lower standard deviation in daily mean temperatures than lakes in Spring, Summer, and Fall. The El Niño Southern Oscillation, associated with variation in flow regimes, influenced thermal regimes depending upon waterbody type and season. The El Niño phase was associated with higher water temperatures in winter among all waterbody types, and the La Niña phase was associated with higher daily temperature ranges in summer among rivers. Air temperature was a strong predictor of variation in water temperature, with model performance highest among streams. Variation in the flow of the Queguay River, ranging from 2 to 860 m 3 s −1 , influenced the sensitivity of water temperature to air temperature variability, whereby high‐flow periods had lower sensitivity than low‐flow periods. Overall, we characterize thermal regimes in an undammed system, showing how extreme climate and hydrological conditions influence thermal regimes. This study fills an important geographical gap in thermal regime data in freshwater ecosystems in South America and suggests that extreme hydroclimate events can have important implications for the thermal variability of freshwater systems.