(1) A survey of intertidal macrobenthic invertebrates at forty sites in SIX estuaries in SW Britain was undertaken to identify the key environmental variables which determine the composition of these communities and to provide a prediction of conditions in the Severn estuary should a barrage for the generation of tidal power be built. (2) Multi-dimensional Scaling Ordination of the faunistic data showed that the sites were separated along two principal axes, one determined by static variables (e.g. sediment grain size and organic content) and the other by dynamic variables (i.e. current velocities as indexed by tidal range and wave climate as indexed by the wind fetch distance). (3) The faunistic composition at sites in the Severn estuary is significantly different from the other five estuaries, which by and large are not significantly different from each other. (4) The environmental variables responsible for the uniqueness of the Severn are dynamic rather than static, and should be predictable precisely in the post-barrage Severn. (5) Implications for qualitative changes in the fauna are discussed.