This thesis concerns the morphology or the Avon-Heathcote estuary mouth, located near Christchurch, New Zealand. While there have been numerous studies of tidal inlet morphology in the United States, there have been comparatively few on the inlets around the New Zealand coast. Morphology was found to vary considerably during the study with substantial changes in both the beach systems and the ebb tidal delta. These changes occurred despite the absence of any major storm events and beach changes were related to the onshore migration of the delta marginal channels. The transport of sediment in the vicinity of the estuary mouth was found to be in a predominantly southward direction. Direct exchanges between the updrift and downdrift beaches however, where prevented by the presence of the main ebb channel. Sediment therefore was transported into the estuary on the flood tide and then out again on the ebb tide in a manner consistent with a tidal bypassing regime. The change in the beach system around the spit of South Brighton spit was of a high magnitude although the pattern of erosion at South Brighton and subsequent deposition in Clifton Bay appears to be part of a larger cycle regularly occurring in this environment.