Abstract: In this study we examine the annual and interannual variations of water temperature and analyzes temperature trends along the river course, and particularly discuss the impact of cascade reservoir operations on water temperature based on measured data of natural water temperature in the upper reaches of the Yangtze River, including hydrological stations of Panzhihua, Huadan, Pingshan, Zhutuo, and Cuntan, as well as locations upstream of Xiluodu and Xiangjiaba dams. Results indicate that peak monthly mean water temperatures across the five stations typically occur in July and August, while the lowest temperatures are recorded in January. Affected by the release of colder water from the reservoirs, both annual mean and extreme temperatures downstream of dam are lower compared to those upstream. Downstream the Xiluodu Dam, temperatures are, on average, 0.6 ℃ cooler than that upstream, and maximum temperatures are approximately 3.5 ℃ lower. Similarly, temperatures downstream the Xiangjiaba Dam are, on average, 0.6 ℃ cooler with maximum temperatures around 1.6 ℃ lower. Notably, Pingshan, Zhutuo, and Cuntan stations experience water temperature rises in winter and decreases in summer. Furthermore, annual average water temperature after reservoir construction are higher than that before construction, with the magnitude intensifying downstream; annual maximum temperatures after reservoir construction are generally lower than those before construction, while the magnitude of decrease diminishes downstream. Conversely, annual minimum temperatures tend to be higher post-construction compared to pre-construction levels.

Key words: water temperature change, measured water temperature, annual average water temperature, cascade reservoirs, Xiluodu Reservoir, Xiangjiaba Reservoir, downstream of Jingsha River