Abstract: Cascade hydropower development is an effective method for fully utilizing hydropower resources, but it inevitably has a cumulative impact on the water temperature rhythm of the watershed. Based on the data series of water temperature monitoring at typical hydrological stations in the lower Jinsha River, this study combines the statistical methods and certain indicators for evaluating cumulative impact, and explores the influence of upstream cascade development on incoming water temperature, focusing on the cumulative impact of water temperature in the dynamic construction process of cascade reservoirs in the lower Jinsha River. Results show the cascade construction in the middle Jinsha River and the Yalong River watershed has imposed a significant impact on water temperature at the inlet section of the lower Jinsha River mainstream. The operation of upstream cascade dams has caused a temperature drop in warming periods and a temperature rise in cooling periods at the tail of the Wudongde Reservoir. As more cascade dams are built and put into operation, a significant difference will emerge in water temperature at the outlet of the lower Jinsha River compared to the natural case, and its influence will become gradually more severe. And the level of hysteresis effect and homogenization of water temperature will be gradually rising. Generally, this cumulative impact manifests different varying trends between months, resulting in an overall process of superposition-stabilization-slowing down-inversion. This study would help deepen our understanding of the impact of water temperature and its cumulative behavior in a reservoir cascade under the changing conditions, and lays a basis for watershed water resource management and regional ecological function maintenance.

Key words: cascade reservoirs, dynamic construction, water temperature, cumulative impact, lower reaches of Jinsha River