Total suspended matter dynamics in the Songhua River over four decades: evidence of anthropogenic decoupling between soil erosion and in-stream regulation

Human activities have profoundly altered suspended-sediment dynamics in large rivers; however, long-term, spatially explicit records that disentangle basin-wide forcing from reach-specific engineering impacts remain scarce. Here, we develop a Random Forest (RF) retrieval model based on Landsat top-of-atmosphere (TOA) reflectance and 542 in situ samples to reconstruct annual Total Suspended Matter (TSM) concentrations along the main stem of the Songhua River from 1984 to 2023. The resulting four-decade record reveals a basin-wide decline beginning in the early 1990s, accompanied by pronounced spatial heterogeneity among river reaches. At the basin scale, the relationship between watershed soil erosion (RUSLE-based dataset, 1990–2022) and riverine TSM is weak at the annual timescale. This weaken the linkage is most evident in the SRH–FRH reach downstream of the Dadingzishan hydro-junction, where in-stream regulation may override upstream erosion signals. Breakpoint analysis, using 2008 as the full-impoundment year, indicates a marked acceleration in TSM decline within SRH–FRH, from −0.22 mg L −1 yr −1 during 1984–2008 to −0.78 mg L −1 yr −1 during 2009–2023. A reach-to-reach quasi-experimental comparison further demonstrates a significant post-2008 divergence between SRH–FRH and a downstream control reach (ANOVA: F = 13.045, p = 0.0001), reinforcing the interpretation that reservoir regulation exerted a dominant, reach-specific influence on suspended sediment dynamics after impoundment. Together, these findings indicate that in heavily managed basins, in-stream engineering can reorganize sediment connectivity and partially weaken the linkage between downstream suspended sediment from basin-scale erosion signals. This process underscores the need to explicitly incorporate reservoir operation into sediment management and water-quality governance frameworks.