The energy strategy 2050 of the Swiss government (BFE, 2016), which includes the phase-out of nuclear power and its replacement with mostly new renewable electricity sources, requires an increase in both the absolute amount of hydropower production (by 1.5-3 TWh yr-1) and the flexibility to produce power at times when the stochastic energy production from new renewable energies is low. The SCCER-SoE is currently developing innovative methods for hydropower operation to support this additional flexibility (SCCER-SoE, 2015). However, the increased flexibility in hydropower production will also modify the discharge regime downstream of storage hydropower plants as compared to present day conditions. Hydropeaking (Bruder et al., 2016) and the related thermopeaking (Vanzo et al., 2015) will likely become more stochastic and more intense, and the already existing seasonal shift of dis-charge from summer to winter months (Meile et al., 2011) will increase.At the same time, implementation of the revised Water Protection Act requests that hydropower produc-ers must mitigate negative effects of hydropeaking with structural or operational measures. An often applied structural measure is the construction of a compensation basin or cavern in order to attenuate peak flows and to reduce the rate of change of discharge and temperature.Earlier studies mainly focused on the temperature along the river continuum (see section 3), yielding important information on large-scale trends. However, key ecological processes such as egg develop-ment of gravel-spawning fish or substrate respiration are also driven by the small-scale mosaic in water temperature that is due to groundwater interactions, stream morphology, shading by riparian vegetation, etc. The effects of hydropower plants and hydropeaking on this spatio-temporal distribution of river tem-peratures are largely unknown.Within the proposed research project, we therefore tackle the following questions:

• How does hydropower exploitation, in general, and hydropeaking, in particular, affect the tem-perature variability across different scales within a river network (spatially, temporally)?
• How will future developments (e.g. new operation rules, new hydropower schemes, climate change) affect the spatio-temporal variability in temperature?
• How does the location of hydropeaking releases and the implementation of structural mitigation measures such as compensation basins affect the spatio-temporal variability in temperature?