Assessment of groundwater - surface water interaction in estuaries that are tidally forced and in addition have transient density conditions in the surface water requires a multi-disciplinary approach. Such an approach has been undertaken in the Swan-Canning Estuary on the coastal plain of Western Australia. Here, the assessment of groundwater interaction has been driven by a requirement to assess the role of groundwater in contributing nutrients to the estuary which is perceived to have had an increasing frequency of occurrence of nuisance algal blooms in recent years. In this work we describe a range of approaches that have been adopted, including measurement of the nutrient and salinity status of sediment pore waters and adjacent groundwater, salinity mapping in groundwater, regional-scale modelling in 2D plan of groundwater flow and discharge to the river, analysis of tidal response in the aquifer to derive aquifer hydraulic properties, density-coupled modelling of groundwater flow in 2D section, analytical analysis of groundwater flow paths near meanders, tracer techniques including radon, radium, CFCs, δ¹⁵N, δ¹⁸O and δ²H and the development and use of an electronic seepage meter to directly measure bidirectional flow across the sediment-water interface. These approaches have identified that groundwater interaction with the Swan River occurs at three length scales. These are at the scale of the river-bed sediments (i.e. < 10 m), at the scale over which tidal forcing of the river is transmitted into the adjacent aquifer (10s to several hundred meters) and at the regional or catchment scale (100s to 1000s m). The resulting interaction is termed a 'mixed convection' since it involves both regional scale groundwater discharge to the estuary (forced convection) and local density-driven circulations (free convection). Measured rates of seepage agree well with model predictions despite the large-scale difference between the model domain and the measurement points. Results indicate that groundwater discharge makes a significant contribution to nutrient loading, especially nitrogen, to the estuary.