A system is adapted to extract energy from flowing liquid. The system includes at least one vertically-extending vane adapted to move in response to the flowing liquid. The vane has a vertical length wherein at least a portion of the vane can be positioned below a surface of a body of liquid such that the vane forms a swept area defined at least partially by the vertical portion of vertical lengths of the vane positioned below the surface. The system further includes a mechanism adapted to selectively vary the swept area.