Energy dashboard with hydropower reservoir filling per region: https://energiedashboard.admin.ch/strom/fuellstaende-speicherseen
Same information but can select previous years: https://www.uvek-gis.admin.ch/BFE/storymaps/AP_FuellungsgradSpeicherseen/?lang=de
Find below the key links about hydrological data and hydropower in Switzerland.
Statistics of Swiss hydropower infrastructure https://www.bfe.admin.ch/bfe/en/home/supply/statistics-and-geodata/geoinformation/geodata/water/hydropower-plants-statistics.html
Visualisation of Hydropower infrastructure on map.geoadmin.ch.
Swiss Energy Charts: https://www.energy-charts.info/charts/power/chart.htm?l=de&c=CH
Current nuclear power production: https://energiedashboard.admin.ch/strom/kkw-ch