Changes in capacity have been examined in the Lune, Ribble, Mersey and Humber estuaries to assess the role played by land-derived sediments in coastal accretion. The engineering concept of dynamic equilibrium is introduced and shown to imply that land-derived sediments should contribute exclusively to coastal accretion. The various factors responsible for changing estuary capacity are also introduced. Capacity changes over the last 100 years show that all the estuaries considered have suffered large scale siltation, with the majority of the sediment being derived from coastal and offshore sources. In the case of the Lune, Ribble and Mersey estuaries, engineering works have been responsible for large permanent changes in capacity. The situation in the Humber is less clear and siltation is probably a result of natural infilling; the estuary having been over-deepened by glacial and fluvial action in the past. A detailed study of the Mersey suggests that the estuary was also over-deepened and over-widened and has only been tidal for the last 9000 years or so. Calculations suggest that a new equilibrium will be achieved in about 250 years time and that the estuary may then be in a state of dynamic equilibrium. The much smaller Lune estuary may already have achieved this state, although detailed evidence is scarce.