Study on Temporal and Spatial Evolution Laws of Water Quality in Canyon-Type Seasonal Regulation Reservoirs During Initial Impoundment Period: Taking Wudongde Reservoir as a Case Study

The impoundment of large reservoirs alters hydrodynamic conditions, which easily triggers water quality issues such as organic matter accumulation and nutrient redistribution. Due to their narrow and long reservoir shape and significant seasonal water level fluctuations, canyon-type seasonal regulation reservoirs exhibit distinct water quality responses compared to conventional deep reservoirs. However, systematic research on the full-stage water quality evolution during the initial impoundment period remains insufficient. Taking the Wudongde Reservoir as a case study, this research divides the impoundment process into four stages (pre-impoundment, 965m impoundment, 965m operation, and 975m impoundment test) based on continuous monitoring data from 2016 to 2023. Meanwhile, it characterizes seasonal differences according to dry season, normal flow season, and flood season, and systematically analyzes the temporal and spatial evolution laws of core indicators (such as NH₃-N, COD, TP, and TN) during the initial impoundment period. The results show that: (1) From the temporal perspective, the concentrations of core indicators in the main stream remain stable overall without systematic abnormal fluctuations; TP and COD in the flood season (June-October) show short-duration pulse characteristics but recover quickly. (2) From the spatial perspective, the main stream presents a stable gradient pattern in the longitudinal direction; the COD concentration is relatively higher in the middle section of the reservoir, while the concentrations of water quality factors all decrease in the front of the dam. (3) The tributary effect is characterized by high intensity, short duration, and locality; the estuaries of tributaries become the key interfaces for pollutant input during the flood season. The research results clarify the key risk areas and main control measures for water quality management during the initial impoundment period, which can provide a scientific basis for monitoring optimization and pollution prevention of the Wudongde Reservoir and similar reservoirs.