Chemical composition of river sediments and dissolved load is classically used to infer controls on continental weathering and, therefore, exert an important role on the understanding of the global carbon and biogeochemical cycles. To date, most studied river basins are strongly impacted by dam constructions; however, the effects of dams on sediment chemical compositions are little known. The Three Gorges Dam is one of the largest dams in the world and began impounding water in 2003 in the Changjiang basin. In order to investigate the impact of this dam on downstream sediment chemistry, temporal variation of sediment weathering intensity is reported here based on analyzed and compiled data between 1997 and 2018. Downstream sediments collected before 2003 are characterized by weak weathering intensity, in agreement with the overwhelming flux and fast transfer of sediments derived from the mountainous upper watershed. After the Three Gorges Dam operation, strong midlower riverbed erosion changed the roles of the midlower reaches from important sinks to major sources of sediments delivered to the East China Sea. This resulted in a progressive change of the sediment chemistry because the eroded midlower riverbed sediments were more deeply weathered, as confirmed by 150-year-old sediment cored in the lower mainstream and by mass-balance calculations. This more intensive weathering may be explained by warmer climate and longer water-rock interaction time in the midlower basin. Thus, this study suggests the need to quantify potential bias in weathering intensity and controls caused by damming activity in large river systems.