Our paper presents data on the annual course of the temperatures of surface sediments in deep bottom areas of a lake that has been assigned a “moderately-warm” designation. It is shown that the highest temperatures of those sediments are reached in autumn when near-bottom waters are involved with the processes of the gravitational mixing of the water column. Mixing conditions of the water column were estimated using additional information on the vertical profiles of standard water variables (pH, oxygen concentration, and conductivity). It is shown that depending on the amount of heat stored in the sediments of the deep bottom areas and the thermodynamic processes of its redistribution, the lake may be designated “super-warm” or “moderately- warm.” This implies that existing thermal classification of lakes is too formal and does not account for the complexity of thermodynamic processes of heat redistribution. Measurement data is evidence of the implication of the inflows from the bottom-feeding sources in the formation of the highly mineralized near-bottom water layer in those bottom areas.