An empirical theory is presented which describes the exchanges between various parts of an estuary as a result of tidal oscillations, and which permits the calculation of the average distribution of fresh and salt water within the estuary. The characteristics of the estuary used in the calculations are the mean range of tides, the river flow, and the topography, all readily available for most estuaries. The calculations are shown to produce results which are similar to distributions observed in three very different estuaries. The theory will permit calculation of the changes in distribution of salinity and fresh water in any given estuary to be expected as a result of variation of river flow.