Climate change has already affected many components of our natural environment which are well described in the literature. Water temperature has received less interest despite the fact that it is recognized as key variable for assessing water quality of freshwater ecosystems in streams and lakes. It influences the metabolic activity of aquatic organisms but also biochemical cycles. Water temperature is also a key variable for many industrial sectors, e.g. as cooling water for electricity production or in large buildings, and for the spreading of some diseases affecting fishes. It is very likely that climate change has and will also have an important effect on the temperature of streams. This study (Michel et al., 2020) investigates first the past temperature evolution and corresponding discharge in Switzerland since 1979, showing an increase of +0.33 ± 0.03° per decade in water temperature. Some differences between catchment type (alpine vs. lowland) and some important seasonal features are identified. In a second step, the response of selected catchments in Switzerland to the future forcing is numerically assessed using the CH2018 climate change scenarios for Switzerland. The approach uses a sequence of physical models including Snowpack, Alpine3D and StreamFlow. The CH2018 scenarios have been down-scaled to hourly resolution using a novel approach based on a delta method which preserves the seasonal aspect of the climate change scenario. The results show an increase in temperature for any of the RCP (2.6, 4.5, and 8.5) and a strong impact of climate change on alpine catchments caused by changes in snowfall/melt and glacier melt. As a consequence, river ecosystems including fish populations will be severely impacted and current legal limits for the usage of water for cooling in the energy production sector and in the industry will be reached more often in the future.