Due to an intensification of anthropogenic activities and climate change in recent decades, the hydrological connections and relationships between rivers and lakes have been significantly modified globally. Poyang Lake is one of the largest freshwater lakes globally and is one of the few that remain naturally connected to the Yangtze River. To investigate the full hydrological conditions (extreme high and low discharge) of Poyang Lake outflow under current bathymetric conditions, a large-scale 1D- and 2D-coupled high-resolution hydrodynamic model of the Poyang Lake basin–Yangtze River system was developed. We simulated the outflow and water levels of Poyang Lake under nine different extreme hydrological scenarios with high precision and computational efficiency. We propose (1) a novel partition calibration method to characterize the roughness coefficient of large water bodies in complicated geographical terrain both for wet and dry seasons; (2) a new method for setting initial conditions for hydrodynamic simulation of large water bodies subject to strong hydrological regulation. Results indicated that (1) maximum outflow and water levels will reach 37,200 m³/s and 22.28 m when Poyang Lake basin floodwater coincides with flooding on the Yangtze River; (2) precipitation over the lake has increased outflow but this has had very limited influence on its changing hydrological pattern; (3) the effect of hydrological conditions within the system differs for both the lake outflow and water level. The research provides important reference conditions for the application of the InfoWorks ICM model in future applications and studies of large river–lake systems.

River regulation by the construction of reservoirs represents one of the greatest challenges to the natural flow regime and ecological health of riverine systems globally. The Danjiangkou (DJK) Reservoir is the largest reservoir on the Hangjiang River and commenced operations in 1967. The reservoir was upgraded in 2012 to provide water resource for the South–North water transfer project through central China. However, the effect of the reservoir operations on the downstream hydrological regime and ecological health of the Hanjiang River following the upgrade (increase in dam height and reservoir capacity) has not been examined thus far. The daily discharge series from four stations along the main stem of the Hanjiang River, including a site upstream, were examined from 1950 to 2017. The study series was divided into three periods based on the difference stages of the reservoir operation: (1) 1950–1966, (2) 1967–2012 and (3) 2013–2017. The nature of hydrological alteration, ecological flow requirement and potential ecological risk during the different periods were investigated. The results clearly indicate that the DJK reservoir has significantly modified the hydrological regime in the middle and downstream section of the Hanjiang River, with most significant modifications recorded immediately downstream of the reservoir. None of the observed ‘Range of Variability Approach’ hydrological indicators fell within the expected range at Huangjiagang following the increase in reservoir capacity. As a result, the ecological flow requirements could not be guaranteed, and the frequency and intensity of ecodeficit increased. The river ecosystem immediately downstream of the dam was observed to be at high risk of ecosystem degradation during the post-dam periods considered.