A system for controllably adjusting spent water levels in the tailrace pool area of a hydroelectric power-generating dam installed into a waterway. The system comprises: (a) a controllably adjustable weir device installed into a downstream portion of the waterway adjacent a distal end of the tailrace pool area. The controllably adjustable weir device is configured for controllably impeding egressof spent water from the tailrace pool area, (b) an apparatus provided with operative components configured for manipulating the controllably adjustable weir device to controllably increase and decrease the impediment therewith of spent water egress from the tailrace pool area. The operative components are serviceable from above the spent water, and (c) a control device configured for communicating and cooperating with the apparatus for manipulating the controllably adjustable weir therewith. The controllably adjustable weir device may be optionally installed into an excavation provided downstream adjacent to the tailrace pool area.