Some problems are described in connection with the survey of currents in an estuary influenced both by tides and runoff (the upper estuary of the St. Lawrence River, Canada). The use of numerical modelling is proposed as a reliable and cheap alternative to intensive hydrographic field surveys. A two-dimensional finite element hydrodynamic model with moving boundaries (drying–wetting) is outlined. An application of this model to the St. Lawrence River between Lotbinière and Sainte-Anne-de-la-Pérade is used as an illustration of the approach. The calibration method is first presented. Then, some results for a typical event (mean tide and river flow) are briefly described. Finally, emphasis is placed on the great potential of this approach to increase our knowledge (at least our insight) of estuarian systems as far as current-related phenomena are concerned (sediment transport, fish habitat, water quality, etc.).Key words: St. Lawrence River, hydrodynamics, modelling, simulation, finite elements, currents, drying and wetting, moving boundaries, tidal flow, estuary.