A Probabilistic Pathway Framework for River–Sea Connectivity in Distributary Networks: A Case Study in the Guangdong–Hong Kong–Macao Greater Bay Area

Hydrological connectivity regulates river–sea exchange by controlling freshwater delivery, salinity intrusion, sediment transport, and biogeochemical fluxes that sustain deltaic ecosystems. In distributary river systems, connectivity is mediated by multiple alternative downstream pathways, yet most assessments rely on network‐averaged indices and overlook cumulative regulation along interacting routes. Here, we develop a pathway‐inclusive, probabilistic framework to quantify river‐sea hydrological connectivity in distributary networks by retaining all feasible pathways and cumulative low‐head barrier effects. This formulation distinguishes structural redundancy from functional connectivity by accounting for the unequal contributions of different pathways to river–sea exchange. We apply the framework to the Guangdong–Hong Kong–Macao Greater Bay Area using multi‐decadal river network and barrier data sets from the 1980s to the 2010s. Total river network length increased from 7.9 × 10 6 m to 8.8 × 10 6 m, while sluice number increased more than sixfold and density twentyfold. Despite network expansion, functional river–sea connectivity declined by approximately 40% across maximum, average, and minimum connectivity metrics. The sharpest decline in average connectivity occurred between the 1990s and 2000s, indicating a system‐wide transition toward reduced pathway effectiveness. Connectivity losses were most pronounced along high‐capacity pathways that historically supported disproportionate exchange, whereas increased pathway number and small‐channel construction failed to offset network‐scale degradation. Spatially, connectivity declined most strongly upstream, despite barrier clustering near estuarine outlets, revealing non‐local, system‐wide regulatory effects. These results demonstrate that explicitly evaluating all feasible downstream pathways is necessary to diagnose connectivity loss, identify critical routes for river–sea exchange, and avoid misleading inferences based on network‐averaged indices.