The authors consider the derivation of an expression for salinity distribution in an estuary, from an instantaneous mass balance equation for a solute in a continuum. The expression is compatible with existing field instrumentation and limited economic resources. Details are given of a prototype survey which included continuous monitoring of the salinity distribution of the Firth of Tay, an estuary subject to a wide range of fluvial and marine influences. The survey results enabled an apparent dispersion coefficient to be evaluated and an estimate to be made of the net mixing.