Observations of the spatial and temporal distribution of temperature in Cannonsville Reservoir over the 1988–1995 period are reviewed, with emphasis on 1994 and 1995. The dominant spatial variations in temperature are in the vertical direction (thermal stratification), with modest spatial variations along the longitudinal axes of the basin. Longitudinal temperature variations were greatest in the spring, with shallow, upstream waters being warmer than at deeper sites downstream near the dam. Diurnal temperature variations of the surface waters of the reservoir were generally less than 2 °C. The variations of temperature of two major reservoir tributaries are also documented. The tributaries are generally warmer than reservoir surface waters in spring, but are cooler through most of the summer and the fall. Diurnal fluctuations in tributary temperature may be significant, with day-night changes as large as 15 °C observed for Trout Creek. A reservoir heat budget calculation for 1995 indicates that inflow/outflow and water surface heat transfer are both important in affecting temperature variations of the reservoir. Direct measurements of incident solar radiation at the reservoir site for a single year allowed calibration of a predictive relationship that may be used in hydrothermal modeling. Estimation of empirical coefficients in predictive expressions for other components of water surface heat transfer was performed using the net water surface heat transfer computed from the heat budget. The implications of these findings for predictive hydrothermal modeling of the reservoir are discussed.