The population of Geneva has just voted in favour of the expansion of the heating network by almost 80%. The use of Lake Geneva as a heat reservoir is central to this. There are also expansion projects on Lake Lucerne, Lake Biel, Lake Zurich and Lake Constance, or larger plants are already in operation. The thermal use of surface waters is gaining in importance, not least in the context of the new federal energy strategy. It includes heat extraction for heating purposes, but also disposal for cooling purposes. This raises the question of the ecological impacts on the lakes and rivers used. Within the framework of an applied project, Eawag has developed a basis for this. A new fact sheet summarises the key points that need to be taken into account.