A system and process for generating hydroelectric power within a body of water relying on the pressure head existing between two depths of the water. A vertically arranged conduit or penstock has an upper water intake and is in fluid communication with a reservoir situated at a lower depth. In a first cycle, water flow is established in the conduit or penstock between the water intake and lower reservoir when the reservoir is substantially full of air. A turbine housing is mounted adjacent the reservoir at a lower depth than the water intake and houses an electric turbine generator having blades mounted within the conduit or penstock to be driven by the flow of water to generate electricity. As water is introduced into the reservoir, air is exhausted out an air exhaust tube to a point above the surface of the body of water. After the reservoir is generally full of water valves are provided to cease the flow of water through the water intake and flow of air out the exhaust tube. An air pump thereafter introduces air into the reservoir to force water out of a reservoir water outlet port. The generating cycle is then repeated.