The hydroelectric power plant of the present invention employs a generating system similar to that employed in my U.S. Pat. No. 4,075,838. As in the patent, there are casing means defining a first chamber filled with a first column of water and a standpipe relatively inclined to the horizontal thereabove to pressurize the first column of water. There is also a housing enclosed in the first chamber which defines a second chamber therein for containing water in a second column horizontally offset from the first column of water. The housing has a series of penstocks about the perimeter thereof which open into the first column of water at a first level above the second chamber, and discharge into the second chamber at a second level below the first level. In addition, there are hydroelectric power generation means including turbines in the penstocks and means for discharging the tailwater from the respective turbines. The tailwater discharge means include elongated hollow-bore, open ended nipples extending into the first chamber from the housing at a third level below the first and second levels and having open ended thimble-like vessels telescopically engaged on the end portions thereof which are relatively remote from the housing, to be extended and retracted in relation to the respective nipples, axially thereof. There are also drive means operable to extend and retract the respective vessels, and float operated check valves in the vessels adjacent the end openings of the vessels relatively remote from the housing, each of which is adapted to permit flow through the bore of the respective nipple in the direction relatively away from the housing toward the first column of water in the first chamber when open, and prevent flow in the opposite direction when closed. As in the patent, control means are connected to the drive means to extend and retract the respective vessels in staggered sequence to one another about the circumference of the housing, with each vessel undergoing an at rest period between the respective retraction and extension stages thereof, when the vessel is fully retracted.