The Yangtze River is the largest fluvial system draining the Tibetan Plateau, yet its time of formation—pre-Miocene versus Pleistocene—has been debated for more than a century, with a particular focus on the incision of the Three Gorges. In this study, we used laser-ablation–multiple collector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) to measure Pb isotope compositions of detrital K-feldspars from Pliocene–Pleistocene sediments in the Jianghan Basin, located just downstream of the Three Gorges. Our new Pb results indicate that feldspars from the Songpan-Ganzi terrane were already being delivered to the Jianghan Basin by ca. 3.4 Ma. Therefore, we suggest that the Three Gorges was incised prior to the late Pliocene. Moreover, the Pb isotopic data also record the first delivery of detritus from the Hanjiang River, one of the Yangtze’s largest tributaries in the middle reaches, into the Jianghan Basin at ca. 1.8 Ma. Most K-feldspar grains from the mid-late Pleistocene samples have similar Pb isotopic compositions to those from the major tributaries in the upper reaches, i.e., Minjiang and Jialingjiang Rivers, indicating that the eastern Tibetan Plateau, as the major sediment supplier to the Jianghan Basin and mid-lower Yangtze River, has played a key role in the sediment source-to-sink transport of this river. This study sheds light on the factors that control the supply of detrital K-feldspar in large-scale drainage systems.