Abstract:The hydrodynamics of many hydropower reservoirs are controlled by the operation of their power plant, but the associated water quality impact is often poorly understood. In particular, significant hydropeaking operations by hydropower plants affect not only the downstream ecosystem but also the reservoir water temperature. This paper contributes to understanding that link. For this, we coupled a hydrodynamic model (Estuary, Lake and Coastal Ocean Model, ELCOM) to a grid-wide power system scheduling model. In a case study (Rapel, Chile), we observe the behavior of variables related to the flow regime and water quality (including sub-daily hydrologic alteration, seasonal and sub-daily thermal pollution of the downstream river, and vertical mixing in the reservoir). Additionally, we evaluate how environmental constraints (ECs) can improve the conditions for a wet, normal and dry water-type year. We found that the unconstrained operation produces a strong sub-daily hydrologic alteration as well as an intense thermal pollution of the outflow. We show that these effects can clearly be avoided when implementing ECs. The current (unconstrained) vertical mixing makes the reservoir susceptible to algae blooms. Implementing ECs may intensify the stratification in the reservoir near the dam in some scenarios. The grid-wide economic cost of Rapel's ECs is a modest 0.3%.