Both the four-thirds law and random process analogy formulas yield effective diffusion coefficients in the proper order of magnitude for the brackish portion of the Potomac Estuary. Either of these formulas should provide useful approximations for other estuaries where good information on salinity variations is not readily available. These formulas should be used with caution in the fresh water portion of tidal rivers, in view of the possible important influence of salinity gradient induced density currents on diffusion. Prototype tracer studies in such locations appear to be necessary to provide reliable estimates of the appropriate diffusion coefficient. (Author Modified Abstract)