Abstract: Variations in bed material composition affect suspended load recovery and movable bed resistance. This paper calculates inter-annual variations in bed material proportion at seven hydrological stations in the lower Yellow River (LYR), based on the 1999-2020 bed material data measured at these stations. We examine spatial and temporal coarsening characteristics of different reaches during continuous channel degradation in the past 20 years, and discuss the influence of channel evolution on bed material coarsening. In time variations, coarsening characteristics varied in different periods after the operation of the Xiaolangdi reservoir, with a sharp trend in the early stage 2000-2006 and a slow trend in the later stage 2007-2020. In the later stage, bed material was coarsened with temporal fluctuations at the stations above Gaocun Station, but changed little below; the reach with the highest coarsening degree moved downstream from the braided reach to transitional reach. Spatially, bed material became finer along the stream. The braided reach was dominated by very coarse sediments and the transitional reach by coarse and very coarse sediments, while the medium and fine sediments were not negligible in the meandering reach. The adjustment of bed material composition was characterized by both medium and fine sediment erosion in the early stage, while by medium and coarse sediment erosion and very coarse sediments deposition in the later stage. At the stations above Lijin, the proportion of fine sediments was less than 5%, limiting suspended load recovery; medium and coarse sediments were still eroded, but very coarse sediments had a good supply of the corresponding suspended load. In post-flood period, the median particle size of bed material was power correlated positively with the cumulative erosion volume of the river bed, but its influence on bed material was limited due to a considerable degree of coarsening in the later stage. The results reveal the current situation of bed material coarsening in LYR and help predict the channel deformation.

Key words: bed material coarsening, different reaches, limited recovery, continuous channel degradation, lower Yellow River