Side-scan sonar, augmented with echo-sounding and sediment sampling, has been employed to examine and map the distribution of sediments, bedform types and bedform geometries in the middle reaches of the macrotidal Tay Estuary of eastern Scotland. Much of this dynamic sector is characterised by mobile, medium to slightly gravelly sands. The most abundant bedforms are small to medium dunes (wavelength 2-10m, height up to 0.5m) of varying sinuosity and superposition the asymmetries of which are indicative of the dominance of flood currents and the marine derivation of sands. However, previously unrecognised abrupt, non-gradational changes in bedform morphology and sediment type occur both with and without apparent bathymetric control. A hitherto unreported acoustic signature, referred to as 'patchy discontinuous', was recorded in many areas of channels. This is attributed to the patchy colonisation of mussels preferentially on the gentle stoss slopes of dunes, with resultant sediment stabilisation, whilst uncolonised sediments are exposed on the steeper lee slopes.