A three-dimensional numerical model was used to predict the timing and the location of shear instabilities in the St. Lawrence Estuary. This model suggests that significant mixing occur during flood tides in the upper estuary. This mixing is associated with a strong bottom density current made of the cold Gulf of St. Lawrence intermediate waters flowing under the St. Lawrence mixed surface waters. Guided by these results, a field experiment was undertaken in summer 1997 to verify this and to document the conditions that favor the development of instabilities. The instabilities were found as predicted and documented from acoustic imaging, current profiler, and density measurements. The instabilities first develop in the form of wavelike disturbances before they break, like Kelvin-Helmholtz instabilities. The unstable waves have wavelength of ≈140--150 m and extend vertically between 10 and 25 m. The fine-scale observations of the semidiurnal evolution of the vertical structure of currents and density at the experimental site are compared with the numerical results. The model reproduces accurately the tidal variability of the currents but underestimates by a factor of 2 the amplitude of the density fluctuations. The general patterns of the shear squared S2 and the buoyancy frequency squared N2 are reasonably well reproduced by the model, but their intensities are ≈2 times smaller than the observations. This difference is attributed to the limited vertical resolution of the model at the pycnocline. However, the modeled Richardson numbers, Ri≡N2S-2, are reasonably well reproduced and appeared to be useful for the prediction of instabilities in such a complex environment. ¿ 2001 American Geophysical Union