In a context of climate change, use of renewable energies should highly increase in the coming decades. Existing reservoirs and hydropower plants could be used to balance intermittent energy sources such as wind power and solar energy by pumping water from the downstream reservoirs and storing it in the upstream ones when electricity demand is low, and releasing it to generate electricity in high demand periods. Lake Suldalsvatn, located on the Western coast of Norway, is the lowermost reservoir of Ulla-Førre, the largest hydropower plants system in Northern Europe. Today, it receives turbinated waters from Kvilldal and Suldal hydro power plants, and supplies water to Hylen power plant. In the future, new operational regimes could include the alternation of pumping phases and production phases through installation of a new pumped-storage power plant. Lake Blåsjø located at a higher elevation could be used as upstream reservoir to store water pumped from Lake Suldalsvatn. This report describes results of a study investigating changes in temperature and current distribution in Lake Suldalsvatn under pumped-storage regime. The 3D-hydrodynamic model GEMSS was used to calculate flow velocity, water level fluctuations, and water temperature for two different pumped-storage scenarios. The results of simulations show that intense vertical mixing through the water column is expected during operation of a pumped-storage power plant, leading to colder temperature in the downstream river Suldalslågen during summer and autumn. In addition strong currents appear next to the pumped-storage plant.