Abstract A comparison is made of the axial distributions of salt and dissolved 137Cs in a seaward section of the Severn Estuary during August 1974. It is shown that these variables were distributed with essentially the same axial dispersion coefficient, and that conditions were effectively stationary. It is also shown that the flushing of 137Cs from the estuary during this period was mainly due to the axial dispersion. The observed concentrations of 137Cs were the linear summation of those due to the North East Celtic Sea and freshwater background levels (0.65 ± 0.06 pCi 1 −1 and approximately 0.08 pCi l −1 respectively), together with those produced by the three nuclear power generating stations which discharge 137Cs into the estuary. The levels due to the Celtic Sea background amounted to roughly 50% of the observed values, taken as an average over the study area; the levels due to the freshwater background were negligible.