The system includes a support structure with upstream and downstream pulleys rotatably supported thereby and with cables following circuits around each of the upstream and downstream pulleys. Prime movers, such as in the form of sail members, are attached to the cable. These prime movers have surfaces which are more perpendicular to the flowing water when on a downstream leg of the cable circuit than when on an upstream leg thereof. The prime movers are configured to rotate as they pass about the downstream pulley and upstream pulley to optimize their orientation to minimize drag when following the upstream leg of the circuit and to maximize surface area against which the flowing water acts when following the downstream leg of the cable circuit. Power is outputted from the system through action of the cable upon a power output such as an electric generator coupled to one of the pulleys.