Long-term ecological effects of altered flow-sediment regimes in the lower yellow river: nonlinear thresholds, channel mediation, and spatial heterogeneity

Introduction Ecological responses to altered flow and sediment regimes may involve interannual lags, nonlinear responses, and channel mediation, yet these features are rarely integrated into a single assessment framework for the Lower Yellow River. Methods We integrated flow, sediment, and channel records for 1960–2025 with ecological records for 1980–2025 in the Huayuankou–Gaocun and Gaocun–Taochengpu reaches. Observational associations, pre-/post-2002 regime contrasts, and mechanism-constrained scenario responses were evaluated using an Almon polynomial distributed-lag model, first-difference sensitivity analysis, block bootstrap, segmented analysis, and evidence grading. Results After 2002, sediment-disturbance intensity decreased by 1.34 and 1.27 standard deviations in the two reaches, whereas flow-pulse intensity changed by only −0.09 and 0.06 standard deviations. The clearest observational association was the sediment–habitat pathway in the Gaocun–Taochengpu reach (CES = −0.148; 95% CI: −0.199 to −0.092; mean lag = 2.08 years), followed by the sediment–water-quality pathway in the Huayuankou–Gaocun reach (CES = −0.236; 95% CI: −0.418 to −0.060; mean lag = 3.21 years). Within the mechanism-constrained scenarios, moderate flow increases produced positive direct, indirect, and total effects on all four ecological indicators, whereas the habitat effect became negative above the P80 flow boundary; above the P75 sediment boundary, constrained effects were negative across all four indicators. P80 and P75 were exceeded in 9/46 (19.6%) and 12/46 (26.1%) years, respectively. Discussion These findings indicate that the channel acts as a critical interface transmitting flow and sediment signals to the ecosystem, and that moderate forcing can be beneficial whereas upper-tail forcing is inhibitory. The exceedance frequencies are descriptive historical quantile statistics for screening and scenario testing rather than independent ecological-risk probabilities. Wetland recovery remained low (ERS = 20.1% and 61.8%), supporting prioritization of floodplain–channel connectivity restoration, moderation of high-sediment-year deposition, and optimization of moderate-flow regimes in future ecological regulation.