The Impact of Cascade Reservoirs Development on the Water Temperature of a River Basin: From the Perspective of Field Data Monitoring and Historical Data Analysis

ABSTRACT The development of cascaded reservoirs significantly alters the thermal regime of river basins, impacting river ecosystems. However, our understanding of the distribution of water temperature in river basins under high‐intensity hydropower development conditions remains limited. In this study, we analyzed historical data and monitored the present water temperature of the Wujiang River to assess the effects of cascade reservoir development on water temperature and fish. The results revealed that reservoirs with regulation capacity exhibited vertical thermal stratification, leading to a noticeable temperature difference between the surface and the downstream water, which in turn caused fluctuations in the longitudinal water temperature of the Wujiang River. Reservoirs have a greater impact on water temperature compared to air temperature, elevation, and river discharge. The critical temperature (CT) threshold was reached between mid‐late May and early June downstream of different reservoirs, which is within the normal time range for fish spawning. The accumulated temperature (AT) value of Pingzhai (PZ) reservoir did not reach the required threshold, while the AT threshold of other reservoirs was met earlier than the normal spawning period of the fish. The continuous construction of cascade reservoirs resulted in a significant reduction in the amplitude of water temperature at the downstream Wulong hydrological station, intensifying the water temperature delay in the Wujiang River. This study systematically revealed temporal and spatial variations of water temperature along the mainstream of the Wujiang River and provided a direct assessment of the cumulative impacts of cascade reservoirs on the river ecosystem, offering invaluable insights into the complex interplay between hydropower development and water temperature dynamics.