The Charef sub-watershed (1735 km 2, 200000 inhabitants) is part of the large Seybouse basin (North-east of Algeria). It contains several reservoirs, of which the Charef dam is the largest (157 10 6 m 3). The main objective of this work was to assess downstream thermal responses to the dam and examine the impacts of the water flow and the water residence time (WRT) on the variability in water downstream temperature. During the period January–December 2012, the hydrological parameters of the Charef dam’s waters varied greatly on a spatiotemporal scale. Salinity, EC, TDS, and water flow all respond similarly, with larger magnitudes in waters upstream of the dam than downstream, whatever the season. Moreover, salinity and EC have a significant positive correlation with temperature, but flow rate and precipitation have a negative one. Besides, TDS and temperature do not have a statistically significant relationship. In terms of flux, the Charef dam retains 59% and 56% of the incoming water and TDS fluxes, respectively. Furthermore, the temperature of the upstream waters is on average higher than that of the downstream in 83% of samples. During the dry season, the thermal response of the waters downstream of the Charef dam can exceed 2°C (downstream cooling). Despite being a small dam, the WRT (2.3 years) in the Charef dam has a significant impact on the downstream water temperature. These temperature changes downstream of dams may cause significant physicochemical and biological issues. This research applies to a critical issue in hydrology: the potential impact of dam-induced changes to water residence time and flow regimes on thermal dynamics, which in turn affect downstream ecosystems. Some restoration practices like increasing discharges during dry seasons could mitigate thermal impacts and minimize negative impacts. Also, riparian cover restoration could enhance riparian habitat, stabilize banks, and maintain cooler stream temperatures.