<p>Riverine boron (B) and its isotopic compositions (<i>δ</i>¹¹B) are commonly used to trace silicate weathering within watersheds, but its sources and isotopic fractionation mechanisms remain contentious. In this study, we collected the seasonal river waters of the Buha River, the largest inlet river of Qinghai Lake, which is sensitive to climate change, and analyzed its seasonal variations of the major ions, B concentrations, and <i>δ</i>¹¹B, to explore its sources and controlling factors. The results indicate that B in the Buha River predominantly originates from weathering of silicate rocks, with significantly seasonal variations in the geochemical behaviour of B isotopes. In the rainy seasons, the riverine B isotopic fractionation is primarily controlled by weathering of silicate rocks under the influence of hydrological conditions, where clay minerals preferentially absorb ¹⁰B, leading to the enrichment of heavier B isotopes in river waters. In contrast, in the dry seasons, the B of river waters may mainly come from the recharge of groundwater (whose <i>δ</i>¹¹B with a notable pH dependence); owing to prolonged retention of fluids, the steady state of <i>δ</i>¹¹B is reached by the isotopic equilibrium through adsorption of clay minerals. This study highlights that the seasonal variations in riverine <i>δ</i>¹¹B in semi-arid regions are jointly governed by silicate weathering, hydrological conditions, and water-rock interactions, with water pH and rainfall as key regulators. Consequently, riverine <i>δ</i>¹¹B in the rainy seasons under semi-arid climatic conditions can effectively trace hydrologically-controlled silicate weathering processes within the watershed.</p>