Flood-ebb asymmetry of a tidal flow has important implications for net sediment transport and the potential extractable resource. The asymmetry of the tide may be understood through the interaction of the M2 (principal lunar) and M4 (first even overtide of the M2) tidal constituents. Through expansion of the non-linear terms in the governing equations it is shown that tidal-stream turbines (TSTs) may augment or reduce both the odd and even overtides with the latter having potential consequences for asymmetry. In this paper, numerical experiments have been undertaken to investigate the impact of deploying a TST in a channel on the harmonics and asymmetry of the flow in the chanel. For the model geometry tested here, augmentation of the M4 tide was seen. Increases and decreases of 15%, 35% and 59% in the asymmetry of the velocity (V ), bed stress ( ) and power (P) in the vicinity of a single TST deployed in the channel were observed.