Study on the Spatiotemporal Characteristics and Driving Mechanisms of Backflow in Reservoir Areas Under Joint Operation of Cascade Reservoirs

The coordinated operation of cascade reservoirs, while achieving multi-objective optimization in flood control and power generation, profoundly alters the natural hydrological rhythms of river systems. The combined effects of downstream water level backwatering and upstream flow reduction can trigger reverse flow within reservoir zones, exerting coupled impacts on environmental processes such as water exchange, nutrient transport, and algal blooms. This study focuses on the Xin’anjiang–Fuchunjiang cascade reservoir system, constructing a high-spatiotemporal-resolution model for May to July 2025 using the MIKE 21 two-dimensional hydrodynamic model. By integrating Lagrangian particle tracking, Spearman rank correlation analysis, and redundancy analysis (RDA), we systematically reveal the spatiotemporal patterns, propagation mechanisms, and driving factors of reverse flow in the Fuchunjiang reservoir area. Results show that during the study period, reverse flow occurred frequently in the middle reaches, decreased toward the upper and lower reaches, and occurred sporadically in tributaries. Reverse flow was predominantly low-intensity and intermittent, exhibiting a bimodal temporal distribution at night and in the morning. Reverse flow events were primarily driven by daily shutdowns of the Xin’anjiang hydropower station, propagating sequentially from upstream to downstream and then into tributaries; fluctuations in the Fuchunjiang reservoir water level and changes in Lanjiang inflow played only minor modulating roles. The three key driving factors—discharge from the Xin’anjiang station, Lanjiang inflow, and Fuchunjiang reservoir water level—together explained 80.1% of the total variation in reservoir current velocity, showing a segmented control pattern. Reverse flow can cause high-concentration nutrients from the Lanjiang River to be transported backward into the main stem of the Xin’anjiang River, affecting an area of approximately 2.5 km. A single reverse flow event increases average nutrient retention time by about 6 h, and the cumulative effect of daily occurrences creates favorable hydraulic conditions for cyanobacterial proliferation. Reservoir reverse flow can be classified into two types: standing wave oscillations and propagating reverse flows. The former corresponds to recurrent water level fluctuations caused by daily regulation at the power stations, with flow direction fluctuating slightly around the mean and producing minimal net transport; the latter refers to the reverse flow propagation following shutdowns at the Xin’anjiang station, advancing sequentially from upstream to downstream and then into tributaries, with a clear direction of water mass transport. The ecological impact of backflow depends not directly on its intensity, but on the extent to which it increases water retention time and, accordingly, on the source and transport direction of the nutrients it carries. Integrating hydrodynamic simulation, Lagrangian particle tracking, and driver factor analysis at time resolutions ranging from 1 to 30 min, this study applies these methods to a cascade reservoir system characterized by “peak shaving at upstream, water level control at downstream, and tributary inflow management.” It systematically quantifies the spatiotemporal patterns and driving mechanisms of reverse flow under cascade joint operation, providing scientific support for optimizing reservoir operations, managing reservoir water quality, and protecting drinking water sources.