Research was undertaken to examine the mixing and exchange of saltwater between a microtidal estuary and a lake connected by an automated box culvert bi-directional gated structure. The gates open four times each day and remain open for a period of 2 h. They are programmed to open once during each semi-diurnal tidal phase, thus allowing alternative and partial exchange between the two water bodies. To determine the mixing dynamics, vertical profiles of temperature and salinity were collected as water flowed from the lake into the estuary and vice versa. The profiles were taken over a number of days and during various tidal ranges to examine the effect of tidal amplitude on the mixing dynamics of the plume. The results showed that the structure permits the exchange of saline and freshwater between the two systems, maintaining the lake system as a saline (brackish) environment. It perpetuated a permanently stratified environment on both sides of the structure under all tidal ranges examined.