The Late Cenozoic evolution of the major river networks draining eastern Tibet has major tectonic implications for the development of the plateau but remains highly contentious. In this study from the southeastern margin of the Tibetan Plateau, we constrain the evolution of the Paleo-lake Xigeda (recorded by the lacustrine Plio-Pleistocene Xigeda Formation) using combined fission-track (FT) dating, U-Pb and trace-element analysis of detrital apatite, along with U-Pb dating of detrital rutile and zircon. Critically, the detrital apatite U-Pb age data from the Xigeda Formation exhibit a major Late Triassic to Jurassic peak that is also seen in the U-Pb rutile and zircon data. When integrated with apatite fission-track (AFT) age peaks at ca. 60–40 Ma and ca. 20–10 Ma and apatite trace-element analyses, these U-Pb ages support provenance from the Songpan-Ganzi and southern Longmenshan terranes to the north. Thus, the Paleo-Xigeda lake was fed by south-flowing paleo-Dadu and paleo-Yalong rivers during Plio-Pleistocene time. Pliocene tectonic activity on the Xianshuihe-Daliangshan-Xiaojiang fault zone likely formed Paleo-lake Xigeda by damming of the Middle Yangtze River; subsequent capture of the Upper Yangtze by the Middle Yangtze after ca. 1.3 Ma likely breached the Paleo-lake Xigeda.