The problems confronting the Siltation Working Party of the Thames Flood Prevention Investigation are stated, the need for an early solution necessitating rigorous limitation of the possible combinations of site, function and design of flood protection structure that could be studied. The reasons for excluding all schemes involving a permanent barrage are given, the Hydraulics Research Station finally being asked to determine the effect on siltation of constructing a barrier at specified sites for use both as a simple surge excluding device and as a continuous half-tide control structure. The methods adopted by H.R.S. in tackling the problem are described. They include four large-scale estuarine surveys; the establishment of stations for continuously monitoring the suspended solids content of the river; field and laboratory tests to determine the properties of Thames silt; the development of a mathematical model to study the effect of the tide control on the distribution of silt which is carried in suspension, and experiments on a physical model to establish the redistribution of bed sediments in the navigation channel likely to follow barrier construction and continuous half-tide control. The data from the field surveys demonstrate how current-velocity, suspended silt concentration, salinity and temperature at different depths along the estuary change throughout a spring and a neap tide during both high and low river flows. The results of the silt monitoring exercise supplement the survey data by indicating how the concentrations of suspended sediment in the estuary vary - with tidal range, with position in the bi-monthly spring-to-neap cycle and with seasonal variations in freshwater discharge. The tests to determine the properties of Thames silt provide values for the four parameters describing the processes of silt movement needed to develop the mathematical model. The indications from the physical model studies are that the construction of a well-designed barrier used only to exclude surges should produce no insuperable siltation problems. Both mathematical and physical model results show that continuous tide control leads to increased siltation, the zone of greatest deposition depending on the barrier site.