The environmental impact of Three Gorges Dam (TGD) in the Changjiang (Yangtze River) is an important topic of concern for scientific communities and the public. However, the changes in river water chemistry in response to the dam construction remain poorly constrained. This study presents the seasonal variability of all major cations and some anions in the lower Changjiang during a full hydrological year from 2010 to 2011. The concentrations of all ions, except for HCO₃⁻, are higher after the TGD operation than before (p < 0.01), implying that the TGD has modified the river water chemistry in the mid-lower mainstream. Dissolved silicate (DSi) fluxes at Datong station thus increase slightly since the beginning of TGD impoundment. The change of mixing pattern of different water sources and alteration of hydrological and biogeochemical processes could cause the change of solute concentrations in the mid-lower Changjiang after the TGD operation. The mass balance model suggests that two factors primarily account for this increase of DSi observed at Datong: 1) an increasing loading of DSi downstream TGD due to erosion by “clean water”, and 2) an enhanced “Source” role of Lake Poyang in the mid-lower reaches. Our study would provide insights into the damming effect on river water chemistry and the complexity of a large river system facing rapid climate change and strong human activities.