Abstract: An apparatus, method, and system of gathering information useful to derive the velocity of the free surface liquid flow in an open channel flow. The method involves recording successive images of controlled surface waves on the open channel flow with sufficient resolution to derive spread of fronts of the controlled surface waves, using image velocimetry to derive celerity of controlled surface waves, and inferring the velocity vector field of the underlying liquid flow using wave theory elements or calibrations. An apparatus according to one aspect of the invention uses an artificial nonintrusive mechanism to set up the controlled surface waves, uses artificial light to illuminate the controlled surface wave to accentuate its affronts, digital camera to capture the successive images. Software can be used to utilizes image velocimetry and to infer the velocity vector field.