Acombined terrestrial and marine seismic investigation has been undertaken of the Inner Mersey Estuary, Halton District, UK, prior to construction of a major new river crossing. Much of the site lies on inter-tidal sand and mud bars accessible only for restricted periods and is unsuited to prolonged physical investigations using heavy plant. The investigation included high-resolution seismic refraction and reflection surveying on the salt marsh and inter-tidal regions, plus sub-bottom profiler investigation of the main river channel. Strategically located boreholes followed the geophysical work. Results show that the current configuration of the Inner Mersey Estuary is largely unrelated to the underlying bedrock geometry. In the area surveyed, the River Mersey is located on the southern flank of a buried elongate depression in the bedrock surface whose approximate position has been investigated previously between Ditton Marsh and Warrington through the interpretation of borehole records. The results also demonstrate a clear increase in compressional (P) wave seismic velocity of the Triassic sandstone with age, allowing the possible detection of an important fault. This paper highlights the potential for saving project capital through a properly designed geophysical site investigation prior to the commencement of major civil engineering projects.