Cryo‐FSC: A Dynamic Functional Connectivity Index Reveals Spatio‐Temporal Shifts in Sediment Transport in the Permafrost‐Dominated River Systems

Cryospheric degradation in cold regions has been fundamentally altering hydrological and geomorphic processes, intensifying sediment dynamics with profound implications for water resources management. Existing sediment connectivity indices lack the dynamic resolution to capture the intricate, seasonally evolving processes unique to cold regions. Here, we introduce the Cryo‐Functional Sediment Connectivity (Cryo‐FSC) index, integrating key cryospheric (e.g., snowmelt, freeze‐thaw state) and surface processes (e.g., normalized river channel steepness) to dynamically quantify hillslope‐channel coupling in the Source Region of the Yangtze River (SRYR). Our analysis reveals that Cryo‐FSC exhibits significant spatio‐temporal heterogeneity, with an overall increasing trend over 2000–2022, particularly in the upstream glacierized sub‐basins (e.g., Tuotuo River: 12.64 ± 3.83%/yr, P < 0.001). Crucially, Cryo‐FSC successfully captures a distinct intra‐annual seasonality, displaying peak connectivity during the thaw season (June‐September) and consistently preceding observed suspended sediment flux peaks by 3–5 days. We further demonstrate Cryo‐FSC's utility as a spatial framework for interpreting localized erosion features such as retrogressive thaw slumps, helping distinguish potentially connected source areas from disconnected deposits. This dynamic index offers an unprecedented tool for deciphering the complex sediment cascades in rapidly changing cryospheric landscapes, providing crucial insights for regional sediment management and landscape evolution.