A 10,000-km2 hypoxic ‘dead zone’ forms, during most years, in the central basin of Lake Erie. To investigate the processes driving the hypoxia, we conducted a 2-yr field campaign where the mixing in the lake interior during the stratification period was examined using current meters and temperature-loggers data, as well as > 600 temperature microstructure profiles, from which turbulent mixing was computed. Near-inertial Poincaré waves drive shear instability, generating ∼ 1-m amplitude and 10-m wavelength high-frequency internal waves with ∼ 1-mdensity overturns that lead to an increase in turbulent dissipation by one order of magnitude. The instabilities are associated with enhanced vertical shear at the crests and troughs of the Poincaré waves and may be correlated with the local gradient Richardson number. Poincaré wave–induced mixing should be an important factor when the Burger number < 0.25. The strong diapycnal mixing induced by the Poincaré wave activity will also significantly modify the energy-flux paths. For example, we estimate that, in Lake Erie, 0.85% of the wind energy is transferred to the lake interior (below the surface layer); of this, 40% is dissipated in the interior metalimnion and 60% is dissipated at the bottom boundary. In smaller lakes, 0.42% of wind energy is transferred to the deeper water, with 90% dissipated in the boundary and 10% in the interior metalimnion. © 2012, by the Association for the Sciences of Limnology and Oceanography, Inc.