Where to Restore: Hydrological Connectivity Differentiates Ecological Restoration Outcomes in Dam-Regulated Rivers

Ecological stabilization of sandbars is essential for maintaining habitat integrity in dam-regulated rivers, yet whether restoration effectiveness varies systematically within project areas and is predictable from design-phase hydrological data remains largely unexamined. Here, we show that hydrological connectivity gradients create divergent spatial patterns of restoration effectiveness, based on field campaigns of a sandbar stabilization project in the Middle Yangtze River. Vegetation established uniformly across the project area (coverage 83 ± 16%) was sharply differentiated within a single flood season along the connectivity gradient (postflood coverage 0-76%), with 65% of the project area covered by flood-deposited sediment. Two independent predictive methods converged on a primary flow-velocity threshold of 0.82 m/s, delineating an effective zone (33%; coverage 47%); descriptive clustering of the above-threshold plots further distinguished transitional (22%; coverage 7%) and limited (44%; coverage <1%) response groups. This spatial pattern persisted through the postflood monitoring period, indicating a connectivity-associated organization of vegetation-cover trajectories. Hydrological data available during project design can identify these spatial boundaries, enabling differentiated strategies that allocate resources where success is most likely. Coupling spatial connectivity criteria with established temporal thresholds, we propose a "when-and-where" framework as a transferable methodological approach for optimizing ecological restoration in dam-regulated rivers.