Burial dating using cosmogenic 26Al and 10Be is an emerging technique for establishing chronologies of fluvial deposits ranging in age from ca. 0.5 Ma to 5 Ma. The determination of burial age, however, requires an assumption that sediment is buried with a known initial 26Al/10Be ratio. Using a sequence of well-dated fluvial terraces along the Yellow River in Lanzhou, China, we demonstrate that this assumption may be violated when upstream sediment sources include Neogene sedimentary basins. Samples of fluvial gravel from six terraces yield burial ages ranging from ca. 0.83 Ma to 2.81 Ma. Comparison of these results to independent ages determined from magnetostratigraphic and loess-paleosol studies demonstrates a high degree of correspondence between ages in the lower three terraces. In the higher terraces, however, burial ages are systematically too old, implying a previous burial history. These results can be exploited to place quantitative constraints on the provenance of sediment in the terrace deposits. Our results reveal that rapid incision and excavation of late Neogene sedimentary basins ca. 1.7 Ma led to injection of fluvial sediment with low initial 26Al/10Be ratios. Dilution of this source through time occurred as incision progressed into upstream bedrock ranges. Our results suggest that cosmogenic nuclides can illuminate the timing of long-term storage and remobilization of sediment in continental-scale river systems.