The preservation of conglomerates far from mountainous sources is commonly interpreted in terms of tectonic or climatic forcing. To relate a depositional signal to changing conditions in source areas, the process and duration of sediment routing from source to sink need to be determined. For the first time, we quantified sediment transport histories using cosmogenic 21Ne in quartzite pebbles from modern river gravels and Neogene conglomerates from the modern and ancient North Platte River of the Great Plains of Nebraska, United States. We demonstrate that at ∼400 km distance from the Front Ranges of the Rocky Mountains, the majority of pebbles were stored in older channel deposits for up to several millions of years before being recycled; this was enabled by very slow to zero basin subsidence rates. This implies that upstream tectonic or climatic controls on surface processes were decoupled from the downstream depositional record—a result supported by the similarities in cosmogenic 21Ne values among Miocene, Pliocene, and modern river channel pebbles despite known changes in tectonic and climatic forcing.