Two one-dimensional (vertical) hydrothermal models of Cannonsville Reservoir, a water supply serving New York City, were developed and tested. A two-layer model, capable of hindcasting temperature dynamics for two (epilimnion, hypolimnion) completely-mixed, variable-volume layers, was calibrated by determining die seasonal variation of the vertical heat transfer coefficient. This model was used only to hindcast temperature for the spring-fall period of 1995. A multi-layer model (average layer thickness 1.5 m over 50 m maximum depth) was developed that has the capability of forecasting stratification and vertical transport conditions in the reservoir based on specified meteorologic, hydrologic, and reservoir operation conditions. As a part of calibration, the multi-layer model was used to hindcast stratification and vertical transport conditions for the continuous period 1988 through 1995. The model accurately reproduced observed temperature profiles and other observed features such as thermocline depth, rate of hypolimnetic heating, and duration of stratification. A sensitivity analysis indicated that vertical transport of heat to the lower waters of the reservoir in summer is largely associated with advection caused by release of water at the base of the dam; vertical diffusion plays a relatively small role. Model forecasts indicate that stratification characteristics are relatively insensitive to intake location (three intakes over a range of elevation are available), but are more sensitive to the rate of dam release.