The hydroelectric power generating system incorporates a man-made dam structure configured to completely enclose a body of water. The dam is preferably filled by pumping seawater into the reservoir defined by the encircling dam. A circumferential canal feeds water to one or more penstocks. Each penstock has one or more hydroelectric turbine generators installed therealong. The penstocks feed an enclosed circumferential channel about the base of the dam. The channel delivers water to a pump that pumps the water back into the bottom of the reservoir. An auxiliary hydroelectric power generating system disposed within the dam utilizes the water exiting from the lower end of the penstocks for additional production of energy. While this system results in a net loss of energy, the system can make use of surplus power to drive the return pump during periods of low electrical demand in order to replenish the reservoir.