Abstract Field measurements of salinity and sea surface elevation are used to quantify the impacts of the developing inlet, Little Pikes Inlet, on tidal transmission and exchange in the bar built estuary, Moriches Bay. These measurements document a peak increase in tidal transmission of 30% for the post-breach condition. This increased tidal transmission led to a 1.4‰ increase in salinity in the region of the bay near the breach with no detectable change in the far section of the bay. It is shown that the tidal transmission of the developing inlet can be reasonably predicted by typical 1-D inlet hydraulic models with an appropriate selection of inlet characteristics. The mean tidal exchange of salt in the system is shown to behave as though frontal exchange at the plume boundary dominates and that the tidal exchange coefficient can be modeled to vary inversely with the square root of the flood tidal volume. Such a model is shown to improve significantly the predictions of the effects of changes in inlet properties.