Context: Dams construction appears to have multi-dimensional effects on the river environment, and it especially affects the thermal condition of rivers. Aims: The objective of this work was to assess downstream thermal responses to the Charef Dam in relation to the water flow and the water residence time. Methods: During the year 2012, the hydrological parameters upstream and downstream of the dam were monitored. Key results: Salinity, electrical conductivity, total dissolved solids (TDS) and water flow all responded similarly, with larger magnitudes in waters upstream of the dam than downstream, whatever the season. In terms of flux, the Charef Dam retains 59 and 56% of the incoming water and TDS fluxes respectively. Furthermore, the upstream water temperature is 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). Conclusions: Despite being a small dam, the water residence time (2.3 years) in the Charef Dam has a significant effect on the downstream water temperature. Implications: These temperature changes may cause significant physicochemical and biological issues. Restoration practices such as increasing discharges during dry seasons and riparian cover restoration could mitigate thermal impacts and minimise negative effects.

Silicon (Si) is the most abundant element in the earth's crust after oxygen; it is found in aquatic environments as orthosilicic acid. This study concerned for the first time the distribution and flux of dissolved silicates (SiO4) in strategic sites (Charef dam, Bouhamdane dam, and estuary) from the upstream of the Seybouse basin to its mouth to understand silicon biogeochemical transformations under dams and estuarine effects. Surface water sampling was taken and analyzed in the dry (September 2016) and wet (April 2017) seasons. In parallel to sampling dates, physical parameters, and water discharge data were recorded. The highest SiO4 levels were recorded upstream and weakened towards the mouth. It was reduced between 32% and 54% for the Charef and Bouhamdane dams respectively, while the retention rate of the estuary was on average 44%. In terms of flux, the Charef and Bouhamdane dams produced 2 t/yr and 17 t/yr respectively, whereas the estuary produced only 54 t/yr to the coastal zone. Dams and estuarine part play a crucial role in changes in the biogeochemical silicon cycle and are expected to have severe impacts on the river system and the receiving coastline functioning.