The applicability of recent theories relating tides and river flow to estuarine bathymetry has been assessed against observations from UK estuaries. These theories, combined with observations, show that variations between estuaries are limited by how tides and river flow control sediments and bathymetry. Typical ages were determined from differences between theoretical and observed depths for Rias (11000 years), Coastal Plain (15000 years) and Bar-Built (100 years) estuaries. The theories can explain the estuarine bathymetry without needing to account for the sediments, suggesting that it is the bathymetry which determines the sediment regime within the estuary.