An Augmented Indicator Framework for Hydrologic Alteration Assessment in Tidal River Networks: A Case Study of the Pearl River Delta, China

River networks influenced by tides represent dynamic, hydrologically complex systems where fluvial inflows interact with oceanic tidal forcing. Conventional flow-based indica-tors of hydrologic alteration (IHA) inadequately capture tidal-driven dynamics, including diurnal water level fluctuations, flow reversals, and tidal asymmetry. This study develops an augmented IHA (AIHA) framework comprising 96 indicators derived from hourly-resolution hydrodynamic simulations (1960–2019) in the Pearl River Delta (PRD), China. The AIHA extracts four daily series—ebb-peak flow and residual, highest, and lowest water levels—supplemented by tidal characteristic metrics. A moving t-test identified 1991 as the significant regime shift, enabling comparison of reference (1960–1991) and altered (1992–2019) periods. Three-dimensional alteration was assessed as follows: deviation from the range of variability (RVA), shift in central tendency (RCM), and change in dispersion (RCD). Indicator importance was integrated via CRITIC weighting and multi-site Borda scoring. Results show that range shifts and central-tendency shifts were generally dominated by low-intensity alteration, accounting for 58.3–71.9% and more than 85% of the indicators, respectively, whereas dispersion shifts were more pronounced, with medium- and high-alteration indicators accounting for an average of 42.4%. Water level indicators exhibited substantially greater alteration sensitivity than flow indicators, particularly in estuarine zones where the alteration degrees of some indicators exceeded 90%. Among hydrological elements, ebb-peak flow indicators responded more strongly in range shifts, with an average comprehensive alteration degree of 29.6%, while water level indicators showed more pronounced changes in central tendency and dispersion; the lowest water level indicators were especially sensitive, with average comprehensive alteration degrees of 19.6% and 77.3%, respectively. Spatially, center-of-distribution shifts (RCM) diverged: positive in western/northern tributaries (increased flows) versus negative in the eastern PRD (decreased flows). Integrated Borda scoring identified low-flow extremes during dry seasons, event timing, and tidal modulation as the most sensitive responses to hydrological stress. The AIHA framework demonstrates that tidal river alteration is characterized by intensified low-flow volatility and amplified tidal influence, with water level metrics providing a superior detection capacity than achieved by discharge alone. This process-integrated approach offers robust quantitative support for ecological flow management and estuarine restoration in tidal environments globally.