A nonlinear model of temperature dependence on lake depth is proposed for interpreting the vertical thermal structure of a lake. The model has two critical temperatures: T₁^* – the maximum density water temperature T_cr = 4ºC and T₂^* – the lake water surface asymptotic temperature in warm seasons. Although the model depends on four parameters, its solution and properties are effectively determined by only two dimensionless parameters: the constant temperature gradient γ and the nonlinearity parameter v. The proposed thermodepth model is applicable to the interpretation of the vertical thermal regime of Lake Tapeliai by comparing the results of theoretical calculations and experimental measurements. Within the limits of our model, the position of the thermocline can be determined theoretically, i.e., the model parameters allow for expressing the thermocline position. A great match has been obtained between experimental data and theoretical dependencies.