Observations of subtidal and tidal flow in the Rı́o de la Plata Estuary

We present the first measurements obtained with a combination of a towed acoustic Doppler current profiler (ADCP) and a conductivity–temperature–depth (CTD) recorder in the Rı́o de la Plata Estuary. Subtidal and tidal flows are described for austral winter and summer conditions. Data were collected at three transects 15 km long for 25 h during August 11–17, 1999 (austral winter) and February 2–6, 2000 (austral summer) at the transition zone between fresh and brackish waters. Two transects covered cross-estuary tracks off the northern and southern coastlines of the estuary. A central transect was oriented along the estuarine axis near the up-stream limit of saltwater intrusion. The observations indicated that the dominant dynamical processes were different at each of the transects sampled. The most relevant feature was the dominance of tidal currents in the southern side of the estuary and the lesser role they played in the northern side (Uruguayan coast). The pressure gradient produced by the freshwater outflow was modified differently at the southern than at the northern sides of the estuary. At the southern side tidal mixing, from strong tidal currents, allowed the development of gravitational circulation. At the northern side weak tidal currents allowed modifications by ambient forcing and surface stresses, consistent with theoretical results of surface-advected freshwater outflows.