Simultaneous measurements of current velocity and density profiles at the Sill of Mainau (western Lake Constance) enabled the observation of the internal response of the lake and the calculation of gradient Richardson number over the entire water column at the sampling site. Periodic changes of current direction corresponded with predicted periods of the two-layer oscillation and could be confirmed with thermocline oscillation data. In general, the current profiles confirmed a two-layer structure. However, an activating wind of speeds higher than 4 m/s produced three-layer profiles for a period of several hours at the sampling site, when the velocities due to the two-layer oscillation traversed zero. At times the implied current shear produced supercritical values of gradient Richardson number on a vertical scale of a few meters. This occurred during winds in the epilimnion and after strong (>4 m/s) winds in the lower thermocline and the hypolimnion. In the 20 m above the lake bed, topographical conditions helped the gradient Richardson number to go supercritical sporadically. Across the thermocline, no supercritical gradient Richardson numbers could be confirmed at the measuring site. A lower boundary for vertical mixing in the measuring site due to current shear could be evaluated. ¿ 2000 American Geophysical Union