Artificial thermal effluent (ATE) is used as an effective measure for solving the problem of ice damage in cold regions; the ATE outlets introduce thermal water (approximately 10 °C) into the diversion channel to increase the temperature of the channel water. This study discusses two approaches for determining the change in the water temperature during the winter operation period at the Hongshanzui diversion channel situated on the Manas River, China. One method involves field observations to determine the temperature change process of the diversion channel. The results indicate that the water temperature of the diversion channel increased from 0.2 °C to 1.0 °C (average 0.5 °C) after ATE outlets were employed. The process of change in the water temperature is segregated into rapid-rising, rapid-dropping, and slow-decreasing periods while the water flows through the ATE outlet. The second approach includes numerical simulations performed with a 3D nonhydrostatic model validated with field-observed water temperature data. The current research explores fundamental relationships among air temperature, diversion channel discharge, ATE out discharge, ATE out temperature, and diversion channel water temperature. The paper provides a discussion on the fundamental interdependency among the heat flux ratio (HFR), air temperature, and diversion channel water temperature. The water temperature attenuation curve is a power function distribution with a logarithmic relationship between the index and the HFR given by y = 0.03ln(x) + 0.2. Further, the air temperature of −10 °C is the turning point of the attenuation law. It is suggested that the changing process of water temperature in diversion channel after ATE outlets could be segregated into sudden-drop, transitional, and slowed-down periods depending on the HFR and atmospheric temperature. Finally, taking Hongshanzui and Jingou River hydropower stations as examples, the calculated unfrozen influence distances in this study are compared with the calculated results of three previous studies, which indicates that each method has its applicability and the calculation formula in this study takes into account the influence of various factors and has a wider scope of application.