Thermal pollution can have considerable influence on water temperature in freshwater systems notably affecting stratification (Kirillin et al., 2013). Consequences can be seen across the entire aquatic food web including benthic organisms (Barnett, 1971), algae (Cairns, 1971) and fish (Sylvester, 1972). The increased usage of aquatic systems as sinks and sources of heat necessitates detailed investigations of the impact of thermal pollution on lakes and reservoirs. Here we investigate the impact of thermal pollution on temperature and heat fluxes in the perialpine Lake Biel (7°10’E, 47°5’N, 39.3 km2 and 74 m deep). The lake has a short hydraulic residence time of only 58 days and is under strong influence of the Aare tributary. The source of the thermal pollution, the Mühleberg Nuclear Power Plant, is located 20 km upstream. The plant emits 700 MW as cooling water into the Aare River. Consequently, as the river enters the lake, it is heated locally by up to 4.5 °C. The aim of our study is to provide guidelines regarding model selection for studies of thermal pollution in similar aquatic systems. Materials and methods The thermal consequences of the Mühleberg Nuclear Power Plant for Lake Biel was investigated with two hydrodynamic models. We used the one-dimensional model SIMSTRAT (Goudsmit et al., 2002) and the three-dimensional model Delft3D-Flow version 6.01.07.3574 (Deltares, 2016). The power plant is up for decommissioning scheduled for 2019. The influence of thermal pollution was therefore investigated by running the models with and without thermal emission. The temporal variability of the lake was analyzed for a cold (April to March 2010/2011) and a warm period (April to March 2013/2014). Results and discussion We observe a strong seasonal dependence in the response of Lake Biel to thermal pollution. By removing the thermal pollution the lake cools down by ~ -0.3 °C between October and March. The corresponding value from April to September was ~ -0.1 °C. The impact was marginally stronger for the warm period compared to the cold period. The majority of thermal pollution (~60 %) leaves Lake Biel through the Aare outflow. This is due to the lake’s short retention time and the short distance between the Aare inflow and outflow (~8 km). Surprisingly we were able to reproduce this throughflow accurately even with the one-dimensional model. The reason being the high discharge of the Aare which quickly flushes the incoming heat out of the lake. The flushing was likewise observed in the three- dimensional model. This model could furthermore resolve the path of the river intrusion within the lake. We could identify periods where the river water travelled along the shoreline directly from the inflow to the outflow, thereby short-cutting the lake. Additionally the spatial impact of thermal pollution was, as expected, better resolved by the three- dimensional model. With observed temperature fluctuations up to –4.5 °C in Delft3D-Flow and –1.8 °C in SIMSTRAT. We argue that three-dimensional models should be used to assess the spatial impact of extreme levels of thermal pollution, which can have severe impact on biota. For overall system assessment one-dimensional models are sufficient. However, care is required for aquatic systems with short distance between the thermal source input and outflow.