Timing Matters: Dual-Track Recovery of Critical Habitats in Dam-Regulated Rivers.

Sandbars function as critical habitats for rare and endangered species within riverine ecosystems, exhibiting ecological significance disproportionate to their spatial extent. Sandbar restoration, including active restoration (AR) and passive recovery (PR), is essential for maintaining riverine ecology integrity, especially in dam-regulated rivers where sandbars are susceptible to hydrological alterations. However, when and how to integrate AR and PR, considering both effectiveness and efficiency, remains a question. Here, we reveal a "Dual-Track Recovery Mechanism" that explains the roles of AR and PR pathways, as well as the optimal integration timing, based on analysis of 20-year sandbar evolution in the Yangtze River following Three Gorges Dam impoundment. Our results show that PR stabilized sandbar morphology by adjusting morphological stability index below 0.05; afterward, AR accelerated vegetation establishment (elevating NDVI to 0.6 and vegetation coverage to 70%). Integration of PR and AR showed excellent resilience after floods. The optimal timing of integration comes when sandbar morphological stability index reaches a moderate level (<0.05) through PR and hydrological conditions are favorable (submergence periods < 60 days and inundation depths < 1 m). These findings provide theoretical understanding and practical guidance for future sandbar restoration, which is fundamental for protecting rare and endangered species in dam-regulated rivers.