Reservoir thermal stratification is often described as a physical and limnological process, but its direct relevance for day-to-day reservoir operation is less frequently quantified using routinely available monitoring data. This paper investigates whether high-frequency Supervisory Control and Data Acquisition (SCADA) records can be used to identify periods in which thermal stratification becomes operationally significant for the Ćelije multipurpose reservoir in Serbia. The analysis combines hourly water-temperature measurements at six elevations, 255, 265, 268, 271, 274 and 277 m a.s.l., with reservoir level and outflow data for the period September 2022–September 2023. Thermal conditions were quantified using a vertical stratification index, ΔT = T277 − T255, while reservoir operation was interpreted through level–outflow relationships and total discharge intensity. Strong stratification, defined as ΔT ≥ 10°C, occurred for 109 days, with a maximum daily value of 17.38°C recorded on 17 July 2023. The longest uninterrupted strongly stratified period lasted 64 days, from 30 June to 4 September 2023. The results show that discharge intensity was primarily governed by reservoir level, but the ΔT index provided essential additional information for identifying periods when increased release coincided with a strongly stratified water column. These operating windows are particularly relevant for assessing outlet use, withdrawal depth and potential thermal effects on released water. The study demonstrates that simple indicators derived from SCADA data can support thermally informed reservoir operation and provide a practical basis for future selective-withdrawal and water-quality assessments.
