THE THESIS This thesis describes the investigation of an estuarine front, the 'tidal-intrusion front, which was initially found in the Seiont estuary, North Wales but has subsequently been observed at other locations. In the Seiont, the front forms after low water, moves a considerable distance into the estuary to an upstream limit, retreats from the estuary and finally weakens, all within the duration of the flooding tide. It separates mixed from stratified regimes and is characterised by its surface appearance; inside the estuary, the front is 'V'-shaped with its apex directed towards the estuary head. Associated convergent flows include collection of all surface buoyant material at the apex of the V. Following a considerable field investigation of the front in the Seiont, two short periods were spent studying a similar feature in Loch Creran, Scotland. In the process of investigating other sites to determine the range of estuarine parameters appropriate to a stable tidal-intrusion front, an alternative estuarine convergence was discovered in the Conwy estuary, North Wales. The Conwy shows similar frontal behaviour to the Seiont during the early part of the flood tidal phase, but increased flood currents subsequently break down stratification producing an estuary of the 'well-mixed' class. 5 Throughout the remainder of the flood tide an axial line of debris forms along a substantial proportion of the estuary length. The persistence of the line indicates a secondary circulation which includes surface convergence towards the estuary axis. However, the line disperses at high water and no similar system is apparent during the ebb. The specific aims of the thesis may be summarised with respect to: a) the tidal-intrusion or T.I. front; i) to understand the parameters which govern the movements of the front. ii) to explain and/or reproduce by modelling, the three-dimensional aspects of the 'V'-shaped frontal boundary. b) the axial convergence or A.C. i) to detrmine the nature of the secondary circulation in the Conwy i.e. the pattern of cross-sectional secondary velocities. ii) to determine a mechanism which accounts for both the production of a sustained secondary circulation and its existence only during the flood phase of the tide. and finally, we attempt to define the stable parameter ranges for both types of convergence.