Between April and September 1982, fourteen current meters were moored at six stations along the St. Lawrence Estuary: two stations each in the lower, middle, and upper estuaries. The experimental data reveal a complex structure in the subtidal salinity and velocity variations, caused by a combination of tidal, atmospheric and density forcing. The results provide an important data base for modeling baroclinic variations in coastal estuaries. The salinity data indicate that the atmospheric forcing induced an 11.5-day oscillation which propagated as an internal Kelvin wave. However, the velocity data provide a different picture of wave propagation. The atmospheric forcing also induced salinity variations with a time scale of 40 to 50 days. The oscillation occurs initially in the middle estuary and then propagates toward upstream and downstream regions. Tidally rectified salinity and velocity oscillations at monthly and fortnightly periods were observed at two stations. From the results of an optimum multiple regression model, it was found that the tidally rectified component dominated the other components in the analyzed currents at these two stations. The freshwater pulse, characterized by a salinity minimum, has two time scales of variation: a long time scale of 73 days and a short time scale of 10--15 days. The long-period pulse was induced by the seasonal runoff. The short-period pulse was induced by two forcing mechanisms: vertical mixing by unusually strong tidal currents, and the advection of low-salinity water by wind-induced currents. The seasonal runoff also induced strong oscillations in the along-channel current in the lower estuary. This forcing of the current oscillation does not agree with a previously suggested forcing by atmospheric pressure. ¿ American Geophysical Union 1989