This study was commissioned to extend the sparse information on the ecology of the Upper Severn Estuary in relation to the feasibility study to place a tidal energy barrage at English Stones. The report is in 3 parts: literature survey, field survey and a discussion of possible changes due to barrage construction. The literature survey examines published information (44 papers) on algae, saltmarshes, fish, birds and invertebrates and discusses the findings. A field survey concentrated on the invertebrates occuring in the major habitats (rock, shingle, sand, mud and saltmarsh) in 5 sectors of the upper estuary from Avonmouth to the riverine stretches above Gloucester. The major conclusion is that tidal scour and its effects together with the different sediment types are equally important to reduced and varying salinity in controlling invertebrate distribution. As expected, major ecological changes are predicted if a barrage were constructed. Increased stability of the system due, in particular, to the reduction in current strength and scour could lead to increases in both the diversity and absolute number of invertebrates. There would also be an increase in the area occupied by Spartina marsh. The report concludes with recommendations for further research required necessary to properly evaluate the consequences of barrage construction. In addition, many samples of saltmarsh invertebrates from the present study still require examination. It seems clear from the engineering and economic studies of the English Stones barrage that the project would not be viable as rapid silting of the upper basin would prevent efficient operation.