River fragmentation and barriers removal priority in the Oujiang catchment: A longitudinal connectivity perspective

Oujiang catchment, Zhejiang province, China. This study provides the catchment-scale quantitative assessment of river connectivity in the Oujiang catchment and establishes barrier removal priorities with the primary objective of maximizing river connectivity to support catchment-scale restoration. Six connectivity indices were systematically compared to evaluate spatial heterogeneity and index-dependent prioritization. Drivers of barrier construction were quantified using Generalized Linear Mixed and Random Forest models, focusing on environmental and anthropogenic controls. River connectivity in the Oujiang catchment was low, with all indices showing near-zero mean values (Dendritic Connectivity Index (DCI) = 0.003; Stream Connectivity Index (SCI) = 0.002; River Volume Connectivity Index (RvCI) and Catchment Area Fragmentation Index (CAFI) = 0.005; Low-altitude Population Connectivity Index (LaPCI) = 0.003 and High-altitude Population Connectivity Index (HaPCI) = 0.001), indicating severe hydrological fragmentation. Headwater reaches and tributaries exhibited lower connectivity than the mainstem, highlighting the dominant role of discharge accumulation in structuring longitudinal connectivity. Barrier removal priorities varied among indices: structure- and flow-weighted indices focused gains on a few barriers, whereas HaPCI highlighted spatially dispersed headwater barriers. Barrier density was mainly controlled by hydrological gradients, especially river volume and stream gradient, with population and road density playing minor roles. This indicates strong hydrological control over river fragmentation and restoration priorities in mountainous catchments. • Multiple indices were applied to assess river connectivity of the Oujiang catchment. • Headwater and tributary barriers have led to severe fragmentation across the Oujiang catchment. • Barrier density increases significantly with land surface runoff and stream gradient. • Barrier removal priorities were established to maximize river connectivity, supporting catchment-scale restoration.