A differential equation is developed to describe the vertical temperature distribution in a stratified reservoir and solved numerically. Outlet temperatures are an integration of temperatures from within the withdrawal layer, the thickness of which depends upon the degree of stratification. A comparison of predicted and measured temperatures from the laboratory and field indicates that the mathematical model is capable of reproducing measured temperature profiles to within an accuracy of approximately 1°C without requiring any parameters that must be determined from prior temperature measurements. A net vertical advection is the primary mechanism by which heat is transported from the surface to the reservoir outlet.