The salinity changes in an estuary, in response to freshwater inflows have been explored by the use of a simple model. The time of lowered salinity is least downstream, where dilution and tidal flushing are most effective, and greatest upstream where river flushing is dominant. These effects may be characterised by the “estuary flushing parameter”, which is the ratio of flushing by the transient river inflow to the residence time due to tidal flushing and the base river inflow. The ratio of inflow duration to tidal period has a limited range of utility. The estuary flushing parameter may be used for rapid assessment or in decision support tools.