We describe the wintertime development and persitence of an anomalous temperature inversion layer in a deep temperate lake with emphasis on the roles of convective mixing and river-induced circulation. The layer is about 20 m thick, 100 km long, and lies at a depth of 60--80 m. The destablizing temperature distribution is compensated by a very weak (0.015 ppt) salinity increase across the inversion layer. We obseve the neither the energy available from the wind nor the buoyancy extracted at the surface via a negative heat flux are sufficient to penetrate the depth of the inversion. Calculations of a thermal diffusion coefficient for this layer indicate a weak turbulent exchange below the halocline. Lake overturn does not follow the classical pattern of surface processes driving complete winter mixing; rather, full circulation is made possible only by the input of relatively saline river water.