Distribution and interaction characteristics of water quality at the stratified confluence reservoirs

The construction of large-scale dams creates tributary bays between the main and the tributary reservoirs. The water temperature and water quality in the tributary reservoirs are inevitably affected by backflow from the main reservoir. In this study, the spatial-temporal evolutions of water temperature, biochemical oxygen demand (BOD) and dissolved oxygen (DO) in the Baihetan reservoir and its four tributary reservoirs are investigated by building, validating and applying the TBD (temperature, BOD and DO) numerical model. The results show that the influence of the main reservoir on its tributary reservoirs show seasonal patterns. The water quality in the main reservoir is determined by inflow, reaeration and temperature-driven stratification, all of which exhibit seasonal patterns. Stratification determines the fate of inflow after it enters the reservoir, which leads to relatively low DO concentration in the middle layer of the Baihetan reservoir in summer and autumn. For the tributary reservoirs, water quality is also affected by the balancing between the upstream inflow and downstream backflow, which varies both seasonally, and as a function of the reservoir geometry. These results have great significance in analyzing and understanding the environmental impact of large hydraulic projects, which can be used as the basis for enacting hydraulic project plans and preserving the aquatic ecosystem in the reservoir areas.