Spatiotemporal evolution and influencing factors of bed material grain size in the Lower Yellow River

Study region: The Yellow River Basin and the Lower Yellow River in China. Study focus: Bed material grain size in the Lower Yellow River (LYR) has undergone substantial changes during 1963–2020, influenced by the dam operations, soil conservation, and water diversion. This study analyzed bed material data from seven hydrological stations along the LYR, comprising 398 groups of annual average grain size measurements. The median grain size, sorting parameter, and compositional variation of the grain size distribution sampled from the river bed of LYR were analyzed to identify the influencing factors on their spatial-temporal evolution. New hydrological insights for the region: This study investigated the spatiotemporal evolution and influencing factors of bed material grain size in the Lower Yellow River (LYR). We found that (1) the median grain size (D50) experienced three phases in the past: minimal change (1963–1985), significant decrease (1986–1999), and notable increase (2000–2020). Bed sediment grain size exhibited longitudinal fining along the river during all periods. The sorting coefficient (σ) remained stable over time but decreased along the river, indicating an enhanced size uniformity. Compositionally, fine sand(FS) and coarse silt(CS) exhibited opposing trends during 1986–1999 (decreasing FS and increasing CS) and 2000–2020 (increasing FS and decreasing CS), whereas very fine sand (VFS)remained relatively stable as the transitional component. (2) Dam construction, soil and water conservation, and water diversion substantially impacted bed material characteristics. The impoundment of the Longyangxia Reservoir caused the fining of downstream bed materials, reducing D50 and fine sand while increasing coarse silt. In contrast, the construction of the Xiaolangdi (XLD) Reservoir caused coarsening. Soil and water conservation reduces sediment load at Tongguan station, increasing D50 when sediment load is less than 5 × 10⁸t due to erosion and selective coarsening in the LYR, but without increasing the sorting coefficient. Declined stream runoff, combined with unchanged water diversion from 1986 to 1999, drove bed material fining. This provides a scientific basis and technical support for river regulation and engineering applications.