A geothermal reservoir that produces mostly hot water is called a "hot water reservoir", the energy of which is exploited in a "flash" steam power plant. The hot water is brought up to the surface through the production well where, reaching the generation equipment the pressure is suddenly reduced, allowing some of the hot water to convert or “flash” into steam in a “separator”.

This steam is then used to produce electricity by powering the turbine. The remaining hot water not flashed into steam, as well as the water condensed from the steam is re-injected into the reservoir. Today flash steam plants are the most common type of geothermal power generation plants.

 An example is the CalEnergy Navy I flash geothermal power plant using the flash steam operation at the Coso geothermal field.