Enormous volumes of sediment are produced in the Central Andes and are then delivered into the foreland basins of Amazon basin tributaries. While cosmogenic nuclides in sediment are a suitable tool to measure the denudation rates of sediment-producing areas, the requirement of steady state between nuclide production and nuclide removal by denudation appears to make this method less obvious in depositional foreland basins, where sediment storage may alter 10Be-based erosion signals from the sediment-providing areas. A published cosmogenic nuclide-based modeling approach however predicts that source-area cosmogenic nuclide concentrations are not modified by temporary sediment storage. We tested this approach in the large Beni foreland basin by measuring cosmogenic 10Be nuclide concentrations in detrital sediment along a 600 km long floodplain reach. The outcome of our study is that the 10Be-based denudation rate signal of the Bolivian Andes is preserved in the Beni floodplain even though this basin stores the sediment for thousands of years. For the floodplain part of the Beni basin, the cosmogenic nuclide-derived denudation rate is 0.45 ± 0.07 mm/yr, and the respective Andean source area erodes at a very similar rate of 0.37 ± 0.06 mm/yr. We thus suggest that any sample collected along a river traversing a floodplain will yield the denudation rate of the source area. This finding opens the unique possibility of constraining paleo-sediment budgets for these large basins using cosmogenic nuclides as the denudation rate signal of the sediment-producing area is preserved in sedimentary archives.