A wave cavity resonator (WCR) is investigated to determine the suitability of the device as an energy harvester in rivers or tidal flows. The WCR consists of coupling between self-excited oscillations of turbulent flow of water in an open channel along the opening of a rectangular cavity and the standing gravity wave in the cavity. The device was investigated experimentally for a range of inflow velocities, cavity opening lengths, and characteristic depths of the water. Determining appropriate models and empirical relations for the system over a range of depths allows for accuracy when designing prototypes and tools for determining the suitability of a particular river or tidal flow as a potential WCR site. The performance of the system when coupled with a wave absorber/generator is also evaluated for a range piston strokes in reference to cavity wave height. Video recording of the oscillating free-surface inside the resonator cavity in conjunction with free-surface elevation measurements using a capacitive wave gauge provides representation of the resonant wave modes of the cavity as well as the degree of the flow-wave coupling in terms of the amplitude and the quality factor of the associated spectral peak. Moreover, application of digital particle image velocimetry (PIV) provides insight into the evolution of the vortical structures that form across the cavity opening. Coherent oscillations were attainable for a wide range of water depths. Variation of the water depth affected the degree of coupling between the shear layer oscillations and the gravity wave as well as the three-dimensionality of the flow structure. In terms of the power investigation, conducted with the addition of a load cell and linear table-driven piston, the device is likely limited to running low power instrumentation unless it can be up-scaled. Up-scaling of the system, while requiring additional design considerations, is not unreasonable; large-scale systems of resonant water waves and the generation of large scale vortical structures due to tidal or river flows are even observed naturally.