Abstract: Underwater power apparatus for harnessing a flowing river or large stream, includes blades or sails moving within a housing and in a circuit on a power leg and return leg. The housing directs the stream flow onto the power leg to drive the apparatus and diverts the flow from the return leg to provide a quiescent area for the return leg. Preferably, the blades or sails are opened when on the power leg and are folded or closed when on the return leg to reduce resistance. The underwater power apparatus may have either arcuate or linear power and return legs. In the latter case, the legs can be of sufficient length to develop a high power output. Since the apparatus is disposed completely underwater it disrupts neither the scenic beauty nor the navigation of the river.