The housing of the power plant defines a chamber at a level below the surface of a surrounding body of water; and also a series of penstocks which open into the water at points between the level of the chamber and the surface of the water, and which discharge into the chamber. The hydroelectric power generating means in the plant include turbines in the penstocks, and means for discharging the tailwater from the respective turbines including nipples on the housing which communicate with the chamber and open into the body of water at levels below that of the chamber. The nipples have open ended thimble-like vessels telescopically engaged thereon, to be extended and retracted in relation to the respective nipples axially thereof; and the nipples and vessels have float operated check valves therein adjacent the chamber and the end openings of the vessels, respectively, each of which is adapted to permit flow in the direction relatively from the chamber toward the body of water when open, and to prevent flow in the opposite direction when closed. Also, there are additional valve means in the tailwater discharge means which are adapted to equalize the pressure in the vessels with that in the body of water when the respective check valves in the vessels are closed. Drive means operate to extend and retract the respective vessels, and there are control means connected to the drive means, which are operative through the same (1) to extend and retract the respective vessles in staggered sequence about the circumference of the housing, with each vessel undergoing an at-rest period between the respective extraction and extension stages thereof when the vessel is fully retracted, and (2) to control the rate of extension and retraction of the respective vessels, and the length of their respective rest periods, as a function of the water level in the chamber. Each of the vessels is adapted in buoyancy to hover in equilibrium in the body of water when at rest, and the number of nipples and vessels is adapted in relation to the number of penstocks and turbines so that the vessels collectively discharge the tailwater from the turbines at a rate adapted to maintain the water level in the chamber between predetermined limits.